A few weeks ago I started having issues with my truck starting, I first had the battery’s tested one of them was going bad so I replaced them both, I still had intermittent starting issues so I replaced the Solenoid on the fender. The next time I drove the truck I noticed the voltage gauge in the truck was low.I checked the voltage and I was only seeing 11.? at the time.I replaced the alternator I the new alternator is putting out the same voltage.I removed and cleaned the battery cables (positive and the grounds), cleaned the battery grounds, cleaned the cable to the starter (main power and Signal to the starter solenoid). After I got everything back together the truck wouldn’t start, after I pulled the starter off I noted the starter solenoid had a bad terminal.I replaced the Solenoid last night and the truck started right up.I was seeing 12.3 to 12.7v depending on the rpm and the electrical load at the time.The truck suddenly shut off, I restarted it and the voltage was back to battery voltage no longer charging.I felt the wires on the solenoid on the fender and on top of the motor none of them where hot.



Also I don’t have a battery light, it come on when I turn the key but once I start the truck is turns off I have checked voltage and the pig tail that connects to the alternator and have 12V on two of the wires.



I’m at a loss at this point tonight I was going to try and jump the exciter wire to the alternator.



I have looked through several of the threads but I haven’t seen where anyone with a similar issue ever found a cause.

 

Check to see if the 2 large posts on the glow plug relay have power on them. If the relay is working correctly, you will have power to both of them when you first turn the key to the run position. The relay should turn off after approximately 2 minutes, then you will have power on just 1 of the posts. If the relay contacts have stuck together you will see low voltage.

 

If you have 12 at the Alt exciter and 12.7 out of it, Alt is defective unless your GPs are staying on all the time

check GPR to see if stuck on, it should shut off after two minutes
can also remove the GPR small control wire if above 70 out side

 

I'm only getting 12 volts (battery voltage) on the post that has the heavier gauge wire on the GPR, the other post has no voltage so that seems to be working. I have it idling now and its at 12.7 to 12.9 Volts it drops down to 12.5 if I turn the lights and blower fan on. Also if I bump the RPM up to a 1000 it will make 13.2.

 

I let it idle for about 15 to 20 min and it stopped charging. could I have gotten a bad alternator.

 

Sounds like a bad one to me

 

I retested everything and when I started truck after it had cooled off the GPR stuck and it was getting voltage on all 4 posts. should the two smaller posts have voltage when the truck is running? I put my old Solenoid in and right now its charging at 12.9 volts I'll let it get how again and see if the alternator shuts off again.

 

Putting the old GPR in worked it make 13v at idle and 12.5 with the lights on blower radio and trailer hooked up. My issue really started with needing the replace the solenoid on the starter itself. My one battery was going but I would have lasted till the winter maybe longer since I only drive the truck a few times a month.

 

Sound like a shorted out GPR

Did you use a GPR or a solenoid? the starter solenoids are grounded, the GPR isn't. Both look like the same except the grounding

If you had 12v on all four posts, then it shorted or was a solenoid installed

GPR is grounded for 2 min after starting

 

12.5 isn't good enough, batteries like 12.6 to charge

 

It depends on the battery state of charge, temperature etc. Electrical systems have to be tested under load. 12.6 volts won't charge anything, in fact that's less than what a fully charged sealed "maintenance free" battery reads at rest. It takes approximately 2 volts above resting battery voltage to reach a full charge. Turn the headlights lights and accessories on to put a load on the alternator and spool engine up to a fast idle. Snap On, with their fancy pants wireless battery and charging system tester specifies measuring output voltage @ 2400 RPM. Kinda simulates conditions when cruising down the highway at night right?

Then measure the voltage right at the battery posts, that's the only voltage measurement that counts. Something like 13.8 to 14.2 volts is right in there what you want to see. Make sure every terminal, cable and ground connection is bare metal shiny bright, and tight, even a paper thin nearly invisible layer of corrosion is enough to cripple any alternator. After 20 or 30 years, the electrical connections will always have corrosion. The voltage might read sorta OK, but the current (amperes) won't flow like it should, and the excessive resistance "fools" the voltage regulator into thinking the battery is already fully charged up. Just a few extra hundredths of an ohm resistance in the wiring or ground connections reduces alternator current capability by about 30%.

 

This or gross battery cables. 3G has a few wires to flip it on and charge.

One from voltage reg. One from VR to +B post. One to switched 12V from chassis. +B to battery. Easy.