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4L80e has me pulling my hair out!

plyr2k

plyr2k

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The trans fuse kept blowing. After much research, I found a quick way to see if it was internal or external was to unplug the the big connection on the driver side. I did and it quit blowing fuses. Telling me it was internal.

I replaced all 4 solenoids and the in trans wiring. That worked, for about a mile. It's all lifted now and I drove it into the garage today. When I went to back it out, fuse blew. Replaced it and through it into drive, blew the fuse. It only blows the fuse when the trans mission builds pressure. When it's low on fluid, it can be put in gear and since it doesn't build pressure it won't blow.

All the solenoids have the recommended ohms. Is it my Tcm? The old solenoids were all the same ohms as the new ones. I don't know what's going on. I'm tired of dropping this pan.
 
All solenoids and in trans wiring is new. I traced every wire to the tcm/ecm under the dash. All are clean and no chaffing. We're still in factory loom. I think I'll see if a buddy has a spare computer just to diagnose if that's it or not.
 
Do you have the wiring pinout? Is this a '95? I've got the '91 4L80E pinout for the R/V's if it would help.

Barring someone that knows what they are talking about posting advice based on actual knowledge of the trans and it's operation, I think I'd take the correct pinout, and using a multimeter, check for a short to ground on every wire going into the trans. If it was a true intermittent problem, you wouldn't be able to replicate it every time like it sounds you can.

That doesn't seem like it will accomplish much if the wiring is new inside, but maybe you will get lucky?
 
dyeager535 said:
Do you have the wiring pinout? Is this a '95? I've got the '91 4L80E pinout for the R/V's if it would help.

Barring someone that knows what they are talking about posting advice based on actual knowledge of the trans and it's operation, I think I'd take the correct pinout, and using a multimeter, check for a short to ground on every wire going into the trans. If it was a true intermittent problem, you wouldn't be able to replicate it every time like it sounds you can.

That doesn't seem like it will accomplish much if the wiring is new inside, but maybe you will get lucky?
Click to expand...

My thought is maybe I did get lucky that it worked at all. I don't have a pin out but one would be appreciated. It is in a 95 burb. At least its not a half ton, where 95 was the only year for 4l60e and TBI. So I've got 4 years to work with. I have a multi meter. I'll try whatever anyone suggests to fix this thing. I leave tomorrow for work but I'll be back Friday.

Also, something that might be important, there's 2 wires coming out of the main plug in on the trans that aren't connected to anything. The end has been changed, but I
Can't find the two wires anywhere in the harness.

Ones solid white, ones tan w/black stripe.
 
Unless someone says otherwise, I would be hesitant to use any pinout other than that specified for your year. I would be leery that internal wiring changed enough that the pinout would not be entirely correct, which could lead you down the wrong path.

You MIGHT check Autozone's repair info online, wouldn't hold my breath, but they do have some wiring stuff up for various vehicles.

Only thing I could find quickly is this for a 1995 Jaguar in reference to the 4L80E, I'm guessing that is what they used? http://www.jagrepair.com/images/AutoRepairPhotos/jagxj1995.pdf

This indicates they did change at least the connector from '91-95 http://www.atraonline.com/manuals/general/tetdv/tetdv-c3i.htm
 
We always run an extra ground from the trans to frame, it is surprising how grounds affect this trans.
 
Do you have an internal wiring diagram? If it is truly pressure related, there could be a wiring problem related to the pressure switches. Somehow the TCM knows which gear you are in and what the line pressure is. Has it stored any trouble codes? Maybe those get lost when the fuse blows.
 
Blue85 said:
Do you have an internal wiring diagram? If it is truly pressure related, there could be a wiring problem related to the pressure switches. Somehow the TCM knows which gear you are in and what the line pressure is. Has it stored any trouble codes? Maybe those get lost when the fuse blows.
Click to expand...

No codes. I think you might be onto something there regarding the pressure switch. I just got work and won't be able to look at it again until Friday.

Thanks for the help guys. Keep the ideas coming please!!
 
Just disconnecting the harness doesn't confirm the issue is internal.

If there is no current in the wires, the external wires may still be the cause.

I unwrapped some factory loom (VSS), and every twisted wire was broken and shorting in the twisted part.

My first suggestion would be to pull all the external wiring off the trans, and inspect it when cold, in good light.

I bet you find the cause. The exterior wiring is way more likely to go bad than the internal.
 
Last edited:
dhcomp said:
Just disconnecting the harness doesn't confirm the issue is internal.
I thought this too, but after looking at the pin out, the TCM GROUNDS the wires. It doesnt get return current.
If there is no current in the wires, the external wires may still be the cause.

