1977 Cherokee Chief S

[TSmith946]

I have recently bought a ‘77 Cherokee 4 door. It has a 401 but has had a top engine fire. So the wiring harness is toast. I went to a salvage yard and found a harness out of a’78 I thought would work but when I got it home some of the wires won’t be connected at the firewall. I have had it running with some “creative wiring” and the engine sounds good for something the hasn’t been tagged in 25 years.

So I am at a crossroads. Should i:
1) buy a painless harness (don’t know which one or how many circuits) and get it running
2) completely rebuild the engine with same painless harness
3) swap for a diesel (I’m thinking buying a 12 valve Cummins, transmission and transfer case out of a salvage yard) for the fuel mileage
4) suggestions….

My intention for this is for an overland hunting/camp rig. I would like to drive it where I’m going not to always trailer it

[tgreese]

The main difference in the '77 vs the '78 is the ignition components. For 1978, Jeep switched from the Prestolite BID (breakerless inductive discharge) system to the Motorcraft Duraspark system. The Prestolite stuff is a problem; it is both an orphan technology and quite unreliable. The Motorcraft ignition is way more reliable, and uses a trigger system widely used even today (variable reluctance or VR). You can mix and match components with the Motorcraft distributor between several other manufacturers that use the VR technology. Not so with Prestolite - you're stuck with the repop BID stuff, if available.

Suggest you get rid of the Prestolite stuff, and go with Motorcraft (my preference) or one of the aftermarket ignitions. The '78 harness is set up for Motorcraft stuff. You can also revert to pre-74 Delco points ignition, if that appeals to you.

I would get the wiring diagrams for '77 and '78 and compare. The bulkhead connector in the firewall should be the same, though some wire locations will be different. I would print out the '77 and '78 diagrams double-size and trace what's different with colored pencils. Map the bulkhead connection numbers between '77 and '78.

Both the '77 and '78 TSMs are online, free to read and download, on the Tom COllins site. The wiring diagrams are also there, in a separate section. Suggest you look at both for the diagrams, since the image quality could be better in one or the other.

https://oljeep.com/gw/elec/GW_wiring.html
https://oljeep.com/edge_parts_man.html

TSM.png

I believe you will find that the differences '77 to '78 mostly are limited to the ignition. The '78 will have a resistance wire to the coil that is not present with the Prestolite system.

A '78 switches to the Delco SI alternator for V8 models, as I recall. You could adapt to the '77 Motorcraft alternator, but the Delco is a significant upgrade, and I'd keep it.

The starting circuit will be slightly different, with the addition of both a wire that bypasses the resistance wire at starting, and a connection to the Motorcraft module that retards the spark while starting for easier starts.

JMO - you seem to have plenty to work with here, and an opportunity to get this Jeep on the road with a bit of research, planning and attentive rewiring. The Painless option wastes your money (expensive) and effort (quite a lot). Nothing wrong with the '78 harness, and it's miles and miles closer to complete and running than any hot rod harness like Painless will be.

Suggest you learn something about the 401 before you consider rebuild. If you have a virgin block, that's a great asset. The 401 pushes the bore diameter to its limit for its block size, and the cylinder walls are thin compared to your typical grey iron USA block. If you decide to rebuild, suggest you do your research and have the block sonic checked for core shift before you bore it oversized. A seasoned engine is a reliable engine - have you checked compression or looked at the hot idle oil pressure? IMO it would be foolish to rebuild any engine that's in good condition just because of its age.

You're in a different universe with the diesel conversion. How much experience do you have with projects like this?

hth! TIm

[TSmith946]

@tgreese Thanks for the input. Sorry didn't meant to oput the same post in multiple spots. I was having trouble with internet.

I have attempted to rewiring the bulkhead connectors, but failed at removing the male contacts (engine side). If I knew how to move them then the rewire would absolutly be easier. Any advice?
Thanks for your input on rebuild and diesel conversion. The only reason for the diesel conversion is to make it more fuel efficient. I would like to be able to drive it 10 hours to the mountains and hunt/camp out of it. I dont know much about the engine other than i have gotten it running, but just long enough to empty the carb bowl so about 20 seconds.

[tgreese]

Hmm. Yeah, I kinds doubt you need to rewire the bulkhead connector at all. If you change to the '78 electrical components, your '78 engine harness should have the right wires and connections for the Motorcraft ignition and Delco alternator.

I looked at these wiring diagrams, and it looked to me like there were maybe two connections that were different for '78, numbers 72 and 75, and I think the 75 is a misprint and is actually 25 and unchanged. Maybe there are a small number that I have missed, but it seems dang close. I had to search to find differences.

