Engine/Transmission (1994 - 1998) alternator does not charge

[JDKammin]

I had trouble with truck starting and found out alternator is not putting out any charge. I know the pcm controls the alternator output. I am getting less than one volt out the two field terminals on the back of the alternator. I checked my other truck and one was getting 8. 5 volts and the other field post was battery voltage. What do I check next?

 

[bmoeller]

There is a 120Amp fuse in the PDC under the hood. It is held in with 2 screws. Could be blown.

 

[wcjp]

On my 96' the alternator made it to 200k. Bought a new or rebuild from Napa.

 

[JDKammin]

I thought I checked that fuse and looked OK, but will check again. The alternator is a NAPA 3 year warranty alternator.

 

[K5IP]

The field terminals are an input (not an output) to the alternator. Even so, the level you see, if actually across the field, is to low to provide any real charge.

Think of your alternator as a big current amplifier. Field input times a big amplifier gain (alternator) = big current output.

Since the input is low, look for input problems.

The PCM will not provide the field current sink (supply) necessary if it does not see an input from the engine speed sensor. First check, does your tach work? If so the VSS is working.

If the Vss is ok, the PCM shoul be applying considerable voltage across the field. (8 to 10 VDC).

It sounds like you do not have the necessary field excitation voltage,

The engine speed sensor (and assorciated wiring) would be the first thing I would look for. If ok, please post and we can go from there.

There is also a relay (ASD) in the circuit which can be problematic.

Keep us informed and we can help.

Rog

 

[JDKammin]

K5IP I think you are on to something. I just installed my Fluidampr and relocated my speed sensor(tach). For some reason my tach does not work and I ordered a new sensor, since I tried changing the air gap and did not help. So, if the speed sensor tells the pcm to supply voltage to the alternator to activate, you have pointed me to the problem. I will let you know when I get my new speed sensor wired in. Thanks.

 

[JDKammin]

I picked up the new sensor today and paid closer attention to what wires went where to the weatherpak connector that comes with the Fluidampr. I started up the truck and now have a working tach again. I will get the alternator on in the morning and will hopefully charge. Thanks for the help.

 

[WilsonF]

I hope the sensor worked for you, if not I just went through about the same thing. GEN light stayed on, volt meter showed battery voltage only. Took the ALT. to a rebuild shop,wanted to keep my ALT. not get a rebuilt. The diode pack, rectifier, had burned out, 200K on it. New rectifier, brushes and new slip rings $80 out the door. No GEN. light good charge and allot cheaper than a rebuild.

 

[JDKammin]

I put the alternator on this morning and started the truck up. The voltmeter went right to 14 volts, so all is good. I guess I should have paid a little more attention to the voltmeter after installing the Fluidampr and the tach did not work. Lesson learned. Thanks to all for the help.

 

[bmoeller]

The diode pack, rectifier, had burned out, 200K on it. New rectifier, brushes and new slip rings $80 out the door. No GEN. light good charge and allot cheaper than a rebuild.

Hope you put in new bearings too.