2nd Gen Non-Engine/Transmission low voltage at idle???

[jkh]

My volt meter drops way down when my rpm's go down. If I put it in nuetral and bring the rpm's back up it goes back up to just over 14 volts. It is fine while I am driving to , runs just over 14 volts on the gauge.

I think it is probably the alternator but does anyone have any thing else to look at in case thats not the problem?

 

[RStankevitz]

i know the crank sensor has something to do with the alternator as well not sure exactly what?

 

[Dave M]

i know the crank sensor has something to do with the alternator as well not sure exactly what?

I just replaced the crank position sensor on my 97. It supplies a signal for the pcm. the pcm has the regulator function in it for the alternater. On my 97 I had no tach, no alt charging, no check engine lite etc and found the crank sensor was bad. Cost about $100. I had also replaced the pcm ($600) but it was not bad. There is a alt fuse in the PDC on the drivers side fender.



I'm not sure an intermitint problem like you have could be the CPS but the CPS does supply pulses relative to RPM since it senses the rotating crank.



I'd have the alternator tested first if you can. Recent post on alt testing have shown most parts places don't know how to test our alternaters very well. Remember the charge/regulator function is done by the pcm (but cps must be functioning first). This tends to confuse some who have had the alt tested and failed due to no regulator working... which our alts don't have.

 

[DieselDoghouse]

And one of those posters is right here! Keep in mind when testing the alternator the diodes should be tested and most shops (electronic/alternator shops excepted) aren't equipped. IF you know a nerd with an oscilloscope or computer that can read waveforms THEY could hook up to the alternator field output. You'll see a bad diode: if all 6 are ok you'll see a steady train of rounded peaks. If one is bad you'll see a 'skipped tooth' pattern. In this condition an alternator can make 14V but only if its spun up enough to compensate for the missing winding. of course then the solution is replacement or someone to press in new diodes or replace the diode array.

 

[DPapworth]

It is normal for alternator output to fall off at idle. Alternator output is proportional to how fast it is spun. If there is a heavy load on the electrical system (e. g. headlights, grid heater, big stereo amp) you'll see that the "voltage drops way down" when the alternator turns slowly, like at idle. (On my 1995, the voltmeter reads about 11 volts with the grid heaters on and engine idling. Completely normal).

The PCM uses engine RPM to shut down the alternator field when the engine is not turning. This conserves the battery -- the alternator puts out nothing at zero RPM, and running field current through it is a waste of electrons. The PCM may also engage in fancier control strategies using engine speed/TPS/etc. You could imagine it squeaking out a few more HP at full throttle by shutting down the alternator, for example.

No CPS => no engine speed input to PCM, and PCM leaves alternator shut down. That's not the original poster's problem, as the alternator does charge at higher RPM.

As DieselDoghouse suggests, a bad alternator diode will amplify the impact of engine RPM on alternator output. You can see the missing pulses with an oscilloscope on the main lug of the alternator (and actually the whole electrical system, though it will be most pronounced right at the alternator). The "alternator field" terminal is what controls the rate of charge, and is an input, not an output. It is not a good place to check for bad diodes.

 

[Toolman9304]

My volt meter drops way down when my rpm's go down. If I put it in nuetral and bring the rpm's back up it goes back up to just over 14 volts. It is fine while I am driving to , runs just over 14 volts on the gauge.

I think it is probably the alternator but does anyone have any thing else to look at in case thats not the problem?

I am having the same problem with my 96. When it is idling, after it starts in cold weather and the grid heat cycles, it doesen't put out enough to charge the batteries back up until I drive it a while . It has left me stranded a couple of times. Have you found what the cause was?