Alternator going bad maybe, or glow plug module? - Diesel Forum

mudking82 · 07-02-2009, 01:43 AM

I have been lurking for a while, but finally decided to join. There is lots of great information here. This is my first diesel, and I love it! But here is my recent problem. I have searched extensively for a similar problem, but have not been able to find one. This is in regard to the truck in my signature.

Sometimes after I have been running the truck for a while, it will stop charging, or not start charging again after I restart it. On the first start of the day, it will charge normally for quite a while. Example: I start the truck, and once the glow plugs shut off, it will charge at around 13.8-14.2 volts, and be putting out at idle with no load 35-38 amps, or with a load at idle be putting out 65-70 amps. After driving for about ten minutes, my digital in cab volt gauge starts to go down, like about a tenth of an amp every five minutes, until it gets to around 13.6 while driving with no loads on.

Then here is where the problem comes in. Usually it happens after about 3-5 starting cycles and a total of around 30 minutes of driving. I will start it up, and it will never start charging, as if the glow plugs are not shutting off to allow it to charge. When this happens I only have 12.6 volts and below at the batteries, and alternator is only putting out from 2-15 amps while this is happening with no loads on. Seems to start working fine again if I shut it down and let it sit for a bit, as if it is related to temperature. Also when this happens the battery light comes on in the cluster. Sometimes after 5-10 minutes it will start working normal again while driving, other times it has to sit for a while. Can't figure out a certain pattern of it yet.

Especially after it has been sitting a while, such as on the first start of the day, the alternator seems to work perfectly fine. It varies the amps depending on load and battery state of charge. While it was working normal I tested it with a Midtronics tester and the charging system tested fine. But then when it warmed up and quit working right, it didn't test good, as there was "no diode ripple detected". Does this sound like the alternator going out, or could the glow plug control module be causing this weirdness?

Please give any thoughts, suggestions, tests, etc. I used to be a mechanic so I have plenty of tools to check stuff, so don't be afraid to point me in the direction of performing some test on it. Thanks in advance for any help!

imsolidstate · 07-02-2009, 05:18 PM

Definitely your alternator. You say that when you first start it the battery voltage is about 14 Volts, and the alternator is properly sourcing current. After a while, the alternator only puts out a few amps, and battery voltage drops. The glow plugs staying on wouldn't cause that; while battery voltage might go down the alternator draw would actually increase. "No diode ripple" is further indication that the alternator is failing to supply power. The diode ripple is the result of the rectification of the phases generated within the alternator.

mudking82 · 07-02-2009, 08:15 PM

Thanks a lot for the reply. So is it just a matter of time before it quits completely you think?

69cj · 07-02-2009, 08:32 PM

Quote:

Originally Posted by imsolidstate

Definitely your alternator. You say that when you first start it the battery voltage is about 14 Volts, and the alternator is properly sourcing current. After a while, the alternator only puts out a few amps, and battery voltage drops. The glow plugs staying on wouldn't cause that; while battery voltage might go down the alternator draw would actually increase. "No diode ripple" is further indication that the alternator is failing to supply power. The diode ripple is the result of the rectification of the phases generated within the alternator.

Agree with above. Diodes can be self healing when they cool down.

imsolidstate · 07-03-2009, 10:41 AM

Quote:

Originally Posted by mudking82

Thanks a lot for the reply. So is it just a matter of time before it quits completely you think?

Yeah. Mine worked marginally for about a year, (put out about 13 volts) before it finally started to drop off in output and I had to change it. Probably the semiconductors are starting to fail like 69cj said. Diodes change so much with temperature that they are actually used as cheap temp sensors in certain applications.

Make sure you buy a good alternator. Read my post: Avoid chain parts-store Alternators!!!