Hi everyone. I am the newest convert to burning oil, and have recently purchased a 1994 GMC C2500 Suburban 6.5 turbo diesel. Having built many race cars, and constant work on Firebirds, Camaros, and Fieros I have to admit diesel is a very different beast.

I love my truck, but I cannot get the thing to charge above 12.5v at idle, and 14.00 at highway speed. I have replaced the following with only slight improvements over the original set-up (previously it would charge at 12.00 at idle and 13.4 at highway speed). Replaced alternator with 220 amp model, replaced pigtail to alternator, added a ground to the engine from the gauge cluster, replaced 5 year old batteries, replaced all battery cables with 000 gauge, and 0 gauge for alternator. I have added 4 additional body and engine ground straps.

I am completely out of ideas (that would normally work with a gas engine), so I need some help with this. Your help is appreciated.

 

Hello and welcome. I may be of some assistance. I am with you on the whole "diesel" thing. They really are an entirely different breed. Everything you thought you knew about mechanical work...just throw it out the door and start over. I run and own ChristianKustomz Automotive & Performance Tuning and believe me when I say that diesels can make the best of us feel lost. So take a deep breath in and breath out. You'll get through this so chin up my friend. I am learning diesels as well and I have been working on cars for over 16 years so dont feel bad.

Ok so back to your initial question...your not getting the voltage you need. First off I will need to know if your Suburban has an internally regulated voltage regulator or an external? Second do not go bigger with the ground wires. Add some yes but do not go bigger. Check and make for certain you have the correct size pulley and alternator. It may sound dumb but very often the parts stores will look up the wrong part and give you the wrong part. Also most times the after market pulleys are smaller then stock and can also cause a problem because they are not spinning the alternator as fast as it should. Lastly make sure there is no drain on the battery. Such drains can also cause the alternator to work harder and lose voltage. You should be at a minimum of 12.4 volts. However 12.5 is still acceptable. Your range should be between 12.4 and 12.8 volts.

Lastly check and make sure the ECU power wire going to the alternator has a good contact and is reading at a minimum of 12.4 volts. If its not replace the ECU. We just had a problem with a customers Buick LeSabre doing the same thing your is and it turned out the ECU was bad. We were reading 11.7 volts and finally realized the ECU was shot after replacing the alternator 4 times with 4 different brands. It was a fried circuit inside the mother board.

Give those things a try and let me know how it goes. I would pretty much put my money on a faulty ECU. But seriously dont use bigger grounds...Go back to stock size grounds.

John

 

@ ChristianKustomz......

I agree with all that you have said in the previous post, except one. I don't necessarily
agree or disagree with you, I just don't understand the reasoning. Why do you say that larger than stock ground wires are not good. Can you clear that up for me? Thanks

 

As a general rule larger ground wires are good because they transfer the heat generated from the voltage/amperage better but larger wires also means more resistance. Some vehicles are more temperamental then others. I'm not saying that's your problem exactly but it can cause a problem. Secondly it's a good idea when trying to figure a problem out to eliminate any possible "what if" scenario's. Go back to stock size and then test again. Adding ground wires is a good thing and always will be but don't mess with the ground wires coming from the battery. Keep them the same gauge.

 

In order to avoid any confusion:
A longer wire will have greater resistance than a shorter one, all else being equal. A larger diameter wire will have lower resistance than a thinner one, all else being equal.

 

I am sorry for not looking at this for a few days, but I was off at Air National Guard duty. I am interested in the ideas that you all have come up with. In this type of truck (pre-OBDII) does the ecu loose a ground internally or through loosing a ground wire connection outside of the ecu enclosure normally. I did use the aldl connector grounding as to how I normally do it with a scanner/logger. I was getting some weird readings on the scanner from some of the sensors, so I did the grounding method. Here is what I came up with...

My scanner did not show any codes even through the SES light was on at the time
My scanner was getting some odd readings off of the inputs from the sensors
With the grounding method I did NOT get a code 12 (this makes me suspect everything after)
I got a code 36 (understand this one I have gone 3 tanks without additive)
I got a code 49 (don't know what this one is at all. Is it diesel specific?)

It is due to the odd readings from the scanner and the lack of a code 12 that I am leaning toward a bad ground inside the ecu, a bad ground outside the ecu, or a fried ecu. Does this sound correct, and what would you guys believe would be the most common.

thanx