How to crimp: Making reliable automotive electrical connections

This post details the proper method for making automotive electrical connections using the crimp method. It is not intended to be an authoritative or comprehensive guide, but rather an introduction to some basic concepts, hardware, and methods that have served me well. The intended audience is enthusiasts who don’t have any formal training in the area but like or prefer to do their own wiring projects, which can range from a simple radar detector installation through full chassis and engine harness fabrication.

In my experience, automotive electrical problems can be amongst the most frustrating to diagnose with loose or faulty connections often being the root cause. However, unlike so many other types of problems that can occur as a result of mechanical stress, driver error, or just plain bad luck, electrical problems can be almost entirely avoided by applying a little bit of knowhow when making connections.

Several years ago, I installed a fuel cell which – amongst other things – involved wiring up an electric fuel pump. For serviceability I incorporated a connector into the design, which involved splicing two wire ends together and applying solder to ensure they didn’t pull apart.

Everything worked well at first but after a while I started experiencing problems with the engine cutting out in the turns (one high speed bumpy turn in particular). My initial hunch was fuel delivery related – which ultimately proved to be true – but the specific cause was time consuming and difficult to diagnose. Random luck helped my find the problem sooner than would have otherwise taken.

While testing a new fuel pump in the workshop I bumped the connector and the pump sputtered. Turning my attention to the connector, I discovered that the soldered connection failed leaving me with two wire halves. The heat shrink insulating material hid the problem from view and kept them mostly in contact with each other but all it took was a little bit of force and/or vibration (like driving over a bumpy turn) to open up a gap breaking the flow of current to the pump.

Like most electrical problems, the fix for this problem was easy once diagnosed but a long an painful journey to get there. Ironically, in my effort to sure up the connection with solder I made it less reliable. Realizing this sparked my interest as to the correct way to make reliable electrical connections.

Update: 2/17/2013

The topic of soldering versus crimping is often debated. Already, there have been several responses to this article denouncing crimping as the preferred method to soldering. That crimping is a preferred method is my opinion (a generalization for which exceptions apply) and I am in the good company of aerospace, military, Formula 1, and medical. If performed correctly, both soldering and crimping will produce reliable connections. Likewise, if performed incorrectly, both will result in unreliable connections. Hence, there is an abundance on anecdotal evidence for and against each method available on the Internet.

A key and significant advantage of crimping over soldering is the ease and speed with which a properly formed connection can be consistently performed. I have seen far too many people get soldering wrong (also my opinion, MSME & having worked in aerospace too) and hence my motivation for writing this article. With proper tooling crimping is nearly foolproof. There are a wide variety of connector types and associated tooling that range from inexpensive to very expensive, but even inexpensive connectors can produce good results. The tooling and connectors demonstrated in this article are on the more expensive end of the spectrum but the same principles and techniques apply to other connector types as well.

Why soldering is bad

There was a time when I soldered all of my electrical connections under the misguided belief that I was making them more reliable. I eventually learned otherwise. I should have known better but I hadn’t thought about it deeply and the practice is commonplace, even amongst some automotive professionals. To understand why soldering is bad, entertain the possibility you could purchase ceramic insulated wire for your next automotive electrical project. Would you use it?

Hopefully your answer to the above question is a resounding no. Ceramic has several properties that when viewed independently make it an excellent choice; however,  its low ductility (brittleness) makes it a poor overall choice. Wire must be able to bend freely as it is routed through the car and is subject to constant flex and vibration. When you use solder to make electrical connections you’re also greatly reducing the wires ductility in the solder region and – like a chain – the wire is only as reliable as its weakest link.

If you take the opportunity to inspect the connections made by the manufacturer of your automobile, you’ll discover that most if not all connections do not have any solder applied. The recall cost to an automobile manufacturer for a flawed electrical connection can run hundreds of millions of dollars, so their engineers incorporate only the best practices for forming durable and reliable electrical connections. Think about this the next time you pull out your soldering gun to make your electrical connection super reliable!

The following video illustrates the effects of solder on a wire connection and compares it to a properly formed connection using a crimper.

