4L80e diagnostic guru needed (kinda long)

[boz42]

First off a little back ground info:
I am working on my 1991 V-3500 Crew Cab. I have owned it for almost 4 years. I bought it from a fellow CK5'er. About 2 years ago the trans went out & I pulled it & paid a GM stealership mechanic to rebuild it. He is well know in the area & has a good reputation for building reliable transmissions. He built the trans for about $200 more than I could buy the parts & build it myself, so I figured I couldn't go wrong. I pulled it & he did a bench job then I put it back in. He installed a new updated wiring harness ('94 & up) in the trans & I spliced a new connector in place of the old trans connector onto the truck. He built the trans 3 times before getting it right. It would only last a day & then fry the clutches the first two times. The third time it stayed together , but had a louder than stock gear whine in first. I was so disgusted I let it go. Two years later I am paying the price because it ate both sets of planetary gears, ring gear, center support, & planet drive shaft. Basically the center support & everything back to the output shaft.


So that was last month when the trans died. I have built several GM trans (350's & 400's), and a lot of bulldozer & back-hoe trans, so I decided to do it myself this time. After a couple of weeks I gathered 2 good cores for the hard parts (the first one I bought was a late model so it would not work), a Trans-Go shift improver kit & a overhaul kit with all new electronics, bushings, bearings, bands & selective washers. I also bought enough Kent Moore tools to make the job easy.

After re-building the trans I installed it & it proformed perfectly. I drove it about 20 miles in 2 days, & turned it back over to Mrs. Boz. She drove it for a week about 5 miles a day. I got in it the next weekend & noticed it did not have Overdrive. My Misses is really good about noticing things like this, but her daily commute is all curvy backroads.

I hooked it up to my buddy's GM Tech II scanner. It had two DTC's: # 28 & 68. It also would not show the correct gear that the trans was in & it showed the trans temp to be 303 & the ECT to be -40 on the transmission side. On the engine side it showed the ECT to match the dash gauge, but still showed the wrong selected gear. I cleared the DTC's in the driveway & after cranking the truck again the two codes immediatly returned. After reading in my GM Service Manual, I talked to a Gm service tech & he told me that a bad PSM can throw the reading off on the Tech II. He said the trans reads the gear selection through the PSM & when it is not right it throws the TCM into limp mode so the outputs I read on the Tech II may not be correct. So, I disregarded 68 & concentrated on code 28. I ran code 28's flow chart & according to that the PSM was bad.

Today I replaced the PSM. While I was in the trans I removed the wiring harness & toned it out. It checked fine. I also did a filter & fluid change. Cleared DTC's with Tech II. Also shecked the data stream, it now showed the correct gear selection but still did the same as before on the temps. Went ahead & drove the truck & it operated normal. Overdrive was back & I was feeling good. I drove the truck to Memphis & back(100mile trip). A few miles from the house, I noticed it did not shift into OD. Got home checked DTC's & I had 28 & 68 again. After a few phone calls I decided to disconnect the battery. After 20 minutes I hooked it back up & went for a 5 mile trip. OD worked fine again.

I am thinking I have an intermittent short in the trans harness. I also do not believe the first PSM was bad now.

I cannot do a road test with the Tech II because it does not have a ashtray power cord. OBD I does not supply power to the tech II. Went by Our loacl mega GM dealer today & they will not sell me a Tech II power cord.

Any body have any thoughts or advice.

 

[nvrenuf]

Man, I gots no 4l80e tech but can offer a bump for the day crew.

 

[boz42]

Well I drove it 40 miles to work this morning & it did fine. I dunno?

 

[nvrenuf]

You know I forgot, my old truck did some crazy shifting junk and it was an injector wire pinched between the air cleaner and throttle body.

Mine was throwing a couple of temp codes and the converter was locked up all the time. When driving it felt like I was shifter a 5 spd like 1-3-5, every upshift felt like I skipped a gear and the truck had no power. Mine trans was under warranty and the shop tried a pcm and a computer but nothing fixed it. I happened to find the wire while doing regular maintenance and never had the problem again.

