Alternator staying on. Running a single Alt using second Alt trigger wire?

[Chevy1925]

Im thinking out loud on this because really i have not dove into the actual issue yet. Just coming up with a game plan if it really is what im thinking.

Back history:

Limo is killing the battery in less than a week unless i have a trickle charger on it. Alternator went out on me a few months back so replaced it with a new napa unit. Plug it all in and i hear the alternator making a slight buzzing noise with the 4 prong connector plugged in. unplug it, noise goes away. Didnt test any wiring but figured the new alt is bad (aint uncommon). Warranty it out with another new one, same issue. Test the signal wire (BRN wire that turns on the alt) and i have 1.2v at it, key off engine off. That aint normal. Key on, its 12.4 or what ever batt voltage is. Disconnect the 4 pin and the truck battery doesnt die out on me.

So, honestly im pretty sure the ECM has an issue BUT i have not confirmed it. Its still possible there is a slight short to power somewhere in the harness causing the "turn on" wire to get voltage. I may have a line on a ECM but before i go that route, i got to thinking...

Would it be possible to use the second output of the ecm for the second alternator to run this alternator? i have a feeling ill get a code for the primary alt but i can turn those off. Other issue is i dont know if the OS already has that pin live, sending 12v out of it even though its a single alt OS. If thats NOT the case, that may be an issue as my OS has oil pressure segments that make my oil pressure gauge read right from Ben when i did the LLY conversion. I dont know how to convert that over for a dual alt OS. I will do my due diligence though and verify wiring and all that before going this route.

 

[1FastBrick]

Curious, did you back probe the pin at the ECM to see if it's shorted when unplug from the ALT?

If you have time, Maybe pull the ECM connection and see if some corrosion got in there.

Sometimes those rubber seals shrink from the heat cycles allowing moisture to get in.

And as we know some times the ECM's just Shit the bed internally. Not super common but does happen.

 

[Chevy1925]

Curious, did you back probe the pin at the ECM to see if it's shorted when unplug from the ALT?

If you have time, Maybe pull the ECM connection and see if some corrosion got in there.

Sometimes those rubber seals shrink from the heat cycles allowing moisture to get in.

And as we know some times the ECM's just Shit the bed internally. Not super common but does happen.

Nope, legit put about 0.2 sec into this issue and that was checking the signal wire lol. Its very possible. I just need to make the time to figure it out. I have that to figure out, coolant leak which is either the radiator or the lower rad hose and a t-case leak coming out of some mysterious place.

 

[1FastBrick]

I hear you. Everytime I touch my LLY lately, something breaks....

I took my work vehicle in for some needed repairs because I don't get paid to fix there vehicles. Drove the LLY to stretch it out... master window switch was giving me fits, I already knew about the pass door latch not recognizing the door being open, now the driver side went out apparently... I went to role down the rear windows after getting the headliner replaced, Loud pop... shit out both regulators in the rear.... one was still able to get closed the other is tapped up. I got the master switch replaced but the regulators are sitting by the door inside reminding it needs to be taken care of. I need to order the drivers side latch now and swap them both out. Just haven't had time lately.


Enough about that...

Since you said NEW alternator, I still don't trust them. I have had more NEW junk parts in the last few years then ever. When you get the time I would double check the wiring and see if you have an issue with it disconnected.

It could be a coincidence that when the last one shit the bed something happened around the same time to the ECU.

 

[Bdsankey]

Another thought, I wonder if you could wire into the secondary alt pin and run it that way? IE move that pin and trick the truck into thinking it has dual alts since the logic is still there in the ECM.

 

[malibu795]

Worse case, wire the field wire to switch hot 12V circuit.

 

[2004LB7]

I'm pretty sure the second alternator wire/pin in the ECM is active on all the OS'. I've read a few posts where people just got the pins and harness, connector and wired in the 2nd alternator without having to program anything

You can use RAP power to toggle a relay on that brown alternator wire for the time being. That way when the vehicle is off it physically disconnects the power to the alternator keeping the battery from draining

I put a second alternator in my LMM. A single wire version and used the RAP power to trigger it. Just straight up battery voltage to the trigger on all the time when the Vehicle is on.

