M1009 Front Battery Voltage Lower than Rear

[79Vette]

Hey all,

M1009 with the factory dual alternator setup. Truck starts and runs fine. Batteries were replaced a couple months ago, as the old ones were wearing out. They were in it when I bought it and served me well for ~3 years. Now the front battery voltage is a bit lower than the rear and I am wondering what I should check.

The dash voltmeter is in the middle of yellow section engine off, and BARELY goes into the green or stays at the top of the yellow with the engine running. But I don't know how accurate that meter is supposed to be...

Voltage readings engine off:

• Front battery: 12.4 (terminal to terminal)
• Rear battery: 12.7 (terminal to terminal)

Voltage readings engine running:

• Front battery: 13.5 (terminal to terminal)
• Rear battery: 14.8 (terminal to terminal)
• Driver Alternator: 14.6 (stud to case)
• Passenger alternator: 29.3 (high stud to case)/14.5 (low stud to case)

So I think what this means is the driver alternator is supplying 14.6v to the front battery, which is dropping to 13.5 in the alt to battery wiring.
And the passenger alt is supplying 14.8V (29.3-14.5) to the rear battery, which is not dropping at all in the wiring.

Does this indicate there is a high resistance somewhere in the wiring for the front battery I need to identify and repair? Alternator outputs at the studs seem fine (14.6-14.8V), but I do think the front battery should be charging a bit better...

 

[Barrman]

Do you have the glow plug resistor bypassed on the firewall? If so, the front battery is doing all the glowing and will take longer to get back up to full voltage.

 

[79Vette]

Do you have the glow plug resistor bypassed on the firewall? If so, the front battery is doing all the glowing and will take longer to get back up to full voltage.

I dont think so, but I will double check anyway. Also these checks were performed after driving ~15 min, so would you expect the battery to be recharged by then?

 

[Barrman]

If nothing else is on, yes 15 minutes should have the batteries topped up.

Go ahead and do the first thing the TM says to do. Have both batteries load tested and fully charged. Everything depends on that at the beginning.