You say you are getting 11.7v reading, so I would presume you would get low readings with a hydrometer because the batteries are discharged.

I'd try disconnecting both batteries to isolate them, charge them both, and take readings again...

Was the 11.7 reading with the truck running?

 

Back to your original question..... 13.5 - 14.5 with no load, nothing over 14.5
except for maybe a few seconds.

I can tell you how to full field the alt and make sure it's working.

1st. Remove the 2 small field wires from the back of the alt, I forget the colors

2nd. Start the engine, then put +12 to one of the field terminals and ground the
other terminal to the back of the alt.

This will bypass the regulator and make the alt go to 100% output.
You only do this for a few seconds just to make sure the alt is humpin it.


I have been going crazy with pulsing voltage from 12 - 16.
I just got it fixed yesterday by making it a "one wire" alt. BEST $30 I spent
in a long time.

 

You guys are on the money...after looking at it a bit more, the alternator is pretty much dead.

 

I replaced the brushes in mine (180K miles) and the shaft the brushes sit on is worn down about 1/8 in. WOW! I'll be replacing my alternator in the spring, if not sooner. The brushes were only $30 and it DID help the charging voltage from 13.8 up to 14.3 @ 2,000 RPM. The voltage meter rides where it used to also, right of 14v.

 

Geico,

Larry B has BRAND NEW alternators for a good price (IMO) - check it out.

 

Well, had the alternator checked and thats not it....

What about the voltage regulator on a Denso alternator??? Is it internal of external??

 

Pretty sure the PCM handles the chore of voltage regulation.

I've heard of folks bypassing that PCM funtion and going with an external regulator though.

Might try a search?


phox

 

Sorry, I should have specified - this truck is a 98 12V


I guess I need to fill out my signature more thorougly

 

Still pretty sure that PCM does the voltage regulating.

Not having read every post in the 12 valve sections completely (just scanning for bad words and such), I remember a couple folks having to get an aftermarket regulator to fix the problem.

Don't think I've heard of the problem yet in the 24 valve trucks.


phox

 

ok, thanks Jerry.

 

Evil if the wiring is good and the fuse is good than you have a bad regulator in the pcm. You will have your choice of adding an external regulator, which can be a pain, or going about $700 for a pcm. Once in and working right the external is a good deal, I like the charging better than the stock system.

There are extensive threads on here on how to convert. Even a picture of the regulator and where the wires go. If you need help, glad to. I run one on mine, and my son has one on a caravan that has the same type of system.

I have also seen on here that the alternator can be converted to a one wire system with the regulator inside, but I know nothing about that system.

 

Well, still have to figure out what exactly is going on here...I cant even seem to pinpoint the problem. Thinking now that I have a short somewhere....

 

I would get the batteries load tested.I have not read where you have done that. Most of the places around here that sell batteries will load test them for free. Load testing is the best way to test the battery.

Just my 2 cents

 

Dumb question, but did you check your charge fuse in the PDC?

Next thing, you reported that your cells all tested at 1.100 - 1.150. This indicates that the batteries are dead. Charge them up and cell test them again. The cells should then be close to 1.250. A dead cell is BY FAR the most common charging problem I see with these trucks. Alternators are usually good for 200 - 250 thousand KM., and the regulator is not real common to fail.
Good luck Dr.

 

The batteries have not been load tested....I have charged them and they measure 12.6 V at present. I am charging them a bit more right now and need to check the specific gravity after charging. I dont think its the batteries - they are brand new Group 31 Commercials.

Alternator tests ok...I was kind of suprised at that.

I am pretty much convinced there is a short somewhere (likely from the previous owner) and its killing the batteries. Just finding it will be a challenge.