Hi all. I have done a motor swap for the exact same motor (6.6 lly) that came out of a dump truck. The only thing I had to change on it was the exhaust and the intake. The motor runs great as long as you don't plug the alternator in. We have taken the alternator off and had it tested and it passed all the tests (regulator, rectifier, and diode). I'm completely stumped as to why it will run just fine without the alternator (until the batteries die) but if you plug the alternator in it will choke the motor out and die. The wiring that goes to the motor stayed with the truck and the alternator is our original alternator off our old motor. If anyone has any suggestions, I would greatly appreciate it.
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help with strange electricial issue
- Thread starter tryn2
- Start date
Motors are electric. Engines burn diesel, gas, or use steam. So it is hard to work electrical when you are calling an engine a 'electric' motor... :rolleyes5:
I would bring the RPM up on the engine and plug the alt in. Load causing a stall that the computer is not correcting? FPR issues not able to correct for load? Maybe full lock on the steering and see if the PS pump load combined with maybe AC on stalls it out. Maybe a plug left off or broken wire where the engine/computer can't correct for load.
Another test would be jumper cables from a running vehicle and see if the higher voltage causes something to fail stalling you out.
My main thought is something shorted. In the alternator or wires going to it. I would test the terminals in the alt plug for "0" ohm readings. I forget the exact year, but, the ECM computer controls the alternator can go as far as turning it off to save fuel when the batteries are charged. A short can kill the ECM output to other things stalling the engine.
Tests are not the final word because the test stands do not hit the alternators with full amp load nor do they have the ~3HP to drive the unit at full load. Employee training etc. all can hand you a doorstop back pronounced "good".
Curious as to why the engine was swapped? Did it grenade with oil and chunks everywhere? The oil and coolant coated alt on my 1995 didn't last long after getting a different engine.
Again, check the not fun wiring harness first for pinched wires, good grounds, test batteries separately, then try another working alternator.
I would bring the RPM up on the engine and plug the alt in. Load causing a stall that the computer is not correcting? FPR issues not able to correct for load? Maybe full lock on the steering and see if the PS pump load combined with maybe AC on stalls it out. Maybe a plug left off or broken wire where the engine/computer can't correct for load.
Another test would be jumper cables from a running vehicle and see if the higher voltage causes something to fail stalling you out.
My main thought is something shorted. In the alternator or wires going to it. I would test the terminals in the alt plug for "0" ohm readings. I forget the exact year, but, the ECM computer controls the alternator can go as far as turning it off to save fuel when the batteries are charged. A short can kill the ECM output to other things stalling the engine.
Tests are not the final word because the test stands do not hit the alternators with full amp load nor do they have the ~3HP to drive the unit at full load. Employee training etc. all can hand you a doorstop back pronounced "good".
Curious as to why the engine was swapped? Did it grenade with oil and chunks everywhere? The oil and coolant coated alt on my 1995 didn't last long after getting a different engine.
Again, check the not fun wiring harness first for pinched wires, good grounds, test batteries separately, then try another working alternator.
- Staff
- #4
Sounds like you have a dead short going to ground from the charge circuit.
So it stalls when you put a running jump truck on it? This is a big factor if it is true. This means the alternator is NOT the cause of the issue if the running jump truck causes a stall without the alternator connected on the truck. If the jump truck isn't running then it isn't a factor.
Higher voltage could do a few things. Examples I can think of: (Battery is 12.6v and charging is 13.5v. So a running alternator will have 13.5v on your truck or the running jump truck.)
Causing bad grounds to act up.
Causing shorts to act up.
Causing ESD damaged components to fail.
Causing troubled positive cables to act up.
Causing a bad ignition switch to act up.
I would put a frame to engine ground cable on and a body to engine ground strap on.
Check the ECM for codes.
Grounds are critical with electric. We never assume ground straps are any good as they fail with internal corrosion etc. Lots of things get fixed here even on my rust free rides with grounds. Anyway it is a good starting place. GM grounds and wire in general is undersized out of the factory...
