Transfer case fuse blows constantly

Hey guys,

My '99 Suburban 350 with the auto-trac 4wd is having electrical problems. The transfer case fuse always blows. I thought it may be the transfer case shift motor, so I took it out and tried and the fuse still blew. I tested the pins on the fuse, and sure enough they are shorted together, but only when the car turns on. This makes me think it's a relay issue. Anybody with a wiring diagram have any feedback?

Thanks,

Alex

Hey guys,

My '99 Suburban 350 with the auto-trac 4wd is having electrical problems. The transfer case fuse always blows. I thought it may be the transfer case shift motor, so I took it out and tried and the fuse still blew. I tested the pins on the fuse, and sure enough they are shorted together, but only when the car turns on. This makes me think it's a relay issue. Anybody with a wiring diagram have any feedback?

Thanks,

Alex

since you have a short circuit to ground you will have to look over the wires to around the transfer case. measure the fuse socket to ground with the battery negative removed. as you disturb these wires you should find the wire where it is damaged. could be the fuse box under the hood also . wires under it could be chewed up.

I doubt a relay would cause a short circuit. I would also give the name of this fuse and its amperage. this may get someone with a wiring diagram the ability to find what it feeds exactly.

since you have a short circuit to ground you will have to look over the wires to around the transfer case. measure the fuse socket to ground with the battery negative removed. as you disturb these wires you should find the wire where it is damaged. could be the fuse box under the hood also . wires under it could be chewed up.

I doubt a relay would cause a short circuit. I would also give the name of this fuse and its amperage. this may get someone with a wiring diagram the ability to find what it feeds exactly.

thanks j_cat, it's the #2 fuse "Transfer Case," 20A. I unplugged the front diff actuator just in case that had anything to do with it but still blows the fuse. However, this time I noticed that after I put a new fuse in and turned the car on, the transfer case control module kept clicking, like there was a relay inside it opening and closing, before the fuse finally blew out after about ten seconds.

measure the fuse socket to ground with the battery negative removed.

I hate being a newb but can you explain what I should be looking for by doing this/what it will tell me?

Thanks,

Alex

Just to clarify somethings.

1. Is your Sub an Old Body Style or New Body Style?
2. You say it has "Auto-Trac". Is that what it really has or does it have a button that says "Auto 4WD"?

If it's OBS with "Auto 4WD" I may be able to help.

Just to clarify somethings.

1. Is your Sub an Old Body Style or New Body Style?
2. You say it has "Auto-Trac". Is that what it really has or does it have a button that says "Auto 4WD"?

If it's OBS with "Auto 4WD" I may be able to help.

It's old body style (GMT400) with the "Auto 4WD" button. I thought this meant Auto-trac but I guess not...

It's old body style (GMT400) with the "Auto 4WD" button. I thought this meant Auto-trac but I guess not...

OK. Let's see if I can help you.

1. The T-Case fuse #2 20a is "hot at all times". It supplies batt voltage all the time to the T-Case Control Module (TCCM) and to the T-case clutch control. This power wire is an orange one going to the TCCM & T-Case.

2. You said it "blows constantly" and then said it blows when the ign sw is turned on. So which is it?

3. If it blows when the ign sw is turned to "run" then I would suspect an internal short in the TCCM. You can try this by removing the C2 connector at the TCCM. It is a four wire connector with 2 blacks, a red & an orange.

OK. Let's see if I can help you.

1. The T-Case fuse #2 20a is "hot at all times". It supplies batt voltage all the time to the T-Case Control Module (TCCM) and to the T-case clutch control. This power wire is an orange one going to the TCCM & T-Case.

2. You said it "blows constantly" and then said it blows when the ign sw is turned on. So which is it?

3. If it blows when the ign sw is turned to "run" then I would suspect an internal short in the TCCM. You can try this by removing the C2 connector at the TCCM. It is a four wire connector with 2 blacks, a red & an orange.

Thanks stickman, and I apologize for the confusion - it only blows when the ignition is turned to run. I'll remove the C2 connector and get back to you with an update!

So I unplugged the C2 connector from the TCCM and the fuse never blew.

I unplugged both connectors, removed the TCCM and took it apart, no obvious shorts, no debris behind the PCB, everything looked shiny new.

I plugged only the C2 connector back in, turned on ign a few times, no fuse blown.

With the #2 fuse out, I then plugged the other connector (the one w lots of wires) back in. When I plugged the fuse back in (both connectors in) and ign OFF, a relay inside the TCCM popped open/close a few times. Fuse still not blown.

I then turned ign ON, relay open/closed abt five times and did that each time I turned ign from OFF to ON. Service 4WD light would show. Still no fuse blown.

I tried turning ign on and off five times to clear/reset the TCCM, and after a few times doing that the fuse finally blew.

