So have had a 1983 Pickup for a couple weeks and have been going through the wiring.

My issue is the parking, dash, and dome lights fusable link keeps blowing when the ignition is turned to ACC.
I have a copy of the 1983 FSM and the wiring between the parking lights, etc. is not even on the same fusable link as the ACC.
I have disconnected everything on the W wire coming from the battery and replaced the fusable link with a standard 30 amp fuse for trouble shooting. Lights work every time until I turn the ignition to ACC. I spent most of the day looking at the wiring diagram to no avail, but have tried a new ignition switch which didn't change anything. I then started looking at components that get powered in ACC and started by disconnecting the CMH relay and for some reason the lights fuse did not blow this time, so tried a different CMH relay which didn't change anything, fuse blew again.
I am still trying to figure out how something on the ACC wiring is blowing the fuseable link for the lights, etc. No other fuses are blowing just the one fusable link.

I am stuck and could use some assistance. I have searched and could not find anything even remotely close. Hopefully someone with more experience on this model can be of assistance.

Any suggestions would be greatly appreciated, I am almost to the point of ripping the old wiring out and rewiring the whole truck.

Dave

 

You have it narrowed down to the CMH relay the circuit this is in.

You just need to trouble shoot this further

It can be someplace in the wiring that is shorted it can be the CMH itself .

loose connection causing a arc the draws to much current??

You did test both relays and both were good??

Added after market load causing that circuit to over load and open the protection.

No telling just what might have been done

Time to break out the Meter and get some real world numbers.

 

Haven't tested the relays, figured they both couldn't be bad, but will check them this evening after work.


Nice to know I am headed in the right direction, just can't wrap my head around why the CMH relay circuit would be blowing the fuse on another circuit. Maybe I am making it to complicated?


Will also check the CMH circuit from the relay to the CMH, but looks like I would also have to disconnect from the Emissions computer. Guess that means the Emissions computer could be bad too.


I will check all the above this evening.


I appreciate the assistance, helped a lot.

 

OK, so I finally had a chance to check the CMH and Relay.

According to the FSM the positive lead of the CMH to the intake manifold should have continuity between .5 and 2 ohms depending on which manufacturer of the CMH you have.

I am getting roughly 50M ohms so almost open for most purposes. This was with the engine cold, since I made the assumption that because when I first turn the ignition to ACC it blows the fuse it would not have to be warm to find the issue. I will try again today with the engine warm, in case it does make a difference.

Checked the CMH circuit for shorts to ground or each other, everything checked good. Even checked continuity for all wires in the circuit, they too checked good.

Both relays checked out good.

I think I must be missing something simple, but have yet to but my finger on it.

Hopefully someone can clue me in to what I am missing or overlooking.

 

OK, so have been slowly working on this more.


I have disconnected everything connected to the CMH circuit and the wire shows open to ground.


I now have to go back through one plug at a time and see which component is shorting to ground. Shouldn't take too long once I get some more free time.


Will keep updating this thread until I fix this problem.

 

The only thing that gets powered by the ACC circuit (C circuit) is the radio and cigarette lighter. The only thing in common with the B Circuit is the memory power to the radio - the radio gets that constant power from the B circuit when the ignition is OFF. Other than an issue there, the only thing I can think of that would cause a blown fusible link would be chaffed wires contacting each other. Since you aren't seeing a problem with the CMH removed then I would assume that somewhere in that wiring you have something crossed. You can try checking for continuity between the B circuit at the battery (Red fusible link) and wire (I believe it is BY) going to the CMH. If you have continuity then there is some sort of short between the CMH and the relay. You can also try to test the continuity between the CMH relay and the fusible link. If you have continuity there then there is a short between the CMH relay and the fusible link. Both tests should be done with the ignition OFF. You are hoping that it is between the relay and the CMH. If not, you have some hunting to do...

 

So the Radio wiring could be it, there is currently no radio installed and I did find a bundle of wires above and to the right of the heater core all wrapped in electrical tape. Guess I need to open that up and see if there are any wires touching or just what's going on in general.


Thank you for the help, pretty optimistic this might be it.

 

You're looking for a light blue wire - that's the memory wire on the B circuit. If a bunch of wires are taped together then there is a good chance that something is bare and getting crossed up. There is a Light blue with black stripe wire for the ACC power to the radio. Check to make sure they aren't crossed, although you'll likely be looking for something that is grounding out as well since you're blowing the fusible link.

You can also check to see what circuit is giving you hell by pulling the fuses to each one to see if it solves your problem. Try the radio fuse first - if the link doesn't blow then it is likely in that circuit. At least that way you can isolate your troubleshooting.

 

Ok well I unwrapped the radio wires so that none were touching, fuse still blows.

Disconnected the BY and LR from the CMH relay, fuse did not blow.

According to the wiring diagram the BY and LR go to the emissions computer and the BY also splices off and goes to the CMH.

Disconnected the emission computer and reconnected the CMH relay, fuse does not blow. Checked resistance between the BY and LR connections on the emissions computer, both the original and the spare were reading roughly 13M ohms.

Does anyone know what goes on inside the emissions computer? Really wish I could Desmog, but that does not work so well in CA.

Also the BY and LR wires shoot open to ground and each other with the emissions computer disconnected.

Other than the radio wiring, everything else appears factory or has not been touched in some time.

Guess I will keep plugging away, still waiting to find the smoking gun.

Thanks for any and all assistance you can provide.

 

So looked at the wiring diagram again and noticed there was not a black dot indicating a splice off of the CMH relay to CMH, and re-shot the BY from the CMY relay to CMH, had continuity.

Re-shot the CMH and had a reading of .9 ohms, which seemed to be within range.

Reconnected all the wiring and fuse blew as expected. Disconnected the relays small connector, fuse didn't blow. Reconnected small connector and disconnected large connector, fuse didn't blow. reconnected large connector and disconnected CMH connector, fuse didn't blow.

So looks like I need a new CMH, even though it seems to be within range.

Will see if I cant source one from the pick-n-pull, unless someone has one for a decent price.

Hopefully that solves that issue.

Now noticed that bright headlights only work when flashing (not the trench coat type of flashing).

Will start on that issue next, once that is sorted will hopefully sort carb issues. Does not apear to be any gas showing in the sight glass at all.

Again I appreciate all assistance I have received thus far.

 

If you need a CMH and can't find one at the pick-n-pull I have the one I took off of my truck for sale for $15 in the classifieds:

https://www.yotatech.com/forums/f189...an-out-291189/

It worked when it was removed and I didn't have any wiring gremlins so it should help you figure out your problem.

As for your high beam issues - that likely IS NOT a wiring issue. I had the EXACT same problem with mine. I could pull back to flash, but I couldn't flip the stalk forward to keep the high beams on. I'd actually have no lights if I flipped the stalk forward. Turns out that I had a bunch of gunk built up on the contacts of the stalk. I just simply took the stalk off, cleaned and lubed the contacts. There is a tiny spring and a detent ball in the stalk - make sure that you don't shoot them across your garage when you take it apart. Get the 30+ year old dust, grease and debris out of there, dab on some dielectric grease in there, re-assemble and you should be good to go.

 

Had a similar issue with an older Subaru I have and forgot that I had to do that. Thanks for the memory jolt, hopefully your CMH solves the parking light issue and I don't run into any more strange electrical problems.


Thanks for the help.