Just saying.
Just saying.
Ed HD
Formerly: Dad's 05 LLY
Run a jumper cable from the grounding place on the alt bracket to a clean spot on the frame. Easy way to test if it's a ground.
Will L.
Well-Known Member
You have a short somewhere. Powerprobe short finder is a nice time saver, especially if your not real usd to tracing them down.
Mac tools et251 another good option.
Otherwise use the old compass trick. Not as good but free.
Mac tools et251 another good option.
Otherwise use the old compass trick. Not as good but free.
ak diesel driver
6.5 driver
never heard that one explain please
Check the brown wire for voltage when disconnected. If it has voltage --> I bet it will and this means the alternator is shorted out. If not we are back to higher voltage causing something to fail. The Brown wire is the charge indicator, but, goes through the ECM.
1. Ignition OFF, test for B+ between the generator B+ output terminal (stud) to ground.
If less than the specified range of battery voltage, test the B+ circuit for a short to ground, an open or a high resistance.
2. Ignition OFF, disconnect the harness connector at the generator.
3. Ignition OFF, test for B+ between the battery positive voltage circuit terminal D and ground. Red Wire
If less than the specified range of battery voltage, test the circuit for a short to ground, an open or a high resistance.
4. Ignition ON, test for greater than 3.5 volts between the charge indicator control circuit terminal B and ground. Brown wire
If less than the specified range of 3.5 volts, test the circuit for a short to ground, an open or a high resistance. If the circuit tests normal, replace the ECM.
5. If the circuits above all test normal, replace the generator.
(GM never updated the term generators to alternators ...)
1. Ignition OFF, test for B+ between the generator B+ output terminal (stud) to ground.
If less than the specified range of battery voltage, test the B+ circuit for a short to ground, an open or a high resistance.
2. Ignition OFF, disconnect the harness connector at the generator.
3. Ignition OFF, test for B+ between the battery positive voltage circuit terminal D and ground. Red Wire
If less than the specified range of battery voltage, test the circuit for a short to ground, an open or a high resistance.
4. Ignition ON, test for greater than 3.5 volts between the charge indicator control circuit terminal B and ground. Brown wire
If less than the specified range of 3.5 volts, test the circuit for a short to ground, an open or a high resistance. If the circuit tests normal, replace the ECM.
5. If the circuits above all test normal, replace the generator.
(GM never updated the term generators to alternators ...)
Will L.
Well-Known Member
Tryn2 follow WarWagons post, this is way down the list of trying to track it down and more of a generic help for the future. Your situation is probably too strong for this technique if you have not done it before.
The wire with an electrical charge will create a magnetic field. Where the wire has a short in it the magnetic field will stop.
Put in a self resetting circuit breaker in place of the fuse, the higher amperage fuse the stronger the magnetic field be generated and easier to trace, but don't get carried away and melt a harness down. Start passing the compass over the wire slowly starting with the fuse block tracing the wire, You will see the magnetic Field by the sweep of the needle. It will go from pointing North to pointing a different direction because of the magnetism in the wire.
Slowly pass the compass over the wiring harness moving a few inches at a time. When compass needle no longer deflects in the different direction that is where you will find the short. This takes practice because as the short causes the auto resetting circuit breaker to heat up and disconnect, the compass will change direction back to north every time it loses power. So when the needle changes directions, stop moving the compass and wait to make sure if it was the circuit breaker tripping or if that is the location of your short.
The best thing to do is to build a fake wiring harness out of two or three pieces of scrap wire cover it with towel on a bench and practice it, you'll start to see how to use it as a diagnostic tool.
You can also use an inductive amp meter Or an inductive voltmeter,to do this, But you have to be careful not to exceed the range on the meter. If you exceed the range on it for too long or doing it too many times will pull the magnetics of the meter out of calibration.
