Electrical issue m1008

[xenocath]

I have had a small issue for quite awhile now and I have been troubleshooting and searching out answers with the help of this forum to try and resolve it. I have not seen an issue quite like the one I seem to be having so I figure to finally post in hopes someone may be able to help.

Here's the done that list:
2 load tested verified fully charged batteries (swapped out twice with different batteries issue still remains)

4 tested good rebuilt alts (swapping these in any combination did not resolve issue)

I have cleaned ,checked, tested all fusible links, grounds, instrument cluster wiring and anything else I could stick a meter into. I have replaced all relays and solenoids done doghead mod. I have installed a manual push button for glow plugs. I have all gen lights acting as they should when you turn key on.

Everything sounds like it should be working correctly right? well to a point that is correct. the problem is I lose voltage when its warm, if the temp outside is above 60 degrees or the truck has run for awhile and warmed up to operating temp. my 24 volt gauge and my 12 volt gauge both sit as they should when its cold (at the white tick on the green and at the 14 volt mark ) when it is warm it drops both gauges down some and that gets worse when adding heater lights wipers etc. and it does not recover unless I turn items off . it drops to almost the yellow on one and almost to 12 volts on the other.



Any ideas on what I might have missed in my trouble shooting that could cause this issue?

 

[Warthog]

Have you split the wiring harness firewall connection and cleaned it? I have seen that cause issue before.

A can of Electrical Contact Cleaner and an old toothbrush does wonders.

 

[xenocath]

I have not done that yet i did check continuity from one side to the other with my meter and got good results figured it was good to go.

 

[Warthog]

But did you check when it was "hot"?

GM used some type of grease on the contacts. Over the 30+ years the grease has turned into s black sticky goo. Some trucks have issues with this goo and some do not. It may be worth an hour of your time to clean the contacts.

A couple of weeks ago another member was having a GEN2 issue. Cleaned everything. Turns out the 24v feed wire in the plug was corroded. It would show it was connected but would not pass any current. He would not have found it if he didn't split the connectors and checked.

YMMV

 

[xenocath]

I surely plan to eliminate this as a cause as soon as possible thanks for the advice.

 

[MarcusOReallyus]

Turns out the 24v feed wire in the plug was corroded. It would show it was connected but would not pass any current.

Corrosion will do that to you, xenocath. It creates a high resistance at that point. You come along with your nice meter, and test for voltage. Since your meter has a very high input resistance, it never draws any appreciable current. So, you measure a correct voltage.

Put a load on that circuit and now that resister (corrosion) drops the voltage. That's why a test light will sometimes show a fault that a meter will not see. It puts enough of a load on the circuit to allow the voltage to drop across that resistance (corrosion), so the light is very dim or won't light at all.

This is the same thing that's happening with the glow plug resister in the stock GP circuit. Measure the voltage at the input of the GP relay when the GPs are not operating, and you'll see 24v. Measure at the same point when the GPs are operating, and you'll see 12v. (Approximately.)

No current flow, no voltage drop.

Infinitesimal current flow (like you get with a meter), and you get infinitesimal voltage drop. Thus the fault can hide from a meter.

 

[xenocath]

i bend wrenches pretty good but not so much on the chasing stray electrons thing. any tips from the more electrical savvy folks are appreciated. i have chased this issue for almost a year now. i drive the truck daily but knowing its not exactly working correct is annoying the heck out of me, enough so that i finally posted to get some help .