The bulkhead connectors are so-called Packard 56 connectors. These are really common to automotive applications of the day. The 10 ga wire for the ammeter uses larger connectors, but also Packard type. You can buy a tool to remove these - I have these https://www.amazon.com/gp/product/B0002 ... UTF8&psc=1 and they work for the Packard and the WeatherPack connectors. There is a tab on the connector that you must release to pull the wire and connector from the shell. Good pictures in this article https://www.motortrend.com/reviews/1506 ... g-options/

Can't tell you what's best for you; however, I expect you really need to map the plug layout for both years, side by side, on paper. This is an interface, and you need to match it, the engine side to the dash side.

The connections are numbered on a picture of the connector from both sides, and the numbers on the connector picture match the circuit numbers on the wiring diagram. I don't think you can do this without some serious desk work using your colored pencils and enlarged copies of the diagrams. One circuit at a time. There is also a list of circuits which you could copy and check off as you compare diagrams. Make notes about each connection that must change, and then go down the list. It will be work, but that's the price of admission.

Regarding the diesel thing, suggest you talk to someone who has done a similar swap in person, or at least privately f2f on Skype or whatever. The longer you talk, the more accurate an idea of where you're heading. I see two major dangers. First and most obvious, you finish it, spending way more than you expect, and it does not live up to your expectations. Second, you get part way into it, and life interferes. This happens a lot. Health, family situation, job, you get the picture. Unfinished projects are cheap. And I wonder if there isn't a better route to what you want, faster, less expensive and less risky.

[tgreese]

I enlarged and copied this picture of the bulkhead connector from the printed copy of the '77 TSM:

Jeep77bulkhead (856 x 864).jpg

This is mostly legible, and could be helpful. I expect the original manual was better. The diagram on the Tom Collins site may not be this good. However, you can buy the '78 TSM on media (CD, USB) at RockAuto under $25 and I expect the transfer will be better. Short money if it solves your problem.

[TSmith946]

Thanks for the help man. Especially the picture of the bulkhead however I can’t access it. I’m new to this forum so can you PM it or email it.

[tgreese]

NP. Appeal to Stuka, the mod. He can boost your status so you can see all posts. Send him a PM if he does not reply to this thread.

[ghcoe]

If the 401 runs good I would leave it. Everyone wants to install a diesel for better gas mileage, but don't add up the cost of a conversion. You can fill a lot of tanks for the cost of just the conversion. Remember that you will have to not only replace the motor, but also beef up the frame, suspension and maybe even the axles. That is a whole lot of extra money just in itself, not to mention if you add up your time lost in the conversion. It just does not add up for me.

I tell people I get about 8mpg. They always say how can you afford that. I say I don't have a car payment! Conversation is done...

[TSmith946]

Thanks for the advice. I will probably just get it running as it sits with new appliances hoses belts etc.
Provided I can figure out the 78 to 77 wiring harness problem. (Getting a little closer… maybe?)
Not doing the conversion will save a ton of money to put more in to the overland plans for it and plan to trailer it for long trips

[ghcoe]

Thanks for the advice. I will probably just get it running as it sits with new appliances hoses belts etc.
Provided I can figure out the 78 to 77 wiring harness problem. (Getting a little closer… maybe?)
Not doing the conversion will save a ton of money to put more in to the overland plans for it and plan to trailer it for long trips

Not sure why you would have to trailer it. I overland my '77 and it does fine.

Can you still see the color codes on the engine side of the firewall on the burned harness? I can't imagine they changed the color codes on the main engine harness. Just pull the pins on the '78 and install in the corresponding plug locations on the old plug. Once you do that you should see what plug locations do and don't have wires. At that point you could rebuild out to the Prestolight ignition, or at least you would know what plug positions would need to be pinned out with a multimeter for the Motorcraft Module. You can do a Chevy HEI conversion to replace the Prestolight ignition module. That is what I did. It would simplify the wiring some.

[Pablo]

"Overlanding" is getting to be a vague kind of a term. I have seen folks on 40's that don't mind body damage that call their rig "overlanders" because they have a roof top tent. Then there are guys with $40k of camping gear strapped to their pickup that look scared to go down a forest service fire road. So not sure what your expectations are for performance there.

