ComicArtist Posted July 29, 2012 Share Posted July 29, 2012 About to pick up a '76 to work on for this girl, has a '78 l28, bone stock, motor runs for sure. I know the car it came out of. It has the '78 ecu as well, but is missing the engine wiring harness. Question #1: Which wiring harness should I use? '78 or '76? The coil harness that runs under the core support has been cut halfway across the core support. Question #2: Should I cut the coil harness off a '76 or a '78 for the '78's coil? Thanks! I figure either engine harness would work, as well as the coil, but I want to make sure before I dig in to it. The girl that came to me about it needs it running soon, so I'll probably end up just fixing it for free. Quote Link to comment Share on other sites More sharing options...
NewZed Posted July 29, 2012 Share Posted July 29, 2012 The 78 and 76 ECUs are interchangeable in some cases. I have a 78 ECU on my 76, the part numbers are the same. The voltage regulation is external to the alternator for 76 and internal for 78. So you'll need to decide which way to go on that. The AFM and its wiring is another difference. The 76 has the fuel pump relay contacts in the AFM, the 78 uses an oil pressure switch and alternator voltage to a dedicated relay for fuel pump control. The 78 AFM might still have the contacts, but the 78 harness won't. The 76 coil and ignition module uses a ballast resistor, the 78 doesn't. All three things can be figured out and worked around pretty easily. Quote Link to comment Share on other sites More sharing options...
ComicArtist Posted July 29, 2012 Author Share Posted July 29, 2012 Hm. I'll probably just run the '78 alt and convert the wiring to run the internal regulator. What would you recommend doing about the coil ballast? I've never had to mess with that before Quote Link to comment Share on other sites More sharing options...
NewZed Posted July 29, 2012 Share Posted July 29, 2012 If you don't want to dig in to figuring out which wire is which, to and from the ignition module up by the fuse box, you could use the GM HEI module with either the 76 or 78 distributor (actually any 74 to 83 distributor). It has its own current-limiting circuitry. Four wires - switched and Start power from the coil positive, the coil negative and two wires from the distributor - and a good ground through the mounting hole. The 76 distributor might have three wires if it has two pickup coils, but you only need two. The old stock ignition modules are all close to death anyway since they're so old. And the HEI modules are cheap. You should consider though, if she'll be using you as her future mechanic, or someone else. If you get too far from stock and something goes wrong, the future guy might be clueless. Quote Link to comment Share on other sites More sharing options...
ComicArtist Posted July 30, 2012 Author Share Posted July 30, 2012 That's a good idea! I might have to, forgot about the ignition module. Are the 76's different from the 78's? And I more than likely will be her future mechanic, but it'll be her daily so I'm keeping it as stock as possible for reliability haha. Quote Link to comment Share on other sites More sharing options...
NewZed Posted July 31, 2012 Share Posted July 31, 2012 The 76es with two pickup coils in the distributor use a different ignition module than those with just one. It has an extra circuit to use the second pickup coil to adjust timing depending on coolant temperature. It's the one thing that you have to give up if you go with the HEI module. The main effect is a low idle until the engine warms up. I think the EGR engines might be when they went to one pickup coil. I had a 78 engine with EGR and a 76 without, one had a single pickup in the distributor, the other had two. Quote Link to comment Share on other sites More sharing options...
ComicArtist Posted July 31, 2012 Author Share Posted July 31, 2012 Hm well I just picked up a '76 harness to use, I might just get a 76 distributor as well to avoid complication. Quote Link to comment Share on other sites More sharing options...
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