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New Alternator not charging, Also no Tach

2K views 16 replies 3 participants last post by  coalroll4power 
#1 ·
Well i have spent the last 6 hours searching the forums and wrenching on the truck and i have managed to get no where :banghead: Heres the summary of whats going on;

Over the weekend i lost charge on the alternator and had a friend bring me a new one out and changed it, seemed to be charging fine and the truck sat. Drove the truck a couple days later and all seemed okay, but about an hour into the drive my Check engine light came on, and voltage was sitting around 12 on the dash. The tach also quit working ( but its been fussy lately, working sometimes and not others but now it seems to be out permanently )

Ran the DTC code and it was a 41 - "Generator Field Not Switching Properly"

Here's what ive done so far this morning.

-Cleaned, checked resistance, dialectric greased all connections on Alt and Batt's.
-Re-charged Batts over night, tested OK
-Gapped the ESS, and even took one off a running parts truck to test with still no tach, but with the truck running the CHECK ENGINE LIGHT goes off when i unplug the ESS, and i have 5v and 8v on two of the harness connectors going into the ESS plug. And the CHECK ENGINE light will come back on when i plug back in the ESS.

Now im no wiring expert, i actually hate it for crap like this but on the alternator i also tested voltage at the BATT supply wire and its the same as the batteries, and resistance checked ok also.
When the truck is running the lower connection on the field terminal has 12v, and the top has nothing. Is the top connection on the field terminal supposed to have some sort of power while running? I tested continuity between that terminal and the PCM and its fine but other than that im not sure what else im looking for. :confused013:

Is there a way to "test" the alternator in the truck without any fancy tools? Its a brand new( not reman ) alternator so i dont really want to bring it back to the store and get it tested if it can be done in the truck by triggering it somehow to charge.

Sorry for the long post, just trying to cover everything i have tried to save you guys time on helping me.
 
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#2 ·
I had a similar problem with my truck after rebuilding it check the ground to the body. .. I just ended up running a wire from the negative battery to the Fender and fixed the alternator not charging
 
#3 ·
As far as the ess in my 97 that I stuffed a 94 motor in there is a ground 5 supply from computer and then a signal wire. . I believe that's correct but it's been awhile and I'm on the road so can't get to my book
 
#5 ·
I'm going to check the rest of the grounds tomorrow and then check continuity between the alternator along with the ESS and the PCM.
 
#6 ·
Make sure your "Check gauges" light comes on when you turn the key to ACC before starting. I believe this is a battery light as well. You need this to excite the signal.

I spent 5 hours on my hotrod diagnosing my alternator issues... I got in the cab slapped the steering wheel with the key on and was like... Wait why is my little red light on dash no on? Grabbed wiring under it and it turned on... what do you know 14.6V
 
#8 ·
Did some more research on another thread and talking with a dodge mechanic buddy of mine. Looks like another culprit can be the battery temp sensor, just need to find how to test that.
 
#9 ·
bring your alt to where you bought it, and have it tested. Part store parts can be faulty.... I use to work at Autozone... its very common.. Try explaining to a mad customer their brand new Duralast gold starter/alt is bad when it takes 2-3 hours to install and remove. And you also dont have another in stock....

I hate customer service.
 
#10 ·
That was my first idea but I just returned a lifetime warranty alternator there and would look pretty stupid bringing one back if it turns out it was good after all. I'm gonna full field the alternator for a few seconds to see if it charges
 
#11 ·
just bring it in and have them test it on the test machine. You wont look dumb trust me. Ive seen people go through 3 or 4 starters on vehicles and they were all bad.

before i buy any parts like that at a big name store I have them test it.

Just the other day we warrantied out our starter on the mud truck and the new one sounded worse than the old one!
 
#15 ·
Sometimes a shorted battery can duck everything up too... my ford focus at this exact problem and it turned out to be the batteryit took me like 3 months to diagnose and the only reason why I found it is because the battery finally completely died
 
#16 ·
I had disconnected both batteries to charge and test them. Both were good in voltage and on load. I full fielded the alternator and had zero charge, back to the store it went. Threw another brand new one in today and its good to go! I guess because my tach was out along with my charging system I just assumed my brand new alt wouldn't be the culprit. Learnt alot about the PCM system though during this pain in the ordeal. Now to figure out the tach, thanks for everyone's suggestions
 
#17 ·
That's what we are here for and improper supply to pcm does about of crap. The newer the truck the worse it gets
 
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