Mopar HEI Conversion
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This idea came about from seeing a basic swap that someone else had done on a 1986 Dodge Van with a slant six that a friend of mine had purchased. The conversion worked when he bought the van, but had some hot-running problems that seemed to be related to poor module mounting. There was no real heat sink for the module and no thermal grease under the module, which was apparently causing the module to run very hot and have problems. It was also somewhat poorly wired - right down to the nasty sticky electrical tape and the hacked together wiring. I'm pretty finicky about doing wiring "right" on vehicles I work on, so I wanted to improve on the original swap as soon as I saw it. After researching the swap a bit more, and in particular finding an excellent discussion about this swap on the site along with another page detailing the conversion, we took some time and improved on the basic work done by the previous owner of the van. It's now a very professional looking installation with a proper heat sink - and due to careful placement and sizing of the heat sink, it required drilling no new holes in the vehicle. The module is easily replaceable from the front of the van in about 5 minutes with just a Phillips screwdriver, yet is hidden from view by the heat sink.

I decided to create this page because all of the existing pages I found on this topic were (to my tastes) a bit weak on "hands-on" details for the first timer as well as weak on wiring details (yes, I am a wiring perfectionist). I'm intending this page to grow to be something any first-timer can use to do this conversion easily and with high confidence that they understand what they are doing so that it will be a success. I also want to call out the proper wiring details in various areas so people can do this without it ending up looking like a hack job. If done properly, it should look like the factory put it there originally.


Standard Disclaimer

If the idea of removing and installing a distributor terrifies you or is something that you believe is beyond your abilities, then stop right now and go find someone to help you do this swap. If the idea of touching wiring scares you, then read on to understand it better, and if this still isn't enough to enable you to feel comfortable doing this conversion, stop and find someone to help you. If you cannot handle basic cutting and drilling of metal (needed to make the heat sink), stop and find someone who can handle it. If you do not posses the quality referred to as "common sense" and/or do not have the ability to think for yourself and decide if something is safe or not, then seek professional help. This page is purely informational, represents what I have learned in reading about and doing this swap, and is not guaranteed to work in your application. If your car fails to start, lets the smoke out of the wires, or has any one of a million of possible undesirable outcomes occur to it - it is not my fault. If you make use of this information to modify your vehicle you are doing so strictly at your own risk. Got it? Good.


Conversion Applicability

Although we did this on a slant six, the same basic conversion should work on any Mopar engine that you can get an electronic distributor for. Basically, you re-use the distributor from a normal Mopar electronic ignition and replace the Mopar ignition controller with a "4-pin" GM HEI module from a non-computer controlled HEI system. The factory vacuum and mechanical advance systems stay in place and should be set up appropriately to match your engine. Four wires hook it all up, and when you're done you've got an electronic ignition setup that is cheaper, smaller - and most importantly, better - than the Mopar setup. If you are a Mopar purist and you really think the Mopar electronic ignition boxes are better, by all means, go pay more to use an inferior product that takes up more space in the engine compartment. Folks smart enough to make this conversion will enjoy having better spark for less money and have a good laugh about the purists all the way to the next cruise night. :-)


