Gen 1 light & alternator troubleshooting

[GentleGiant]

Hello good people,

Just replaced the fuel pump and now it seems the ole truck has an electrical problem. The Gen 1 light is on continuously.

Having read some helpful threads on this, here is what I have done so far:

I cleaned all the wire terminals on the Gen 1 (except the one under the very secure little boot).

I tightened the belt.

I ran some tests:

Battery Voltage:

rear, truck off: 12.63 rear, truck running: 14.96
front, truck off: 13.05 front, truck running: 12.33 (I had just run the trickle charger overnight - that's why 13.05)

Alt 1 (drivers side):
big red wire, truck off: 13.05 volts
small red wire from plug: 13.05 volts

plug off, key off, exciter wire (brown wire on plug): nothing
plug off, key on, exciter wire: 12.02 volts (battery read 12.6 after the test: glow plugs took it down from the 13.05, I think)

The dashboard voltmeter sits exactly in the centre on the line between yellow and green.

Questions:

1. Can anyone confirm that it must be something in the alternator because the wire voltages seem to be correct?
2. Could it also be a bad battery? That battery is pretty old - it came with the truck when I got it 10 years ago. I have load tested it and it shows just a teeny bit weak, but not terrible.
3. If I don't use the lights, heater blower, etc... is it OK (safe) to drive the truck with Gen 1 not working, as long as I don't turn it off and maybe carry my battery charger along with me? I'm going to fix this... but got some errands to do.

Thanks,
GentleGiant

 

[cpf240]

From the voltage readings, I'd say Gen 1 is bad. Pull it and have it tested, or swap it with GEN 2, assuming they are both still isolated-ground alternators...

 

[Rvitko]

I would fix it ASAP, don't end up like I did, frying your starter, batteries and fusible links from starting with too low a voltage and having the starter solenoid weld up. It is a pain in the butt to repair as your starter and alts have to be rebuilt, many fusible links replaced, the batteries and cables replaced and all told it is more about the time than the money and the heartache of knowing that if you were less stupid (me not you) you would still have the money and time.

 

[richingalveston]

with the truck off you should have over 24 volts at the back battery. I think you have a bad rear battery and bad passenger side alt. if the battery were good you would get 24volts on back battery with truck off. your second alt was probably taken out by the bad back battery.

I would by two new batteries and one alt. rebuild kit. put the batteries in first and test, you will probably find alt 2 bad.

all cells can go dead in a battery and not get a short the battery will show good but never take a charge or handle a load. The second battery quit taking a charge and burnt the rectifier bridge and diode trio in second alt. This caused the second battery to quit charging all together.

My 2 cents.
Start with the batteries, you need them due to age if nothing elze.

Rich

 

[Tow4]

You can start and drive a long time with good batteries even if the driver's side alternator is inoperative. If you have one battery that is weak, put it in back so the operating alternator will keep it charged, it is only needed for starting.

I would have the original alternator rebuilt, if you don't feel up to doing it yourself. If you are handy, the alternator is easy to rebuild. There is a link to my rebuild thread in my signature, and I also sell rebuild kits.

 

[cpf240]

with the truck off you should have over 24 volts at the back battery. I think you have a bad rear battery and bad passenger side alt. if the battery were good you would get 24volts on back battery with truck off. your second alt was probably taken out by the bad back battery. ...

Rich

I think he was measuring across just the rear battery terminals, not between rear + and the frame, so he would only see 12v, not 24v...

 

[GentleGiant]

Yes, cpf240, that's correct, across the battery terminals.

reading threads here, I see that there are many connections to be cleaned. Warthog posted this in 2013: (at http://www.steelsoldiers.com/showthread.php?117594-Cucv-m1009-drivers-alternator-not-charging )

I have seen the GEN1 circuit not function properly because of dirty connections at the firewall plug along with dirty connections at the GEN1 idiot light.

Clean the bulb and socket of the GEN1 light, split the firewall plug and clean with Electrical Contact Cleaner and an old toothbrush and clean the 2pin plug at the alternator.

I guess I might start with that additional cleaning work.

I have already cleaned the two prong plug on the back of the alt.... and to be honest it seemed wiggly and loose when I put it back on. That might be the problem itself. What's the easy fix for that?

Next idea is: If I take the alt to have it "bench tested" (for free) at a good local alternator shop, will that give me a clear answer whether the alt is the issue? Or are there internal elements not tested by "bench testing"?

Thanks all,
GentleGiant

 

[GentleGiant]

Also, I have recently replaced the GP relay with the ST85. It has the plastic isolators... was just reading a thread where the wrong GP relay was causing a drain continuously and resulting in the voltage drop on Alt 1.

How to test the GP relay to see if its defective?? Is it just checking continuity between small terminals and ground/mounting brackets?

Thanks,
GentleGiant

 

[Tow4]

You can pull the plug off the regulator and use a jumper to go from the alternator B+ terminal to the terminal the exciter wire goes to. That will turn on the regulator. Since the light is working and you have 12 volts on the exciter wire with the ignition switch on, I think the circuit is working properly.

If you take the alternator to be bench tested, make sure they know it's isolated ground (if it's the original alternator) so they hook the ground to the ground terminal and not the case.

 

[GentleGiant]

Alternator bench tested dead.... its being rebuilt at a good local shop. Thanks for all the help.

GentleGiant

 

[mattsmoski]

I have a question, if the alternator is bench tested and ground is to case will you get voltage? I had both of mine tested and 11.5 wqs the voltage. I did not know about the isolated ground terminal to tell them.

 

[GentleGiant]

Hello All:

After the alternator rebuild, the truck runs perfectly. No Gen 1 light any more.... and voltmeter way up in the green. It makes me want to have the other one rebuilt, too... just to have it in the same perfect condition. Thank you all for your help.

Mattsmoski: I am not one of the real hotshots on this forum, but when I tested for continuity between the ground terminal and case on my alternator, I got no continuity. That's what isolated ground means, as I understand it. So I don't see how one could get a voltage reading if one is using the case instead of the ground terminal. In any case, I suggest taking them back and having them tested again, using the ground terminal. If people who really know this stuff don't respond to you, I suggest starting your own thread, which people may notice more easily.

 

[MarcusOReallyus]

In any case, I suggest taking them back and having them tested again, using the ground terminal.

Yep. And if you still get 11.5 volts, it's rebuild time. There's a member here who sells rebuild kits (Tow4, who posted above) and ASP Wholesale is also highly recommended.

I suggest starting your own thread, which people may notice more easily.

Yep, again.