I think the codes for the alternator only trigger if the voltage falls below a certain threshold. There is no communication between the alternator and ECM. Just a one way turn on signal. You can unplug the trigger wire with the engine running and keep the battery up with a charger or jumper cables and the code will never set

 

[Chevy1925]

Ah this is good shit. Sweet!

Brad, that’s exactly what i had planned to do

 

[DAVe3283]

Subscribing since my Duraburb started doing this EXACT same thing (down to the same 1.2V and buzzing noise when the key is off), but usually only when it is cold outside.

So very curious if moving a pin and turn off some DTCs is all that I need to do to fix it. Keep us posted!

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[Chevy1925]

UPDATE:

Pulled pin 4 from the c2 connector on the ecm, ohmed it and 0.00. Checked for short to ground, nothing. Checked for short to power, nothing. Checked all the pins for any green crusty and same with the computer. Clean as a whistle.

Moved the pin to pin 57 on the c2 connector (for the second alt) and plugged the connector back in. Surprise surprise. Still puts out 1.1v.

Thought maybe the cluster was outputting a signal to the ecm through pin 15 on c1 connector (serial data wire). Pulled it from the connector and plugged back in, nope. Still have 1.1v

So i guess its ecm? Think my buddy nate has a spare i can try.






In other news, you can de-pin and pin in the single alt to the second alt location and it will work lol. I fired the truck up and no batt light while the brown wire from pin 4 was in pin 57, no reflash of the tune and no CEL but i think its a 2 trip code for that or i just didnt let it run long enough

 

[2004LB7]

I'm pretty sure those alternator control pins in the ECM are just a turn on or off signal to the alternator. 12V = on. The voltage regulation is still done by the alternator regulator. I have a second alternator I put in and have RAP power turning it on or off as needed when cycling the key. The "on" signal is only used to keep the alternator from being on when the engine is not running or in some circumstances when cold and engine is starting and coming up to idle speed. Newer ones might do a bit more but at least LMM and under is just an on off

 

[Chevy1925]

Honestly i dont want to “bypass” the issue. I’d rather fix it right since this truck aint going anywhere. Yeah i know, aint like its all factory as is lol



At any rate, swapped a known good ecm in… same damn thing. Even checked nates lly to see what his brown wire on the alt shows and its 0.


Sooooo something is commanding it on but how and what? Fucking weird…

 

[Chevy1925]

Updated the title too since this kinda changed gears

 

[DAVe3283]

Perhaps relevant, but I've been trying to diagnose this on my LB7 as well, and I did end up swapping ECUs, but both ECUs behaved exactly the same.

Thinking back, this also started when I swapped alternators after the OEM one died. Maybe the newer regulator modules they're building alternators with are more sensitive and staying on when the OEM ones wouldn't back in the day?

Next time I'm at the junkyard I'll grab a (hopefully) original 2001-2004 alternator and see if the problem magically goes away with the alternator swap like it started with the alternator swap.

Sent from my CPH2655 using Tapatalk

 

[Chevy1925]

Perhaps relevant, but I've been trying to diagnose this on my LB7 as well, and I did end up swapping ECUs, but both ECUs behaved exactly the same.

Thinking back, this also started when I swapped alternators after the OEM one died. Maybe the newer regulator modules they're building alternators with are more sensitive and staying on when the OEM ones wouldn't back in the day?

Next time I'm at the junkyard I'll grab a (hopefully) original 2001-2004 alternator and see if the problem magically goes away with the alternator swap like it started with the alternator swap.

Sent from my CPH2655 using Tapatalk

If it werent for the fact i checked nates truck and it shows 0.0v on the brown wire (05 lly), I’d agree. Its possible the ecm i tried has the same issue but that seems unlikely.

Something else is causing it that im missing imho. Like there is a serial data line im missing or something even though prodemand says thats aint the case

 

[DAVe3283]

Yeah, it's weird. I suppose it could be a 2nd ECU with an issue that I bought.

Hmm, I'll measure my blue truck and my brother's truck and see what they show for voltage on that wire to compare. Neither of them has a battery drain, so that should be telling.

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