The Service 4WD light still showed after cycling ignition, so I'm guessing that it's actively throwing a code. I was going to try and pull the code by shorting ALDL connector #13 to GND and observing the 4WD switch light blink sequence.

So I unplugged the C2 connector from the TCCM and the fuse never blew.

I unplugged both connectors, removed the TCCM and took it apart, no obvious shorts, no debris behind the PCB, everything looked shiny new.

I plugged only the C2 connector back in, turned on ign a few times, no fuse blown.

With the #2 fuse out, I then plugged the other connector (the one w lots of wires) back in. When I plugged the fuse back in (both connectors in) and ign OFF, a relay inside the TCCM popped open/close a few times. Fuse still not blown.

I then turned ign ON, relay open/closed abt five times and did that each time I turned ign from OFF to ON. Service 4WD light would show. Still no fuse blown.

I tried turning ign on and off five times to clear/reset the TCCM, and after a few times doing that the fuse finally blew.

The Service 4WD light still showed after cycling ignition, so I'm guessing that it's actively throwing a code. I was going to try and pull the code by shorting ALDL connector #13 to GND and observing the 4WD switch light blink sequence.

since the fuse did not blow when the c2 connector was removed from the encoder motor large ga. wires I would say the encoder is in lock mode. unless the computer gets feed back to the encoder position or the control for the lock signal is broken this will occur. the other reason would be a damaged transfercase which means the encoder can not move because of damage. the encoder could be defective many have had the position sensors [small wires ] of the encoder get damaged then it will not move. the clicking could be the motor over current relay.

Thanks for the reply j_cat, I'm sorry I should have made it clear earlier that the encoder motor is unplugged. Actually, it's not even on the vehicle right now. I unbolted it to diagnose it and couldn't get the t-case selector shaft to fit back into the socket on the encoder motor - i'll cross that bridge again when i come to it.

So basically all the stuff above has been happening with the encoder motor physically off the vehicle. I'm going to go check again for frayed wires and try to trace some grounds.

the other reason would be a damaged transfercase which means the encoder can not move because of damage. the encoder could be defective many have had the position sensors [small wires ] of the encoder get damaged then it will not move. the clicking could be the motor over current relay.

When I was first trying to diagnose the problem, I unplugged the c2 connector from the encoder motor and blew on the contacts, plugged back in, and replaced the fuse. To my surprise, it seemed to work. The 4WD button lights came on, indicating the t-case was in 2WD mode. I couldn't resist tempting fate by pressing the 4WD-high button...I was afraid the fuse would blow leaving me locked in 4. The t-case successfully switched into 4, which I verified by driving for a bit, and then I tried switching to 2WD. The 2WD button light blinked for a bit then went off and the 4WD high light went solid again (as if it tried to switch to 2 and couldn't). At this point, the fuse blew, but luckily going into reverse somehow knocked the t-case back into 2WD.

So it appears the encoder motor/transfer case does work, but I've got this electrical demon which I need to chase down. :banghead:

Thanks for the reply j_cat, I'm sorry I should have made it clear earlier that the encoder motor is unplugged. Actually, it's not even on the vehicle right now. I unbolted it to diagnose it and couldn't get the t-case selector shaft to fit back into the socket on the encoder motor - i'll cross that bridge again when i come to it.

So basically all the stuff above has been happening with the encoder motor physically off the vehicle. I'm going to go check again for frayed wires and try to trace some grounds.

When I was first trying to diagnose the problem, I unplugged the c2 connector from the encoder motor and blew on the contacts, plugged back in, and replaced the fuse. To my surprise, it seemed to work. The 4WD button lights came on, indicating the t-case was in 2WD mode. I couldn't resist tempting fate by pressing the 4WD-high button...I was afraid the fuse would blow leaving me locked in 4. The t-case successfully switched into 4, which I verified by driving for a bit, and then I tried switching to 2WD. The 2WD button light blinked for a bit then went off and the 4WD high light went solid again (as if it tried to switch to 2 and couldn't). At this point, the fuse blew, but luckily going into reverse somehow knocked the t-case back into 2WD.

So it appears the encoder motor/transfer case does work, but I've got this electrical demon which I need to chase down. :banghead:


so from the connector you unplugged work back to the fuse box area. if those two wires short together the fuse will blow. many have had wire damage along the transfercase as the fasteners/loom fail. then the casting is abrasive.

After all you've done and posted (great job on that) I'm still leaning towards an issue with the TCCM. I would suggest a salvage yard try. There are 2 different TCCM's for '99. One for the older "selectable" 4WD with one connector. The one for yours has the 2 connectors. Give it a try and let us know.

After all you've done and posted (great job on that) I'm still leaning towards an issue with the TCCM. I would suggest a salvage yard try. There are 2 different TCCM's for '99. One for the older "selectable" 4WD with one connector. The one for yours has the 2 connectors. Give it a try and let us know.