Like many things in life this is the cheaper way out, and therefore is harder, takes longer. When fighting electric problems the best electronic tracking tool you can get your hands on is your friend. The Mac tool that I mentioned works of this principle but has a power generator to do it safely so you cannot burn out the electrical circuit, And the inductive pick up for it is more delicate to read the EMF generated by the wiring to give you better readings and work much quicker.
The wire with an electrical charge will create a magnetic field. Where the wire has a short in it the magnetic field will stop.
Put in a self resetting circuit breaker in place of the fuse, the higher amperage fuse the stronger the magnetic field be generated and easier to trace, but don't get carried away and melt a harness down. Start passing the compass over the wire slowly starting with the fuse block tracing the wire, You will see the magnetic Field by the sweep of the needle. It will go from pointing North to pointing a different direction because of the magnetism in the wire.
Slowly pass the compass over the wiring harness moving a few inches at a time. When compass needle no longer deflects in the different direction that is where you will find the short. This takes practice because as the short causes the auto resetting circuit breaker to heat up and disconnect, the compass will change direction back to north every time it loses power. So when the needle changes directions, stop moving the compass and wait to make sure if it was the circuit breaker tripping or if that is the location of your short.
The best thing to do is to build a fake wiring harness out of two or three pieces of scrap wire cover it with towel on a bench and practice it, you'll start to see how to use it as a diagnostic tool.
You can also use an inductive amp meter Or an inductive voltmeter,to do this, But you have to be careful not to exceed the range on the meter. If you exceed the range on it for too long or doing it too many times will pull the magnetics of the meter out of calibration.
Like many things in life this is the cheaper way out, and therefore is harder, takes longer. When fighting electric problems the best electronic tracking tool you can get your hands on is your friend. The Mac tool that I mentioned works of this principle but has a power generator to do it safely so you cannot burn out the electrical circuit, And the inductive pick up for it is more delicate to read the EMF generated by the wiring to give you better readings and work much quicker.
Will L.
Well-Known Member
How far can you follow the wires that are coming off of the alternator? I really have to suspect those wires are pinched and grounding out somewhere. Or they're making a connection to some of the wires controlling the IP.
Maybe try taking off the serpentine belt, hooking up the wires to the alternator, then starting the truck. If it won't start, disconnect the wires then start the truck, then connect the wires and see if that Acts up.
Maybe try taking off the serpentine belt, hooking up the wires to the alternator, then starting the truck. If it won't start, disconnect the wires then start the truck, then connect the wires and see if that Acts up.
Will L.
Well-Known Member
I forgot to mention -one common place for electrical problems is the large electrical Connector that has a bunch of wires going into it. there is a lever lock on top of it. it is usually on top of the engine Towards the driver side. Disconnect it & make sure all the pins or correct not bent over Or any corrosion present. Compare it to the one that was on the original engine that came out making sure they're the same number of connectors in the same locations as the new harness. After you verify everything is good,clean and the same as the original put it back together and try it one time. If not then do the following.
Engine off. Remove the belt that drives the alternator. Disconnect the electrical wires going to the alternator. Start the engine. Plug in the wires to the alternator. Does the engine 'misfire' or die while the wires are plugged in but the belt is not turning the alternator?
Engine off. Remove the belt that drives the alternator. Disconnect the electrical wires going to the alternator. Start the engine. Plug in the wires to the alternator. Does the engine 'misfire' or die while the wires are plugged in but the belt is not turning the alternator?
Will L.
Well-Known Member
Yes but not very long 1 minute is enough never more than 2 minutes. The belt would drive the water pump and without it the engine will overheat. A quick test like this is ok, just take the belt completely out. After the test reinstall the belt.
When you had the other vehicle & the jumper cables, was the other vehicle engine running? Did your engine die without connecting the alternator wires that time?
When you had the other vehicle & the jumper cables, was the other vehicle engine running? Did your engine die without connecting the alternator wires that time?