Thus far the focus has been on reliability as reason to avoid solder but there are other practical considerations as well. Soldering can take significantly more time than crimping (a topic we will get to shortly). For large projects, this can mean added days. It also requires a higher degree of skill to execute properly if used (e.g., not enough heat, wicking past the contact into the insulated area, etc.). For enthusiasts, electrical work is often performed in compromising positions with poor lighting such as under the dashboard, underneath the car, inside the engine bay, etc., which can make applying solder a challenge. Proper ventilation in tight quarters can also be difficult to achieve but is necessary in order to avoid breathing in toxic fumes. Care should also be taken to avoid letting solder (or solder residue) come into contact with the mouth, cuts, and sores. Always wash your hands after handling solder!

I suspect many resort to soldering connections because they perceive crimps as being weak or otherwise insufficient by themselves. A common scenario it to take a crimp-style connector, crimp, and then apply solder to strengthen the connection. However, a properly executed crimp can be as strong as or stronger than the wire itself is properly executed. By adding solder you are ultimately undermining your desire to form a more reliable connection.

You might be wondering why soldering connections is bad when modern cars contain scores of electronic printed circuit boards (PCBs) each having hundreds of soldered connections. Unlike wires being routed throughout your car, the components soldered onto a PCB are not subject to flex and handling because they are mounted to a rigid plan (the PCB board) contained within an enclosure. If vibration is a concern, the PCB it can be mounted on vibration isolators and epoxy can further be used to mechanically secure the components in place. None of this is true for most automotive wiring projects. Furthermore, as with any rule, there are exceptions and the application of solder in forming connections is no exception. Used properly in conjunction with crimping, solder can be used to build a more reliable connection but is unnecessary in most cases and – in my opinion – the risk of getting it wrong does not outweigh the benefits and therefore is not discussed.

If done with care, solder can be applied to a crimp connection as an added measure of reliability by applying only in the area near the very tip of the wire. The solder must not flow into the region below the crimp compression as you want the wire itself to remain flexible so it does not break apart from the connector. In my opinion, the risk of making a mistake is far greater than any improvement in reliability (marginal at best). It also adds a significant amount of time for each connection to be made (2-3 times) and it therefore not advised.

Crimping

The figure below shows a wire properly crimped a solid contact pin. Unlike other connector types, this pin doesn’t lend itself to soldering even if we wanted to. Visual inspection shows 8 compression points (only 4 are visible in the figure). A cross-section view is shown in Table 1.  The stranded wire can freely flex at the base of the connector with little or no fatigue and the wires are held in place by compressive forces that exceed the strength of the wire itself, making for a connection that is strong, durable, and virtually impervious to vibration. It took only a few seconds to form this connection with proper tooling using a foolproof procedure.

A solid contact socket properly crimped to wire takes only a few seconds to achieve. This is a good crimp, but ideally there should be a little bit of uninsulated wire showing before it enters into the connector pin (about 1/2 the diameter of the wire).

Same pin as shown above after applying tension to the point of failure. Visual inspection shows that the wire strands broke before the crimp failed confirming that the crimp is stronger than the wire itself.

Same pin as shown above after applying tension to the point of failure. Visual inspection shows that the wire strands broke before the crimp failed confirming that the crimp is stronger than the wire itself.

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Close-up view of same pin shows the tip of the wire showing through the inspection hole, providing further proof that the crimp is stronger than the wire itself.

The perception that crimping is inadequate may stem from the commonplace crimper available for about $10 at your local hardware, automotive, or electronics store. This tool has no place in forming automotive electrical connections and if used, adding solder probably will serve to better the connection.

The commonplace hardware store crimper has no place in forming reliable automotive electrical connections.

The key to a reliable crimp connection is having the right tool for the job, and knowing how to use it. The right tool depends on the type of connector being used, of which there are many. Even an aerospace quality crimp tool will not work or produce proper results if used on the wrong connector type. Some shops limit themselves to using a small handful of connector types because they don’t want to or cannot afford to purchase tooling for every type. I’m partial to Deutsch DTM style connectors for reasons stated below and which will be illustrated extensively throughout this article. For these connectors, crimp tool like the one illustrated by Figure 5 is required. If you choose a different type of connector, be sure to ask the vendor or do your research as to the proper tooling required and learn how to use it properly.

A proper crimp tool for automotive applications (Astro Tool M22520/1-01) for use with connectors having Amphenol style circular contact pins.

Video Demonstration

Tooling and Connector Hardware

I am going to describe and illustrate how to make a proper connection using a Deutsch DTM style connectors. Again, this is the type of connector type I prefer partly because they are used by Motec in the construction of their ECU harnesses so I needed to become familiar with them have acquire the required tooling. By no means do I attest these to be the only or even the best connectors for making durable automotive connections. There are several good alternatives available such as GM Weatherpack, Metripack, Deutsch Autosport, and many more.

