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4L80E 2nd and Rev Only

You can easily test almost everything with a meter. I would do all the testing as the PCM connector. You can amperage test the solenoid wiring. You know the solenoids are good, so if the amperage readings are good, then so is the wiring. The fluid temp sensor you should be able to OHM out and compare that to a temperature/resistance chart for that sensor. For the VSS/ TSS put your meter on 200v AC and watch for the voltage to increase with vehicle/ engine speed depending on which sensor your testing. The Range sensor should be easy, just test for power at the PCM in the corresponding gear. Power and ground I always recommend using a headlamp bulb to confirm the circuit can carry a load, but you can use you meter if you don't have a bulb. (Best home made tool in my box for testing power and ground circuits). The only thing you need a scanner for is to test the pressure control solenoid. The wiring can be tested with a meter, but you need a scanner to control line pressure. The solenoid should be 3.5 ohms if you want to test it. Of course if you have access to a scanner you can easily test everything I just mentioned in minutes.

Hope this helps, I know I'm not the transmission encyclopedic like THEFERMANAOR, just a lousy ASE master tech.
 
Well if an ASE master tech is all I can get, I guess it will be ok. :)
Thanks for the testing info.
This weekend I hope to have the chance to get these tested. And internal harness installed.

What are your thoughts about the PCM not throwing codes when in limp mode and not even when the transmission is unplugged?
It reads only 12 when AB shorted.

Thanks again!
 
Recap-
1994 P30 7.4L 4L80E went to limp mode at stoplight on camping trip.
Checked plug to transmission, small amount of T-fluid, contact clean, no help.
Solenoid resistance check ok
Limped home
Wired direct switches to Sol A, B and TCC. It shifts great manually.
Replaced external pigtail/plug
Test drove, still in Limp Mode.

Update-
Have internal harness to replace
Wanting to check everything I can before dumping fluid, I only want to do it once.
TPS test ok (5v supply, 1.3v at no pedal to 4.85 full pedal) tested at PCM
MAP test- 5v supply, 1.45v at idle, 4.2v at quick rev, tested at PCM
here is an the interesting part.
The pressure switches. The chart I found is at the below link. Page 147
So if I understand it correctly.
Range switches off would be open and result in a 12v value ground to PCM pins F1, E4 E5 depending on gear selected.
I get 13.11v all 3 pins and all gears, this indicates D2.
If a switch closes it goes to ground as my diagram shows, then the voltage drops to 0v.
I'm wondering if I lost the ground to the transmission.
Not sure how, if the motor is grounded and bolted to the transmission, seems like it should be grounded.
But, where do the switches get their grounding?
This definitely seems wrong, but if it is in Limp, would the switches work as they should?
I have Rev and 2nd in limp. Rev switch condition should be 0v, 0v, 12v, but it is 12v, 12v, 12v.
So I would imagine that Rev should put the switches into the correct conditions.

The temp sensor has it's own ground and the Solenoids also get grounded thru the PCM to activate.

The plan is still to swap out the internal harness this weekend. I don't think this will fix the lost ground, if that is the case.
Inspect grounding and improve it.

Thanks in advance for the input.
 
You should have some harness wires with eyelets mounted on bolts up around where the block and bellhousing meet. on our diesels they are on the rear passenger side area of the intake near the trans dipstick, GM put those grounds in various places over the years. give your entire engine bay a real good look locating any grounds to the block, tranny, inner fenders, firewall, and on + around the battery. I would honestly remove them all one at a time cleaning the contacts and putting back together. croding grounds is the #1 thing to check for when diagnosing electrical issues.
 
Hold the phone.
Bad information provided.
I had unplugged the transmission from the PCM last weekend to prevent the new plug from being contaminated.
So
I hooked it back up
Made some measurements.
The switch function I will verify later.
But the ground is good to the case and pan and any other metal portion I could find.

Sorry for that.

So, test the switch function with the engine running, maybe tonight, but by weekend for sure.
test harness before removal.
install new harness
test
test drive.
The harness and plug needed to be done, but I don't have much confidence that the change will correct it.
I will likely swap the front/rear speed sensors.

