Electrical Guru Welcome...Please!!

OK so I just finished a build on my Cj-5 which is a 76 so I hope I'm in the right forum, I'm trying to break in the motor but its one problem after the other. I will be driving it and it runs fine for a bit with the head lights on, then after probably just a couple miles the Jeep begins to cut out and won't run with the lights on also when its doing this the lights go very dim. I can't figure out why it would take a minute for a short to act up if thats the case. Has anyone else experienced anything like this? Or know where to look?

 

I have never experienced that, but on our old van we would not be able to start it after it was warmed up and shut off. We would let it cool and it would fire right up. Ended being a short in the battery cable, but it took it to get warm and expand before it showed up. Perhaps you have a wire or connection that needs to heat up and expand before an issue occurs.

Thanks,
Scott

 

I would have the alternator and battery checked first.

That said it often times comes down to not having ,or a shorting ,ground.

All connections should be clean.

Headlights often ground to the body but the body isnt grounded properly.

 

I know when I kept losing my headlights it was due to the mounting bolt on the bottom of the grill coming loose and since tha point was the only metal to frame contact, the lights would flicker and go out. Everything else is fiberglass.

I would definitely check your grounds. Battery cables are really cheap. Make sure connections are clean.

Good luck,

I hate electrical gremlins

High5

 

Download the Fsm. Do a google search for it I thought there was one for a 74 on this site ??? If you cant find one ,the 71 renegade would give you an idea what wires go where. Your ignition is another good starting point but after "battery, alternator". Points were obsolete by then, correct? If so, i second a loose battery connection or real bad short. GL

Guru J.r.

 

I am guessing as well, Its possible you have a short somewhere but my first guess is not a short but high impedence to ground. Basically a bad frame / body ground.

My second guess is the alternator is not doing so well. If its the alternator, with the engine running you should measure 14-18 VDC at the battery. It should increase slightly with engine RPM. If you are reading 12 volts or less the alternator is not charging. If you have the AC declo alternator there is a test point in the back. If you stick a screw driver or other metal object in it it will bypass the voltage regulator and let you run "full field" The faster the engine turns the alternator the more voltage you will get. With the engine running above 2500 you will easily get 25-30vdc from a good alternator with the test points shorted.


Assuming its not the alternator and it is a weak ground or poor ground...
When you built the jeep I assume you connected the big black wire to a bolt hole on the engine. Probably a nice new painted engine. Did you sand the paint off where the ground is ?

If you know how to use a DVM or Ohm meter. Measure between the battery teminal ground and a scratced metal piece of the engine. The meter leads will need to get through the paint and touch metal. It should measure 1 ohm mayby 2. If you have 10-20 ohms you have paint that needs to be removed. If you have 500-1 M ohm you have alot of paint or just a bad connection to the engine.

Second if you are like me you grounded the engine but did not connect the ground cable to the frame or the body. you need to do both. Usually there is a braded copper ground strap that runs from the engine to the body and one to the frame.

It does not matter where or how its done, but you need a big heavy wire or metal to metal contact between battery, engine block, frame and body. Again use the DVM. 1 ohm battery terminal negative to any scratched metal surface on both fram and body is good. Much more than 5 ohm and you have a bad ground.

The result will be physically hot power wire. Either near the alternator, or more likely at the starter relay, or possibly under the dash at the big red wire to the ignition switch. Where it gets hot does not matter, but somewhere in the jeep a wire will get hot fairly quickly, 5-10 minutes tops. It will eventually get hot enough to melt the insulation and cause a short. The short will cause a lot more heat untill it blows a fuse or starts smoking badly.

From experience the ignition module on the driver fender will probably burn out first and kill the spark to the engine.

 

so does the jeep run fine if the headlights are left off? -or- does this issue only occur when the headlights are on? well if that is the case, like everyone else has already stated, your most likely looking at a bad grounding/shorting issue, and if it is only when lights are on, well, that should help you narrow it down to that area of the harness.
also, it was mentioned to get your battery and alternator tested, that is a great starting point for this problem. but may be tough if you can only drive a couple miles.

let us know how it goes. i love electrical!

 

When the alternator in my jeep went bad one night the lights got very dim and the engine started sputtering. Have you tested the voltage on the battery while it's running? It should be between I'd say 13-14 volts give or take

 

I've found on many different flavors of old iron that chassis grounds tend to fail over long periods of time. Whenever I rewire a vehicle, I run an extra ground wire in the harness from front to back tapping anything that draws current like headlight buckets, tail lights, horn, fuel cells, etc. This wire is just supplemental, you should still ground the panels, too.

Also check the field sense circuit on your alternator.

 

Sounds like a bad ground to me. It's always best to make sure every single thing on there is grounded good. Motor to the frame, body to the frame, body to the motor. If your headlights are grounded off the grill, make sure the grill is grouned to the frame. Kinda look over your grounds and make sure the places where they are grounded are cleaned off good and they don't have paint on that spot. If everything looks like it's in place, check the alternator.

 

Thanks all!! I will go to work on it now and let you guys know what I find.