Deutsch DTM Connector

In addition to the reasons stated above, reasons why I like Deutsch DTM connectors and have stayed with them are several:

  • Incorporates circular contact pins facilitating fast and virtually foolproof crimps.
  • Wedgelock insert fixes pins precisely in place for smooth and easy plug / unplug action.
  • Suitable range of wire sizes (16-22 AWG) for most connection needs.
  • Available in 2, 3, 4, 6, 8, and 12 pin connector arrangements.
  • Inline and flange mount available.
  • Parts readily available from a wide range of suppliers in gold or nickel plated.
  • Pins are easily removed from connector housing and can be purchased separately.
  • Receptacle and plug assembly halves can be purchase separately.

Deutsch DTM connector parts consists of pins or sockets (sockets shown), housing (plug assembly shown), and Wedgelock.

Close-up of 4-pin plug assembly having part number DTM06-4S. Not shown is corresponding receptacle assembly having part number DTM06-4P.

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DTM solid pin drawing. Source: Deutsch Industrial Product Catalog.

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DTM solid pin drawing. Source: Deutsch Industrial Product Catalog.

I order my connectors through Motec USA because they’ve never failed me and they also offer kits that make ordering parts a little easier (housing, pins, and wedge sold packaged together). The kits can be found in the Motec catalog available on their website. For a complete list of distributors in the USA and worldwide, visit the TE distributors webpage. I found the complete Deutsch connector catalog on the LADD Industries website. LADD is a Deutsch distributor.

In addition to complete product line, the Deutsch catalog contains lots of useful information such as produce line overview, tooling, electrical characteristics, operating condition tolerances, and how-to instructions.

You should select the housing configuration that is correct for your application. With the DTM connectors, you also have the option to select pin type. I like and use the solid pins because they’re very easy and virtually foolproof to crimp, so this is what I’ll be going into more detail on when describing the assembly process. Should you choose to also use a Deutsch connector, I recommend reading the catalog sections that describes details regarding assembly, electrical characteristics, and operating condition tolerances.

With the DTM connectors you can choose solid barrel or stamped pin types. I prefer the solid barrel pins because they result in fast and virtually foolproof crimps. This pins seen in the illustrations are the solid barrel type. Stamped pins have wings that curl onto the wire. Either pin type is fine for most applications. Just be aware there are two different types of pins, each requiring a different crimp tool. Refer to the Deutsch catalog for additional details.

Crimp Tool

The correct tool is critical for achieving a proper crimp, else you’re probably better of soldering (as bad as that is). Check with the manufacturer or distributor for your connector of choice for information on tooling requirements. For the Deutsch DTM series connector, Tyco (manufacturer) recommends the DMC MH860 crimper, which is a circular indent style crimper conforming to MilSpec M22520. Its design accommodates a turret that can be removed and replaced by another to accommodate different contact types making it a versatile crimper (I also used this same crimper to crimp connector for MilSpec bayonet connector shown by Figure 8).

The MilSpec M22520 crimper incorporates an interchangeable turret design that can accommodate different connector types such as the MilSpec (MS3116F2041S) bayonet lock bulkhead connector as shown here.

For DTM connectors, I use the M22520/1-01 “large MilSpec crimper” manufactured by Astro Tool (Figure 5), also available from Daniels Manufacturing Corporation (DMC). These cost about US$250 new. You’ll also need to purchase a crimp head (a.k.a., “turret”), which is M22520/1-02 for the large crimper and about US$80 (Figure 9). In all, expect to pay about US$330 new. These are also commonly available on eBay for about ½ the cost, which is how I purchased mine. It sounds like a lot of money for a crimper, but its foolproof design will save you tons of time and you’ll get perfect crimps every time. I’ve used a number of different contact types and associated crimpers and this is by far my favorite, so I seek out connectors incorporating a solid round pin design for all of my wiring projects where applicable.

M22520/1-01 crimper incorporates an interchange crimp head (turret) for accommodating a variety of contact designs. Use M22520/1-02 for the Deutsch DTM solid contact. Each gauge holes shown (12, 16, and 20) actually accommodates a range of gauge sizes, and adjustment for the desired gauge wire is performed using the wire size selection dial shown by Figure 11, and as specified on the turret label (see next photo).