Parts Needed

  • GM 4-pin HEI module from a non-computer controlled application. If you favorite parts source insists on a specific vehicle application to find parts, a 1975 Chevy Caprice Classic with a 350 engine and AC will get you the right part. Try to avoid the cheap "offshore" versions of these as they are prone to fail and have poor high-RPM performance. Various aftermarket versions are available from places such as Summit and Jegs, but they cost a bit more than the genuine AC Delco or other American made versions of the module.
  • Thermal grease. This should come with the module you buy. It goes between the module and whatever you choose to use as a heat sink.
  • Mopar electronic distributor for your engine. These have a two-wire plug to hook up the Mopar electronic ignition systems, as well as the usual vacuum and mechanical advance systems. If converting from an existing Mopar electronic ignition, then the right distributor should already be in the engine.
  • Mopar electronic distributor connector. This is the connector that is part of the wiring harness and mates to the one on the distributor. You'll have to pirate one out of the wiring harness of a Mopar product that came with electronic ignition - I have not been able to find a replacement for this as of yet, and you don't want to alter the distributor wiring in case it needs replacement later on. If you really hate your existing Mopar electronic ignition or it's DOA, you can cut the connector from there. If it's working, I'd recommend that you leave it intact and working Just In Case something goes awry in this conversion. Murphy's Law applies to this work as much as any other work.
  • Wire. You should have red, (power), black (coil), white (pickup), and green (pickup) wire to make troubleshooting the harness easier in the future. The wire should be 18 (or 16 - larger is OK) gauge for the white and green pickup wires, and 14 gauge for the red and black wires for power and the coil.
  • Terminals for HEI module. You'll need the proper terminals to hook up to the HEI module - three 1/4" blade terminals and one 3/16" blade terminal, and optionally/preferably plastic connectors to put them into. The 1/4" blade terminals are readily available from many sources. The 3/6" blade terminals are much harder to find - some car speakers use these "skinnier than normal" terminals, and so do some arcade games. You can also try Terminal Supply - they carry part #42282-2 which is for a relay base, but it seems like it should work.
  • Uninsulated ring terminals + heatshrink tubing to hook up the power/coil wires. These need to be sized to fit over the studs on your coil and to fit 14 gauge wire. You can substitute insulated terminals in a pinch, but they tend to be bulkier and not work as well over time - the plastic insulation of the cheap ones tends to get very brittle and falls off over time.
  • Uninsulated crimp-on butt connectors + heatshrink tubing for the Mopar electronic distributor connector wires. You'll need to splice them into the wires coming from the W/G terminals on the HEI module.
  • Power/Coil connector for the HEI module. The B/C terminals use common 1/4" blade terminals and match up with a common spacing used on many GM connectors such as the backup light switch on the steering column (for automatic cars), some older "hydraulic" brake light switches, and the "older" style GM AC compressor coil connection. You should be able to get a connector and pigtail to use by cutting the right one off of a GM vehicle at your local salvage yard (the AC compressor coil is probably the most common and the easiest one to grab), or by looking at the replacement connector + pigtails available at your local parts store - I believe a standard GM internally regulated alternator plug will work here. You may even have something in your stash of space parts. The factory GM HEI terminal should be arranged such that it cannot be plugged in "backwards", and as such it is recommended that you use it in this case if at all possible.
  • Pickup coil connector for the HEI module. The W/G terminals use about the same spacing as the B/C terminals and the W terminal is the standard 1/4" blade terminal, but the G terminal is a smaller 3/16" terminal. GM HEI distributors use a special connector and terminal to hook up to this - you could pirate the connector and wiring pigtail from a GM HEI distributor in a junkyard, look for it as a replacement part, or simply use individual connectors to hook to each terminal. Because the terminal widths are different, using single connectors on this end of the module will be much harder to mis-connect, and if you do misconnect them, no damage to the module should result and you can just reverse the wires. The 3/6" blade terminals are much harder to find than the 1/4" terminals. Some car speakers use these "skinnier than normal" terminals, and so do some arcade games. You can also try Terminal Supply - they carry part #42282-2 which is for a relay base, but it seems like it should work. NOTE: You can also crimp the business end of a standard 1/4" terminal down a bit to make it work in a pinch if needed, but it's not a very good long-term solution - it can vibrate loose and fall off somewhere down the road. It's best to find and use the right size terminals whenever possible.
  • Harness tape. You'll need some tape to wrap up the harness to and from the HEI module. Don't use the typical sticky electrical tape - get the proper non-adhesive vinyl harness tape and you'll thank yourself down the road when the harness isn't a gooey sticky mess. Eastwood sells the correct tape - search for "wiring harness wrapping tape" and get the vinyl one.
  • Aluminum for the heat sink. The mounting surface for the HEI module should be about 2" tall by 3 1/2" long by about 1/4" thick. Larger is fine, and we used a piece that was a chunk of extruded aluminum angle bracket so we could mount it more easily with the module open to the air for better cooling. We were trying to solve a hot-running problem, after all, so we went for the "overkill" factor on the heat sink we built.
  • Small sheet metal screws to mount the module to the heat sink. Use the holes in the module as a guide and get the proper size from your local parts store or Home Depot-type of place. The factory used small machine screws to hold the module down, and you can do that if you have access to a tiny tap to cut the proper threads into the heat sink.
  • Small sheet metal screws to mount the module to the car. Length and size dictated by the thickness of the heat sink plus what you are mounting it to. If re-using an existing hole or screw on the car, you may need to match that screw size.


