OK, this one has me baffled. In the last 2 days about 20 minutes after start up while I am driving the GEN light comes on and the voltage is down to 12V on the dash. The first time I came directly home and checked out the voltage to the alternator and then at the battery and didn't shut the truck off. By the time i fished around and got readings the voltage was normal on the batteries and the volt meter showed 14.. After a re-start, the gen light went away and didn't come back off or on however many times.

Today the same thing. Only I was cruising in traffic when it quit. Just for fun I put in the clutch and killed the engine while rolling, then restarted. Everything back to normal and never did it again..

Tach was fine so I know the ESS should be OK and the Alternator is only 2 years old, OEM replacement.

What to check? Is my PCM starting to die?

 

Not uncommon at all for the regulator portion of the PCM to fail but the rest of the circuits go on fine for years.
Might be time to wire in an external regulator soon.

Rule out the grid heaters coming on when they shouldn't by unplugging a small wire from each of the grid relays.

 

I will try that Bill.. I removed and cleaned the connections last year when I was having some voltage oscillations.

This time though I do get the 4-7 code on the CE lite.

Some data I took this afternoon..

Battery Voltage at idle (~850rpm with laser tach) 13.68V
Excitation voltage at alternator 2.85V
About 65 mV drop from the output of the alternator to the batteries.
~2.5 mV drop from negative post to the engine block/alternator area.

Everything seems to check out. Which bothers me..

 

Yes, mine did that a while back (4 years or so). It was an early sign of an eventual failure, I wound up replacing the alternator. Had I known about the option of the external regulator that I could wire on, I may have gone that route. But my OEM is (know on wood) still going strong four years later. hope you can get that sorted out...

 

Same thing w/mine, I had it rebuilt @ a rebuild shop, wanted to keep my OEM replaced the slip rings and diode pack in the alt. has worked great for two years.

Floyd

 

So you figure its an alternator issue? I would rather that than a PCM issue at this point.

I took some electrical contact cleaner to the end of the alternator to clean it up some, but I keep my engine pretty clean. Maybe thats an issue too.

EDIT: Is there an recommended regulator for the Denso alternator? I really don't want to get a 1st gen one as I had nothing but problems with them on my 89. I see online ones that bolt right onto the alternator, are those OK?
Thanks guys!

 

A regulator is a regulator.
About the only choice you have is mechanical or electronic.
A certain regulator specified for a specific model/year is so only because of the bolt down pattern and connector style.
Any 12 volt regulator will do.

When my buddy needed one in a pinch for his '94 CTD I borrowed one from my '63 IH dump truck.
Been working fine for him for at least five years.
I finally replaced the dump's missing regulator with one from an '80s Ford sedan that an immigrant farm worker abandoned on the neighbor's farm.

 

Make sure the ASD relay is working correctly and tight in the plug. My truck was doing the exact same thing and that was the prolem.

 

Did you finally get everything working right?

 

I took the alternator off, pulled it apart and cleaned the brush contact areas on the rotor and ohm checked the fields on the rotor and outer fields with no problems. Also checked the diodes without any issues. So far its been OK.

I did musical chairs with the relays early on to be sure that wasn't the case, thanks for the tip!

 

When you pulled it apart, did you make sure that the brushes aren't sticking or binding. Sometimes they will be a little loose in their guides and this will cause them to seat incorrectly on the commutator, this creates erratic charging problems that look like a bad regulator. They only stick when the alternator is turning and after some time the charging stops, when the engine is shut off the brushes reset and the alternator will work fine for a while. I use to see this with aftermarket brushes that were slightly smaller the OEM.
I'm not a re-builder, just have allot of experience with this and similar situations.

 

Another great point. Yes they were sticky and seemed a little worn. It looked like the more they wear the more they are forced up or off the commutator. I took them out to gently dress up the commutator contact area and cycled them against the spring a few times. Probably due for a set of brushes.