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The crimp head (turret) specifies the wire size selector setting to use for the gauge wire being crimped to the contact. (Only part of label is visible in photo.) For example, for 24 gauge wire, rotate the red hole (20) into position as shown, and set the wire selector into the 2 position.

The wire size selector wheel configures the crimp tool for the specific wire size (gauge) to be crimped. As shown, the tool is configured for 20 gauge wire.

Wire

Although not required in order to achieve a good crimp, I use Tefzel insulated wire whenever possible. It is military spec (MilSpec 22759) and used by F1 racecar and airframe constructors. Definitely overkill for stereo and radar detector installations, but if building engine or chassis harnesses, or routing wire through harsh environments (temp, oil, moisture, etc.) then I recommend you consider it if long-term, trouble free performance is desired. I indiscriminately use Tefzel wire for all of my jobs given the choice.

Like a quality crimper, you’re not going to find Tefzel wire at your local hardware, electronics, or automotive store. You’ll need to order it online but there are plenty of suppliers. I get mine from Pegasus Auto Racing Supplies.

For automotive use, always use stranded wire for flexibility. Solid core wire has no business being in an automobile.

Wire Stripper

A wire stripper tool is used to strip the insulation from the endpoint of the wire being inserted into the electrical contact to be crimped. When stripping, leave the insulation being stripped from the wire covering the tip of the wire so that it can be easily removed by hand (), but so that it protects wire strands from fraying at the endpoint. This will help to ensure a smooth insertion into the connector housing. You’ll also want the strip length be correct for the contact being crimped. A quality wire strip tool will help you easily achieve these goals.

As described in the illustrations below, I use the Ideal Stripmaster wire stripper. It’s expensive but only absolutely required if using Tefzel wire, else less expensive strippers of similar design can be used with good success.

Use a wire stripper to strip the insulation from the wire. Pull the insulation away from the wire but keep the tip covered until just prior to crimping to protect the wire strands from fraying.

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The Ideal Stripmaster wire stripper for MIL-W-22759/16 Wire (26-16ga). At around US$250, it’s very expensive. Suitable for wire gauges 16-24, but only required if stripping Tefzel insulated wire. Else, lower cost strippers of similar design available at your local hardware store will suffice; however, its small form factor and precision operation will have you appreciating it even if not using Tefzel.

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The Ideal Stripmaster incorporates hardened removable dies that are more precise than your garden variety (cheaper) wire stripper having a similar design. They are required for stripping Tefzel insulated wire. Cheaper strippers do not cut cleanly into Tefzel wire insulation otherwise lack the precision to get a good strip.

Crimping: Step-by-Step

Finally, I’m going to describe how to execute a proper crimp using a solid round contact for a Deutsch DTM connector. Different contact types involve different tooling, but the overall process remains largely unchanged.

Strip the wire end using a strip tool. The strip length depends on the pin type. For the solid barrel pin, you ideally want it so that there’s just a little bit of bare (uninsulated) wire showing when the wire tip bottoms out into the barrel.

It’s best to not pull the strip end all the way off the wire until just before you’re ready to crimp. This will help keep the wire end from fraying, which makes it more difficult to cleanly insert the wire into the pin barrel.

Insert the stripped wire end to be crimped into the barrel end of the connector pin. Ideally you want a little bit of bare (uninsulated) wire showing at the base of the pin so this is OK but not true to spec.

Set he crimp head dial for the pin size being used. I’m using a 20 gauge pin so I set it to 20 here. Note that a 20 gauge pin accommodates gauges 20-26. The selector dial on the tool frame configures the tools for the exact wire gauge as shown below.

A label on the side of the crimp head (turret) specifies the selector setting for the pin size and wire gauge to be crimped. Since I’m using a 20 gauge pin (contact size) and crimping 24 gauge wire, my selector setting should be 2.

Use the dial on the crimp tool to set it for the gauge wire to crimp. I’m crimping 24 gauge wire so I set the tool to selector setting 2 as per the table shown in the previous step.

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Insert pin and wire into tool as shown.

Using the wire, push the contact pin into the tool until is stops. Keep just enough push force applied so that the wire tip stays burred into the barrel socket, then crimp.

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The finished crimp should look something like this. Again, ideally I would have had a little bit of bare wire showing at the base of the connector pin. There’s an inspection hole that can be used to verify that the wire is all the way in. If you cannot see the wire through the inspection hole, snip off the pin and discard then try again. (Inspection hole is not visible here but seen below with wire end showing.)