I can't remember my codes, give me a minute...

 

[nvrenuf]

I found an old post of mine on Pirate, it listed one code - 58. I thought there was 2 codes, I'll check when I get home I think I wrote them down in my repair manual.

 

[boz42]

Update,, PSM removed yesterday was bad 3 of the 5 pressure sensors were depressed & would not pop back out.

 

[Greg72]

Where's Greg Ducato when you need him....?

Maybe send him a PM.

 

[Greg Ducato]

By PSM I guess you are referring to the manifold pressure switch which is bolted to the valve body. This can cause the problems you are having but having two bad ones in a row is very unusual. Being that the harness was spliced in with crimp connectors would lead me to think that there may be some kind of problem in a splice where the new harness end was connected, and that would be my first area of attention. Also, make sure you run a good ground from the trans housing to crossmember or frame, bad gounds will cause some funny things to happen. if the computer detects a high temperature it will shut down overdrive and default back to third gear. The temp sensor is in your internal harness. If you suspect the harness may be bad, let me know and I will send you a good used one to swap out and experiment with. Kudos to you for building your own 4L80E and making the investment in tools and knowledge to build this yourself. I'm sure your problem is a minor one, but tracking these down can be time consuming.

 

[1-ton]

I know this sounds simple, but are you sure you are getting your Torque Converter in all the way? Whenever I install a Torque Converter, I stand the Trans up on its tail shaft, and drop the TC down into the Trans while spinning it. You should feel three distinct clunk and drops while it is going in.

It is not good to install a TC while the Trans is sitting on its side. The Trans pump gears fall out of alignment, and the TC only goes partially in. Believe it or not, you can bolt in a Trans to the engine smoothly with the TC only partially engaged, and never know it. I learned the hard way from experience and burned a “bench built” Trans that way.

 

[Greg Ducato]

One more thing, check wires on the input speed sensor and check the sensor itself, this can cause the code 68 and no upshift to 4th.

 

[boz42]

By PSM I guess you are referring to the manifold pressure switch which is bolted to the valve body. This can cause the problems you are having but having two bad ones in a row is very unusual. Being that the harness was spliced in with crimp connectors would lead me to think that there may be some kind of problem in a splice where the new harness end was connected, and that would be my first area of attention. Also, make sure you run a good ground from the trans housing to crossmember or frame, bad gounds will cause some funny things to happen. if the computer detects a high temperature it will shut down overdrive and default back to third gear. The temp sensor is in your internal harness. If you suspect the harness may be bad, let me know and I will send you a good used one to swap out and experiment with. Kudos to you for building your own 4L80E and making the investment in tools and knowledge to build this yourself. I'm sure your problem is a minor one, but tracking these down can be time consuming.

Yes I am talking about the pressure switch manifold. The wiring harness is soldered & heat shrunk not crimp connected. I should not have used the term "spliced". No independant ground from trans to crossmember. There is the factory one on the engine. I will install a ground to the transmission case. I have fought bad grounds before & that is no fun. Thanks for the wiring harness offer, but I bought a new one today. Also my Tech 2 is reading an input speed signal close to the engine RPM. I do not know how far off & if it is within normal operating range. I will check that also.

I know this sounds simple, but are you sure you are getting your Torque Converter in all the way? Whenever I install a Torque Converter, I stand the Trans up on its tail shaft, and drop the TC down into the Trans while spinning it. You should feel three distinct clunk and drops while it is going in.

It is not good to install a TC while the Trans is sitting on its side. The Trans pump gears fall out of alignment, and the TC only goes partially in. Believe it or not, you can bolt in a Trans to the engine smoothly with the TC only partially engaged, and never know it. I learned the hard way from experience and burned a “bench built” Trans that way.

I am aware that the TC can be hard to install, and the trans will bolt up with out the TC being fully engaged. I am sure that the convertor is installed correctly.

