Engine/Transmission (1998.5 - 2002) Charging issue

[BoeingDiesel]

This morning while driving my '99 I got the "check gauges" light. The dash voltage gauge was far below what is normal. I got off the road and tested the batteries with my multi meter. The voltage at the batteries was 12.5v with the key off. It's been quite some time since I replaced the alternator so I installed a new Alt (not remanf.) No change in voltage to the batteries...the best incoming from the alternator at 1500 rpm was 12.66v.
I thought that maybe the grid heaters were stuck on so I disconnected the wires from the battery but that didn't help. Anyone ideas are greatly appreciated....thanks gregg

 

[Road Dog]

Measuring the voltage of the battery tells you almost nothing. You need to load test the battery -- that is measure the voltage while drawing 500A. Most battery shops will test for free and they can do it with the batteries installed on your truck. Checking the charging system is a little more complicated. The voltage regulator is built into the ECM and there's a temperature sensor under the left battery. The charging voltage goes up and down depending on the state of charge of the batteries. I think it checks the crank sensor too; no charging unless the engine is turning over.

 

[DavidC]

Just to clarify the VR is in the PCM. If the crank sensor has problems your tach will show that if a DTC is not set. The battery sensor is under the Drivers battery as stated and should ohm from 9 to 11k at room temperature.

Dave

 

[BoeingDiesel]

Thanks guys, I appreciate the feed back. I replaced the 140-amp alternator fuse today and that seems to have corrected the problem. No DT codes were active and tach is normal,
however, it does seem a little slower than in the past in coming up to the normal 14v. I'll try it for the next couple days and see where it goes.
thanks gregg

 

[K5IP]

In this case, measuring the battery voltage tells you everything. The battery voltage is normal for a
slightly discharged battery. The fact that that the terminal voltage does not increase when charging confirms what you suspected, a charging failure. I would next measure the voltage supplied to the field terminal of your alt. If you find the voltage near, or close to the battery voltage, this is normal and your alt should be charging. If the field voltage is not present, the cpu, or associated wiring is suspect.

In some systems the alt is protected by a fuse/fusable link.
this is on the output of the altinator. You can check this link by measuring the altinator output and verifying that is the same as the battery voltage. They should be the same, if not, the link is bad. all of this must be done under charge.
Good luck.

 

[Bob4x4]

It is common for that fuse to be blown while changing to oil filter without using the proper care

 

[BoeingDiesel]

It is common for that fuse to be blown while changing to oil filter without using the proper care

Yep, I can see where that could happen with the filter being so close to the alt charge wire but I can't complain as it has never been changed since I drove it off the lot Aug 1998.