I unwrapped some factory loom (VSS), and every twisted wire was broken and shorting in the twisted part.

My first suggestion would be to pull all the external wiring off the trans, and inspect it when cold, in good light.
Pulled the wires all the way into the cab where the ECM/TCM are. All were clean and looked good.
I bet you find the cause. The exterior wiring is way more likely to go bad than the internal.
Click to expand...

I now have a pinout, but im at work. If i get home and cant figure it out, ill either drop this one and pull it apart, or take it to a shop. God, i dont wan to take it to a shop.

Looks like theres a "pressure switch manifold" I dont know. This thing is driving me crazy.

For anyone curious, heres the site i found to be the most useful
http://www.msgpio.com/manuals/mshift/4L80E/4L80e.html
 
The pressure switch manifold is a normal part in all of these autos. It's just a bank of switches mounted to the valve body to show where the pressure is. If you ever looked inside an 700R-4 or something of that vintage, there were separate, screw-in switches on the valve body for lock-up, etc. This manifold is just a more compact way of getting multiple switches in there.

Do you have a current clamp? You should be able to measure the current actually flowing to the transmission. P, N, R and 1 seem to all share the same solenoid logic, so these must be selected by position of the manual valve. With the PCS and 1-2 solenoid, the tranny should only be drawing about 2A. That would be like 0.4A into pin E and 1.5 or so through Pins C/D (for PCS). You could also measure the current into the TCM and compare, but you might have to catch it when the fuse blows. A DMM with max hold would be useful. What size fuse is on the TCM? Is this the stock transmission for the vehicle or was it swapped in?

Does your DMM have diode check? Sometimes these solenoids have internal flyback diodes. I don't know if that's true of your PCS, but if that diode was failing open, the spikes could mess up the TCM. If it was failing short, you would get higher line pressure and maybe a DTC? It sounds like you replaced all of the solenoids with no ones and no change, though.
 
Blue85 said:
The pressure switch manifold is a normal part in all of these autos. It's just a bank of switches mounted to the valve body to show where the pressure is. If you ever looked inside an 700R-4 or something of that vintage, there were separate, screw-in switches on the valve body for lock-up, etc. This manifold is just a more compact way of getting multiple switches in there.

Do you have a current clamp? You should be able to measure the current actually flowing to the transmission. P, N, R and 1 seem to all share the same solenoid logic, so these must be selected by position of the manual valve. With the PCS and 1-2 solenoid, the tranny should only be drawing about 2A. That would be like 0.4A into pin E and 1.5 or so through Pins C/D (for PCS). You could also measure the current into the TCM and compare, but you might have to catch it when the fuse blows. A DMM with max hold would be useful. What size fuse is on the TCM? Is this the stock transmission for the vehicle or was it swapped in?

Does your DMM have diode check? Sometimes these solenoids have internal flyback diodes. I don't know if that's true of your PCS, but if that diode was failing open, the spikes could mess up the TCM. If it was failing short, you would get higher line pressure and maybe a DTC? It sounds like you replaced all of the solenoids with no ones and no change, though.
Click to expand...

It's in a 1995 2500 burb. The one in my sig.

I have multiple high end meters, but this time I'm at work for a job, and won't be home Friday. I'm thinking march maybe.

It'll be impossible to check when the fuse blows. As soon as the trans is plugged in and the transmission builds pressure, it blows. I believe I can go park-neutral real fast and it'll hold.

All solenoids are new. No change. All the old ones tested good too.
 
I was at YellowK20's earlier and I think we might be on to something.... Out of the pigtail that plugs into the transmission has 2 wires that don't have anything to connect them to. I believe the burb has 10 or 11 wires out of the TCM. Gus had a complete harness and 4l80e on his garage floor, both from a 91. They have 11 wires. So I have 12 or 13 wires on my transmission, but 10 or 11 out of the TCM. We came to the conclusion the MAYBE the transmission swapped in is from a newer/older rig.

So, tomorrow I'll verify wire count, and I was wondering if anyone actually had a Pin out for a 1995 TBI burb 2500 with a 4l80e, as well as a newer diagram to compare wire counts to. Maybe, if I'm lucky, I can just re wire it and this transmission will work. Just need to know what wires do what.
 
Can't help with the specific wiring diagram, but you might try the local library. I've been pleasantly surprised in the past at what they had available regarding factory service manuals. Long shot, but it's free, and getting your hands on those really large diagrams is much easier than trying to view them on a screen. :)
 
It ended up being a 4L60 Plug, which is the same plug as the 4L80E but is pinned different
 
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