Another Chevy electrical question.

[Flyinfool]

Her is a new one that I can not figure out.

 

I have a 96 GMC K1500 P/U, pretty much loaded with all of the toys.

Whenever the engine hits 4000 rpm the alternator shuts off. The voltage drops from the normal 14.2 down to 11.5 and all of the lights dim noticeably and the truck starts to run rough. As soon as the rpm drop back under 4K the voltage comes back, The voltage changes like you threw a switch, it does not come in and out gradually.

This voltage drop is being measured on both the dash gauge and a separate digital voltmeter.

It does not matter if I am driving or parked, it still does this right around 4K rpm.

It has been doing this for a couple of years now. I have since replaced both the alternator and the battery (yes they were bad) but that had no effect on this problem.

 

Any ideas???

 

As long as I stay under 4k the charging system is fine.

Typically the only time I do hit 4K is when I pull out to pass a slowpoke on the highway and the trans downshifts.

[Squidley]

Just rambling here, but it sounds like perhaps the main computer is having an issue if you have replaced the battery and alternator

[skydoc_17]

I seem to remember that part of the HD Trailer Towing Package is an Alternator "Cut Out" under heavy loads. (like passing or towing a trailer up a hill) I am sure if you call your local Chevy Dealer and give them your VIN number they can tell you if this is an option on your pick-up.

Just a thought,

Earl

[Flyinfool]

I do have the HD towing package.

I never noticed this for the first 14 years (200K Miles) that I owned this truck.

I never heard of an alternator cutoff, the voltage drop will reduce the spark voltage right when you need it most.

I will do some more research into this.

I know that there is an A/C cutoff on hard acceleration.

[Flyinfool]

Good call Earl

 

I did do some more research into this and found that there is indeed a high rpm cutoff for the alternator.

The reason that I am noticing it so much right now is that my battery is dying of old age, so when the alt cuts off, the voltage is really dropping down low.

 

Now to see if there is a way to eliminate the cutoff or at least raise the rpm of the cutoff to a more friendly number.

 

I did a 100A load test and the battery dropped down to 7.9V.

 

I guess it is time to go battery shopping. :mo money:

[saddlebum]

On that model there are 2 pos cable's to the battery connected one on top of the other. After a while the cables develop a bad connection between them. Every one tends to clean the cables but never fully separate the two positve cables. Secondly groves tend to wear where the raised tabs from the one cable wear holes into the other cable.

Remove the cables, separate them and clean all surfaces. When you put them back together slip a stainless steel washer between the terminals to get a tighter connection.

I have seen maney alternators and starters, replaced unnessarily because of this simple little issue

[MiCarl]

On that model there are 2 pos cable's to the battery connected one on top of the other. After a while the cables develop a bad connection between them. Every one tends to clean the cables but never fully separate the two positve cables. Secondly groves tend to wear where the raised tabs from the one cable wear holes into the other cable.

Remove the cables, separate them and clean all surfaces. When you put them back together slip a stainless steel washer between the terminals to get a tighter connection.

I have seen maney alternators and starters, replaced unnessarily because of this simple little issue

 

The part that wears is a lead insert and is replaceable. I got new ones at the corner auto parts store.

[Flyinfool]

For as long as I have owned this truck, since '96, I have had to service that connection between the pos cables about every 3K miles due to corrosion build up and poor connection.

 

I have been researching to see if I can find a way to disable the Alternator cutoff.

No luck so far.

[Shamue]

FROM WHAT I REMEMBER THIS TRUCK HAS A REV. LIMITER THAT CAUSES THE ENGINE TO SHUT DOWN OR FIRE IRRATICALY TO KEEP THE ENGINE FROM BLOWING UP. I would talk to the dealer to locate this piece and see if there is a test to see if it is working properly.

It may have shifted, gotten dirty, gotten loose, may be old and just wearing out, causing the engine to think it is running at too high an rpm causing to shut down.

 

(Another thing you might check is the speed sensor pick up on the drive shaft may be out of time with the sensor give false info. to the computer causing this. Very rare for the speed sensor cause the problem you have described.

Just a thought.

 

 

 

 

 

Her is a new one that I can not figure out

 

I have a 96 GMC K1500 P/U, pretty much loaded with all of the toys.

Whenever the engine hits 4000 rpm the alternator shuts off. The voltage drops from the normal 14.2 down to 11.5 and all of the lights dim noticeably and the truck starts to run rough. As soon as the rpm drop back under 4K the voltage comes back, The voltage changes like you threw a switch, it does not come in and out gradually.

This voltage drop is being measured on both the dash gauge and a separate digital voltmeter.

It does not matter if I am driving or parked, it still does this right around 4K rpm.

It has been doing this for a couple of years now. I have since replaced both the alternator and the battery (yes they were bad) but that had no effect on this problem.

 

Any ideas???

 

As long as I stay under 4k the charging system is fine.

Typically the only time I do hit 4K is when I pull out to pass a slowpoke on the highway and the trans downshifts.

[SilvrT]

Regarding the battery cables and corroding .... many folks might clean "what is visible" but the corrosion works it's way down the cable, inside the cover and in between the cable strands. Rip back some of the covering to check this and if there is corrosion and the cable is long enuf, cut it back to where the corrosion ends, clean what's remaining, solder it and install a new battery clamp.