Just remember a 6bt is 1250 pounds for a dressed engine (give or take). Plus the transmission and transfer case that can live behind it. Then 1 ton axles all the way around to live under it. That is going to be lots of weight to contend with off-road. Not good for the mud. Mount it too high and it will get very tippy. Frame modifications recommended. Not a simple drop it in and wheel it kind of a thing. Consider keeping the 401 and modernizing it a bit (maybe fuel injection for off-road). Or consider a smaller lighter diesel engine or some other gas engine swap.

[SJTD]

Overlanding. Pretentious word for camping.

[ghcoe]

Overlanding. Pretentious word for camping.

Actually, camping is where you drive to camp for the whole time you are there. Overlanding is traveling during the day and camping at different spots most every night. Close, but different. Although, some people who camp think they are overlanding. I can see where the confusion would come from.

If you go by the gate keepers idea of overlanding you have to be out at least a week otherwise it is "outdoor adventure". Whatever....

[TSmith946]

Ghcoe i have a few inches of insulation left on original 77 harness. I have also printed wiring diagrams from Tom Collins page. There are a few more connections in the 78 for the motorcraft ignition system mostly. I’m planning to move pins to match the 77 and go with a new motorcraft ignition at least to start. The thing I’m not sure about is what all else I need to change to make that work.

[tgreese]

The following is expressed as advice, not telling you what to do. This is how I would approach this project.

You have the '78 harness. Map out the connections to their circuit numbers. The circuit numbers are nearly identical '77 to 78. You don't have to take the wires out of the loom - use your multimeter to test continuity for each wire and determine its location in the connector and at its distal end.

You have the diagram I posted of the enlarged bulkhead connector for '77. It does not matter what the bulkhead layout for '78 is, since you will be re-pinning the '78 connector to match the '77 bulkhead.

I would stress, you really need to follow all of this on printed copies of the diagrams, and keep track of what you have changed. Methodical, stepwise, documented.

Most multi-conductor connectors have a number for each pin on the plastic shell, likely only visible with a magnifying glass. I suspect this will not help you, since the dash side of the connector will remain in the firewall and be hidden by the fuse panel.

However, the pins have a layout that is different top to bottom, left to right, and which will be swapped left to right depending on which side of the connector you look from. Top and bottom will always be top and bottom. Again, you need to careful and methodical here, and keep track of the pins by circuit number.

The Motorcraft ignition parts will match the distal ends of the '78 harness. The ignition will be easy. Install the distributor and module and plug them in.

The other main difference is the alternator. The '78 harness will plug in to the Delco 10SI alternator used that year; "Systems Integrated" with the regulator inside the alternator. The '77 uses a Motorcraft alternator with an external regulator. I went over this in your previous post, but it may help to go over it again. Adapt the '78 wiring to the inferior '77 alternator/regulator, or adapt the mounts to the more modern Delco alternator. Again, a replacement Delco for '78 will plug in to the '78 harness, with no issues - if we know the whole story.

[TSmith946]

Wow tgreese that’s a lot of good information. That will be the plan when o get back to days off. I’ll keep you posted. We’ll see if I’m smart enough to figure this out

[ghcoe]

The following is expressed as advice, not telling you what to do. This is how I would approach this project.
if we know the whole story.

I can add that the Motocraft Module will have a extra wire that energizes a start circuit in the module. The start circuit retards the timing during starting to relieve battery and starter strain against engine timing. You'll have to see how that is wired on the wiring diagram. It may come from the ignition switch. If so that may need to be address on the inside wiring. Or it may get the signal from the starter relay on the fender. If that is the case then the wiring should all be there.

[TSmith946]

So quick update on the 78 wiring harness. Me and my son repinned it I’m 95% sure it’s right, but I still have 2-14 ga wires that go to a 78 alternator but I don’t have anymore circuits currently. The 78 had 19 pins and the 77 had 13. I was able to eliminate 4 (Quadra Trac was on the other half, one wiper, and not sure what the fourth one was)

I have a motorcraft ignition, alternator, distributor ordered from rock auto.

Could I try starting it now? What else do I need to think about before I burn my barn down. My wife is mad enough because I spend all my free time working on the Jeep I don’t need her mad because I burnt the barn.

[TSmith946]

So I got it to crank from the key tonight. I can’t tell y’all how thankful I am for all the help. Still have to rebuild the fuel and vacuum lines but that is by far east than building a Frankenstein wiring harness.
Now I can change the distributor

[TSmith946]

So a BIG THANKS! To everyone tgreese gchoe etc. today she ran on her own till she ran out of fuel in the bowl (waiting on a new fuel tank, radiator, etc).
Huge learning experience happened throughout this wiring

Not sure how to add a video or I’d send one

Anyway thanks again for your help