One more thing, check wires on the input speed sensor and check the sensor itself, this can cause the code 68 and no upshift to 4th.

I will check the input speed sensor & its wiring.

On my Tech 2 I can see the false input on the ranges that the PSM is reading. So I know that is causing code 28. I do not know why for sure. In my service manual it says code 28 will cause the trans to go into limp mode with limited 4th. Could that have caused code 68?

Thanks for your help.

 

[1-ton]

The reason I ask about your Torque Converter is because no electronic problem is going to fry your whole trans the way it did (it will cause it to shift funny, but not fry it). Starving the planetary gears of fluid will, and not having a properly intalled TC will cause the pump do just that.

 

[sledheadak]

my fresh built 4l80 lost od and it was the tps.shifted fine otherwise.

 

[boz42]

Update....

I gathered up a new internal harness & a new PSM. This time I bought the PSM from GM. All other times it had came from Dacco.

Checked ISS wiring & sensor, it is fine.

Removed PSM. I have the same result from last time I changed it. 3 of the five pressure sensors are collapsed. They will not pop back out to their original form.
The red "X's are marking the ones that I believe are collapsed. I believe those to be faulty.

The PSm on top is the first one I installed into the trans during the rebuild. It was new from Dacco. I did not check the pressure sensors before installation. When I installed the second one I slapped it in also. It is the one on the bottom of the pic. I pulled it today. After the truck was back together I cut the metal backing plate off & pulled the O-rings from the first one to see how it worked. That is when I noticed the collapsed pressure sensors. When I bought the third PSM from GM (not yet installed) I noticed that all the pressure switches are not collapsed.

Before removing the second PSM I thought maybe Dacco has a bad batch of switches. After getting it out I saw that the same sensors are collapsed. I am now thinking that my trans is killing them. I mean what are the odds of 2 PSM's having the same positioned sensors being bad.

I feel I am right in my thinking that the switches are bad if they are collapsed? What is your take on that?

Any suggestions?

I am going back to the garage to drop the valve body. I am gonna check it for trash & sticking valves. Check back in later.



******Edit*****

I checked my line pressure before removing the pan this morning. They are:

AMPS = PSI
.10 = 170
.20 = 170
.30 = 160
.40 = 150
.50 = 140
.60 = 120
.70 = 100
.80 = 90
.90 = 85
1.0 = 70

According to my service manual those pressures are really close to the high side of factory spec except the .8,.9, & 1.0 amp pressures. They are 20-30 PSI higher than the suggested numbers. I credit that to modifing the front pump. Do you think those pressures could be a problem?

 

[Greg Ducato]

Did you replace the epc solenoid during the build? Any type of shift kit installed?

 

[boz42]

Did you replace the epc solenoid during the build? Any type of shift kit installed?

EPC?? Is that the force motor?

If so yes it is new. Also I did install a Trans-Go shift improver kit.

I changed all internal electronics, A & B shift solenoids, PSM, Force motor, & TCC solenoid.

 

[Greg Ducato]

Yes, that is the force motor, and the TransGo kit will not allow more than 175PSI line pressure with the little spring and valve plate they have you install.

 

[boz42]

I did replace it, It is a black bodied one. I know that the early models call for the silver one, but it had a black one in it when I tore it down so tha is what I went back with.

I posted my line pressures in an edit above, in case you missed it. You are right 170 is the max I got. I was not impressed with the dinky pressure relief valve plate spring thingy They have you add the the V-body, but I guess it works.

 

[Greg Ducato]

You can add a slightly stronger spring for more pressure if you want as long as it isn't so strong the little plate will bend. We add an extra plate and stronger spring for 200psi, if we need more, we leave that gizmo out of the build.

 

[boz42]

Well I don't know if I need more pressure or not. I like how it shifts so far. Nice & fast but not neck snapping.

I really just need to get what is killing these PSM's figured out. That is all I have thought about the last week.