1985 C10, low voltage, battery not charging

[midnitewarya]

Hey guys,

My other truck, an '85 C10 with a 305, isn't running so great. I recently took it on a 300 mile road trip. On the way back, I watched the voltage drop from 12 to 9, where it's been stuck for a while now. I replaced the alternator and the battery. It seems to be running directly off of the battery. Voltage drops until the engine is barely running, right around 7-8 volts. The alternator doesn't seem to be getting to the battery at all. If I charge the battery up, I can get a few days of driving out of it, then the battery is dead. While it is running, it reads ~9 volts.

Anyone else have issues like this?

Thanks!

 

[6872xtc]

I would check to see if the charge wire has a fuse link in it, or there is some other break in that circuit. If you disconnect the charge wire from the stud on the alternator, you can either hook the battery up and check for battery voltage on the charge wire, or check the ohms from it to the positive cable.

You also need battery voltage on one of the small wires on the 2 prong connector of the alternator, then slightly less than battery volts on the other.

 

[diesel4me]

Your alternator & battery were probably still good--it's likely the alternator isn't getting the "signal" to turn on and start charging..

On an '85 the alternator has 3 wires--one connects to a stud on the back of it (thick red wire),and the other two are in a plug that plugs in on the side of the case towards the back of it--both those wires in the plug must get 12 volts when the engine is running in order to activate the alternator and provide the right amount of charge..

If you know how to use a test lamp,its easy to check to see if all the wires are getting the proper voltage--the test lamp has two wires,one goes to the negative battery terminal,the other end (usually looks like an ice pick) is touched to the wire or terminal you want to check for voltage..you could use a volt meter or multi-tester too,I prefer a bulb,because these wires need to deliver some amperage,and sometimes a meter will read "good",but in fact the connection or wire is not able to deliver the full amount of amps to operate the alternator--it takes a bit more "oomph" to light a bulb up,vs make a multi-meter read voltage..

With it running,test for 12V at each wire on the plug on the alternator...my guess is you'll find the thin red wire has power,but the other wire,usually a tan or white one,wont--and that is the wire that tells the alternator to charge..that wire has to get 12V with the ignition in the run position..

-one quick way to test for this is to use a jumper wire to connect both of the wires on the plug for a few seconds,then remove it--you will hear the engine slow down a bit and the alternator whine some,when it starts charging,if it wasn't charging due to lack of voltage at the tan or white wire...(It may continue to charge after you "activate" it,until you shut off the engine--next time you start it,it is likely to not charge again..



if no voltage is present,you'll have to trace that wire tan or white wire back to where it enters the firewall on the back side of the fuse box (next to the steering colum)...if it has power there ,with the key "on",engine not running,,then somewhere between that junction and the alternator plug ,the wire has failed somewhere between the fuse box and alternator..and its easier to just run a new wire from there,snip off the old wire,than rip apart the harness..

The thick red wire on the rear of the alternator has 12V all the time,even with the key off--that wire should light up the test lamp if all is OK with its circut...if not,you'll have to trace the wire back to where it goes to the harness behind the engine--there it a "T" splice in the harness for that wire--one side goes to the starter solenoid where the positive battery cable goes--that wire has a fuse link ,that is where it gets power from the battery--if the fuse link burnt out or the wire gets fried from a short,they will need to be replaced..

The other side of the "T" runs behind the engine and ends up at a junction block near the brake booster ,that is where the cab and rest of the truck gets most of its power from...

I don't know if your able to do these tests yourself,but its pretty easy if you have even limited electrical skills...

 

[6872xtc]

The tan wire shouldn't have the exact same voltage as the other. It has to have a slight difference.

 

[diesel4me]

True,the white or tan wire has a resistance in the circuit and will not get a full 12 volts---but to simplify things I didn't add that,it might just confuse him..
There should be enough voltage at both wires on the plug to light a test lamp--you may notice it is not as bright on the tan or white wire as it was on the red wire,a multi-meter would probably be needed to determine the exact amount of volts in each wire..

 

[dremu]

I vaguely recall that the "other" wire on the alternator plug is driven by a lamp, so the tan wire (taking 6872xtc's word for it, I forget what the colors are) should be hot, then to a lamp or resistor, then to the other one.

Also, duh, make sure that the blades are clean and are connecting properly -- sometimes it's the easy stuff

-- A

 

[diesel4me]

I've had quite a few GM's that the plug either corrodes,or the tabs on the internal regulator they plug onto get rusted up,or the wires on the plug rot internally when they get folded over at the plug,making a poor or no connection--also I have had to chop out about a foot of the white or tan wire from the plug,back into the harness--the copper inside the wire turned green,then into dust that far back,probing it with the test lamp showed current only up to that point where it was still good shiny copper....

 

[midnitewarya]

Follow up, it was a combination of problems, all having to do with the wiring harness. At the connector, the large red wire had pulled out and wasn't making a connection. The smaller wire was ok, but it wasn't getting 12v. I ran a new line as far back as the fire wall, and now its perfect.

If it hadn't been this thread, the 15 minutes it took to fix this would have taken me eons.

Thanks guys!

 

[diesel4me]

..glad if anything I posted helped you..

Wires corrode faster up here--in SoCal,your trucks wiring harness probably has a longer life span than in MA--wiring woes I was chasing decades ago,are probably just starting to appear in your climate !..