Thanks will do. Haven't had time to get to this, had to fix some things for MD inspection, including replacing input shaft seals on the steering box to fix a small leak. I'll re-check the wires by the t-case for fraying and also post back about the junkyard TCCM.

Well it's been a long time coming but I finally got around to fixing this today. After many hours of foolin around with this, I figured out that the transfer case wire loom was hanging on the driver side exhaust header; three of the wires were burnt/severed, including the ORN/TAN encoder motor wires. I spliced in new wire and voila, t-case/selector/encoder motor work! No Service 4WD light either!

However, the front axle actuator seems to be stuck in the engage position (maybe because the truck was in 4HI for a year?) The front tires still bind/chirp when turning. Will removing the actuator disengage the front axle? Can the actuator be tested plugged in but dismounted?

Thanks,

Alex

Well it's been a long time coming but I finally got around to fixing this today. After many hours of foolin around with this, I figured out that the transfer case wire loom was hanging on the driver side exhaust header; three of the wires were burnt/severed, including the ORN/TAN encoder motor wires. I spliced in new wire and voila, t-case/selector/encoder motor work! No Service 4WD light either!

However, the front axle actuator seems to be stuck in the engage position (maybe because the truck was in 4HI for a year?) The front tires still bind/chirp when turning. Will removing the actuator disengage the front axle? Can the actuator be tested plugged in but dismounted?


so this is what I thought with some that had the loom damaged. only diff is the wires got into the exhaust. something was not put back correctly...

I would remove that front actuator and see if it is damaged or stuck.. clean it up if it looks good and see if it will work.

so this is what I thought with some that had the loom damaged. only diff is the wires got into the exhaust. something was not put back correctly...

I would remove that front actuator and see if it is damaged or stuck.. clean it up if it looks good and see if it will work.

Yep, some previous owner didn't put the loom back into its hanger and I paid the price! I'll remove the actuator and clean it up and post back.

I took the actuator out and it was stuck open (out/engaged). I checked the voltage readings at the wire harness per alldatadiy and they seemed OK. I need to check some resistance values but I also tried to bench test the actuator with no success. I applied 12V and GND at the C and E terminals, respectively, and tried to pulse the B terminal and there was no actuator movement.

At this point I think it's the actuator, I went ahead and ordered a new one, let you all know how it goes.

Thanks,

Alex

I took the actuator out and it was stuck open (out/engaged). I checked the voltage readings at the wire harness per alldatadiy and they seemed OK. I need to check some resistance values but I also tried to bench test the actuator with no success. I applied 12V and GND at the C and E terminals, respectively, and tried to pulse the B terminal and there was no actuator movement.

At this point I think it's the actuator, I went ahead and ordered a new one, let you all know how it goes.

Thanks,

Alex

sounds good to me .. stuck in out position ...

sounds good to me .. stuck in out position ...

New actuator solved problem. Now onto the rear diff (G80, doesn't lock). I think i'm going to have it rebuilt.

New actuator solved problem. Now onto the rear diff (G80, doesn't lock). I think i'm going to have it rebuilt.


this can be from debris .. remove the cover and clean it out.. 75-90 wt synthetic gear oil.

what I have found is as the lube ages the locker does not engage quickly. see if it works with a good cleaning.. if looks good give it a try.

this can be from debris .. remove the cover and clean it out.. 75-90 wt synthetic gear oil.

what I have found is as the lube ages the locker does not engage quickly. see if it works with a good cleaning.. if looks good give it a try.

OK I'll give that a try. I suspect the clutch packs are gone or a spring is loose/broken b/c it's acting completely like an open diff, jacking up the rear I can't get the wheels to lock whatsoever.

I already order a Detroit Trutrac, so if I can't fix it I'll just pop that in.

OK I'll give that a try. I suspect the clutch packs are gone or a spring is loose/broken b/c it's acting completely like an open diff, jacking up the rear I can't get the wheels to lock whatsoever.

I already order a Detroit Trutrac, so if I can't fix it I'll just pop that in.

if you do see no damage and clean out the box good. you will not be the first guy to have it work .. with the wheel off the ground rotating one wheel will get it to lock....

as the oil ages and debris build up it will stick and not slide to lock up the both axles together.. I have found with my use when the slip causes a delayed engagement this means replace the gear oil and clean out the box... 40K miles is when I get this issue..

if you do see no damage and clean out the box good. you will not be the first guy to have it work .. with the wheel off the ground rotating one wheel will get it to lock....

as the oil ages and debris build up it will stick and not slide to lock up the both axles together.. I have found with my use when the slip causes a delayed engagement this means replace the gear oil and clean out the box... 40K miles is when I get this issue..

OK fair enough, I'll give it a try and post back!

OK fair enough, I'll give it a try and post back!

with the cover off and cleaned see if it is broken . if not it should work.