Distributor and Other Ignition System Parts

To make this all work, you need a properly working Mopar electronic ignition style distributor installed into the motor that is timed correctly as per your application. The vacuum advance should be hooked up just like it always would be, and you should have a cap, rotor, spark plugs, and spark plug wires that are in good condition. Particularly if you are changing from a points style ignition system, you will be putting out a much hotter spark that before, and this will tax weak components, find gaps/cracks in the spark plug wires, and a variety of other issues. A hotter spark can't fix bad ignition system parts, so do a tune-up first with high quality parts. (OK, the link is slant-six specific, but that's what we did this conversion on.) If you are replacing the distributor, make sure the timing is set right, and  be prepared to fiddle with it the first time you start the engine after this conversion is done as most folks seem to have trouble setting the base timing when removing the distributor and installing a new one.

After doing this conversion, you may need/want to adjust the timing, spark plug gaps, and idle mixtures on the carb. A hotter spark burns the fuel more completely, which could translate into needing less timing, enabling a larger plug gap, and adjusting the idle mixture to be correct again.


Make A Heat Sink

Some folks not used to the electronics industry may be asking "what's a heat sink?" right about now. Simply put, it's a chunk of metal that pulls heat away from an electrical device. Think of it like a radiator for electronics. This is needed because many electronics devices put out a lot of heat in a very small space, and by mounting them directly to a larger chunk of metal, that heat is pulled away from the single hot point on the electronics device and helps keep it cool enough to work properly. The larger surface of the heat sink can then dissipate the heat to the surrounding air. It's also very common to use something called "thermal grease" between the device and the heat sink. This is simply a grease that is designed to fill in any voids between the electronics and the heat sink and this help conduct the heat better. Aluminum is a good choice for a heat sink. It conducts heat well, it light, easy to work with (cut, bend, machine, etc.), and is readily available.

Decide where the module and heat sink will be mounted, and what the orientation of the module will be - the terminals come off at angles and sometimes reversing the module direction results in a cleaner wire routing later on, so stop and think about it now. Make sure there is a reasonable path for the wiring to take from the heat sink + module mounting location back to the distributor and coil. The mounting location should be reasonably cool (aka, don't strap it to the exhaust manifold or to the radiator :-) and can be anywhere that is reasonably close to the distributor.

Cut and shape your aluminum piece to be of roughly the proper size and shape to fit where you need it to, keeping in mind which side the module will mount to and what the orientation will be. Drill the desired mounting holes in the heat sink and make sure it will mount to the vehicle properly and that the HEI module and wiring will mount to the heat sink without hitting the heat sink mounting screws.

Look at the back of the HEI module. Most of the high quality ones will have two small plastic "pins" that protrude out the back. These are designed to help locate the module on it's original mounting surface, and I recommend you drill your heat sink for them. They do not have to be very deep, just in the right places and large/deep enough to help locate the module.

Once you have the positioning holes drilled in the correct locations, hold the module against the heat sink and mark the location of the mounting holes using a small pen, screw, or whatever. Remove the module and drill the proper size holes through the heat sink for your mounting screws. If using machine screws, tap the holes with the proper threads. If using self-tapping or sheet metal screws, screw in your mounting screws so that they will cut the right threads in the heat sink.

Do any final shaping and cleanup of the heat sink, and you're ready to mount your module. Do a final "dry" test fitment of the module on the heat sink and the heat sink to the vehicle to make sure everything works and fits properly. Double check for wiring clearance. Remove the module form the heat sink, apply a liberal coating of thermal grease to the back of the module, and mount the module to the heat sink.

Some folks have successfully mounted the module directly to flat places on the body such as the inner fender and it works OK for them. I prefer not to do this, but you can if you want to. Just make sure it's flat and you use some thermal grease under the module. And if it acts up on you during really hot cruise nights, well, I warned you. :-)


Wiring the HEI Module

First, check out my article on Reusing Plastic Wiring Plugs for details on how to wire up GM style connectors without any extra splicing needed.

As you wire things up, leave plenty of wire until it's in place, and make the final wire cuts after you have mocked up the system on the car. Excess wire is unsightly and can be a hazard. Run the wire harness for the module as far away from heat and moving parts as possible. Secure it to the vehicle or engine in a similar manner as the original wiring harness, possibly re-using the original wiring harness clips in some cases.