Perform a pull test to ensure a good crimp. A tester like the one shown here is overkill for the auto enthusiast and I don’t have one, but you should at least give a good tug (harder for higher gauge wires). It only take a second and could save you a lot of headaches down the road. For a sense of how hard to pull, refer to the table below.

MIL-T-7928-01

MIL-T-7928 specifies minimum pull test values. If you’re life depends on a reliable connection, then follow it. If not, then be aware of the values and use your good sense when performing a casual pull test. It would really slow me down if I actually pull tested each and every crimp for spec conformance, so I don’t. A quick and sturdy pull is all I do.

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Shown here is crimped wire pull tested to point of failure. Point here is to illustrate (again) that this was a good crimp because — amongst other things — the wire itself broke before it was able to pull free from the crimp. (Note short amount of bare wire exposed relative to barrel length.)

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Further evidence that crimp was stronger than the wire being crimped. Wire was pull tested to failure but did not pull out as proved by presence of tip end still being seen through inspection hole.

The following steps are DTM connector specific and have nothing to do with crimping. They illustrate pin insertion and connector housing assembly. I’m using different wires (Tefzel insulated) crimped from a previous job so don’t be confused about why not red 24 gauge wires like the ones crimped above.

LADD is a Deutsch distributor.

Insert wires into DTM connector housing, pin head first through the silicone seal.

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Continuing from previous step, push the wires all the way in until they click into place. If the wire is very thin (18 gauge or higher) you might need to use an insertion tool to help push the pin in from behind. (I just use a small flat blade jewelers screw driver but be careful not to push on the wire if you do the same.) Once clicked in you will not be able to pull the wire back out using pull force alone, so give a quick tug to verify.

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After all of the pins are locked in, insert the wedgelock. It pushes straight in to the housing and snaps when flush. The wedgelock is what I love about the DTM connectors over some other types. They keep the pins perfectly aligned and from moving around when mating the two halves so there’s no need to nurse them together like cheaper connectors.

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Connector fully assembled. A quality job that will provide years of reliable service even under harsh operating conditions.

Be sure to document your pin assignments and save them someplace safe.

Document and save your pin assignments as I did here for a recent job. You’ll thank yourself down the road.

Wire Labeling

I am going to additionally discuss wire labeling. It contributes nothing to reliable electrical connections but is something that goes hand-in-hand with it and therefore worth discussing.

It’s important to label your wires for future reference if not using color coded wire. A label like the one shown here reads all the way around the wire making it easy to read from any angle.

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I use a Brady BMP21 label maker as shown here. It produces labels like the one shown above. It has a lot of nice features including lighted display, symbols, different font sizes, small and large cap characters, and much more. You set the wire gauges size and it’ll spit out more rows of text so you can get a full wrap of the wire so text goes all the way around the wire. I highly recommend it and I used it for all my electrical jobs, not just automotive (home networking, home automation, etc.)

The BMP21 incorporates a removable tape cartridge, which is available in different tape widths. Label itself is durable nylon cloth which does not smudge, even after being printed.

Label shown hot off the printer. Simply enter in the text (WYSIWYG), press print, and then shear off the label using a button.

Label is very easy to peel off from backing which is wider than the label part. No time is wasted trying to get the label and backing pulled apart like some other label makers I’ve used.

To attach label to wire, start by placing it on wire as shown. I try to get the top edge of the wire to align with the wire run so when I start wrapping there’s no overlapping edges. If this doesn’t make sense it will after you’ve tried it a few times. Not critical. Just depends on how OCD you are.

Continuing on from the previous step, wrap the label around the wire. If you specified a larger gauge wire than you actually have you may get an extra wrap or two than needed. No harm in doing so but you’re throwing money out the window by doing so by wasting label material on something you cannot see.

Optionally add some clear shrink wrap to keep the label protected from dirt and chemicals that could make them otherwise difficult to read after years of service. Be sure to plan ahead if you want to go this route by slipping the shrink on before your attaching your connector ends, else you will not be able to get the shrink onto the wire.

Apply heat to shrink wrap for final labeled product. If only all the connections in my car were as good as the one created here for a couple of microphone line inputs. Seems like overkill but I know no other way when it comes to electrical.

About Feature Image

E30 racecar that I built. I incorporated a Motec M600 ECU and I adapted car from wasted spark to coil over ignition. Note custom coil frame mounts that I milled from aluminum. All techniques described in this article where used in building the ECU and chassis harness.