An GM HEI module has four terminals - two at each and - that should be labeled C and B on one end, and W and G on the other end, with G being the "smaller" size terminal.

  • C = Coil (negative side)
  • B = Battery (positive side of the coil, or any appropriate power source that is live in ignition and start)
  • W = positive lead (+) from the pickup in the distributor
  • G = negative lead (-) from the pickup in the distributor

Connect the red 14 gauge wire to the B terminal, the black 14 gauge wire to the C terminal, the white 18 gauge wire to the W terminal, and the green 18 gauge wire to the G terminal. Use plastic connectors whenever possible - especially on the B/C end of the module. The original HEI terminals should make it impossible to hook things up backwards, but the wire colors help make it somewhat obvious too - "B stands for battery and the red wire goes to the battery" is easy to remember. Leave plenty of wire to make up a harness out of the resulting wires that will reach the distributor and coil.

Make sure the module and heat sink are properly grounded. As long as the heat sink itself is already grounded (such as mounting it to to the vehicle frame or body), then mounting screws on the module should handle this. Some folks like to run an extra ground wire from one of the HEI mounting screws to a nearby ground just to be sure. If you have mounted the heat sink to something that isn't grounded, like something made of plastic or fiberglass, then you will need to add an extra ground wire.

Using the proper non-adhesive harness tape, wrap all four wires from the module (plus the ground if needed) into a single harness. Leave enough slack in the wires to allow them to be unplugged from the HEI module without unwrapping the harness, but not so much slack that things will be flopping around. Wrap the harness together until you are within a few inches of the coil and distributor. Do a test fit of the wiring into it's final location and decide when the power/coil and pickup wires will "break out" of the harness. In our case, the wiring ran past the coil first, so the power/coil wires exited the harness there, then the two pickup wires continued along in the harness for a few more inches. Leave extra wire coming out of the harness and make sure you have enough tape to do the final harness wrapping.

If using a separate grounding wire on the heat sink + module, decide on the final hookup for that, have it "break out" of the harness where appropriate, and hook up to your preferred ground point as needed

Decide on the final wire lengths for the power/coil wires, and cut each wire to length. Crimp the ring terminals onto each wire and put heatshrink over the crimp area. The heatshrink is added insurance against vibration and it helps insulate the terminal.

Decide on the final lengths for the pickup wires to connect to the harness end of the distributor pickup connector, and cut each wire to length. STOP - do not crimp everything together yet. Why? You need to make sure the pickup is wired correctly before making the final connections. Usually this is by connecting the green HEI module wire so that it goes to the black wire on the pickup in the Mopar distributor pickup, and then the white GM module wire goes to the orange Mopar distributor pickup wire. Note that the wire colors for the distributor pickup refer to the ones coming out of the actual distributor - the wiring harness connector and stub you have may or may not have matching colors, so beware.

Hook up the coil connections and make temporary connections for the pickup wires (making sure they are properly connected and will not short out), and test-start the vehicle to make sure it is working right. If they are backwards, the engine will not run well or may not run at all. If all seems to be running right, stop the engine and move on. If the engine is not running right, reverse the pickup wires and try again. If it's still not working right, start troubleshooting things to check for bad connections, bad grounds, and other such fun. If you opted to try out a used module (I've done it - sometimes you get lucky, sometimes you get unlucky) then swap the module for a known good module and try again.

Once everything is working correctly, make the final crimp connections to connect the Mopar distributor pickup connector to the white/green wires coming from the HEI module. Be sure to slip the heatshrink tubing on the wires before you crimp the terminals in place. Finish the harness wrapping down to the pickup connector such that you wrap the splices into the harness for protection. Secure the open end of the harness tape with one single wrap of adhesive electrical tape to keep it from coming unraveled. Do any final routing of the wires into the proper harness clips, and you're done. Fire up the engine to make sure it's still working right, and enjoy your new electronic ignition system. You should have many trouble-free miles out of it, and if something does go wrong, replacing the HEI module is a cheap and simple job using parts available at pretty much any local parts store.


Still To Do for This Page

I still need to get some pictures of this and draw some wiring diagrams to put up here, but the basics are here, on the discussion thread I linked to above, as well as on the other page I linked to.

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

Pretty much everything on this website is copyrighted, if you want to use something, ask first.

Page last updated 01/02/2009 01:51:39 PM