Other suggestions?
 
Would you have access to anyone near you with a GM tech 2 scanner? being that your not getting any codes logged, not sure which scanner your currently using but a quick scan with a tech 2 might reveal trans codes. of course you'd have to drive it to them and for the codes to show up again after disconnecting the battery. I've learned that on older GM vehicles the tranny codes won't clear by a aftermarket scanner, only by disconnecting the battery.

If someone has one in your area, it might help point you in the right direction. you can also see live data determining if the PCM is commanding the trans to shift comparing to how it responds. limp mode can be caused by a multitude of things including engine sensors. it might be within range for the engine to operate normally, but things like the CTS and TPS can send it into limp mode. even if the TPS is miss-adjusted but still good. I was reading on the posts where you listed the reading from the TPS. it seemed rather high for idle. iirc the voltage should be lower when your not pressing on the gas pedal, I may be wrong though.

When testing the TPS, you should see a very low voltage like 0.4~ and when slowly pressing on the pedal it should steadily increase to 4.8ish without any dropouts or fast to full 5v's. again I may be wrong about the low end readings, you'd have to look up the specs on that. you should be able to find the exact readings searching how to adjust or replace the TPS on your model
 
Thanks for the TPS alert. The information I found for the TPS is a starting voltage .58-.59v at closed throttle, so I will check again and adjust/replace as needed.
So I plugged in the transmission, checked the pressure switches, they worked correctly in all gear positions.
Ground good as mentioned.

Test drive.
gear selector in D4, starts in 1st, won't shift out of 1st. Pulled shifter into D1. It holds. Shift to D2, I feel a change. Its not a full shift. I have to rev it up hi to get it to catch. but it is still in 1st (I think, maybe). Pull shifter into D1 I feel a downshift bump. The reason I think the shift I feel isn't real is at 20-25 the engine is high revs.
Back home in driveway, check pressure switches and shifter selection, they are correct.

Back tracking,
Original--At stoplight it went into Limp mode, 2nd and Rev
At campsite we cleaned the plug with contact cleaner. (batteries were disconnected at camp, normal process)
Plugged in when we were leaving it was stuck in 1st.
Unplugged it and limped home in 2nd.
Hooked up switches to shift solenoids and TCC, worked great, smooth, up shift, downshifting.
Replaced external pigtail
Testing items electrically, results above.
Likely needs TPS adjusted or replaced (Saturday, and replace internal harness)
Tested Pressure switches, correct.
Plugged in transmission, test drive, stuck in 1st.

With PCM 1st gear solenoid is on.
Without PCM no solenoids on, default to 2nd.

So PCM doesn't seem to be calling for Limp Mode anymore. (actually only the very first time)
The other Limp Modes have been due to being unplugged.

Reading codes have been with a paper clip shorting AB and counting. Only 12's. OBD1.
I didn't even realize there were OBD1 scanners. Actron CP9690 does OBD1, Live data stream, $356 at Amazon, GM Tech 2 scanner on ebay for $378. Like mentioned, "if you can find someone". I'm planning to keep this vehicle for many years, so it will be needed again. But I can also buy all the likely sensors and PCM for that or less. I like tools...

Still hoping for ideas from the crowd.
 
Even if there is a local shop that uses a Snap-On scanner they might be able to read the trans codes for you. I am curious about line pressures, not that you should go out and plumb in a gauge, but if those are low it won't shift.

the flea bay chineesium tech 2 scanners will work fine same as a real GM scanner but NEVER update it or it will break. some folks here in the forum have used them with good results.

another thought might be depending on if there is a local pick a part yard near you, you could try searching for another PCM to try. being your's is a reman unit. I found out my original PCM was failing my accident. I kept getting a code for the boost solenoid (which I wasn't too worried about, just more of a nuisance to me), I just so happened to be out in the junk yard and stumbled across a rig with my same engine and trans, it still had the PCM along with a few other small items. I ended up snagging up on the PCM for about $12. got home and tried connecting it just to see if it was a good one (keeping as a spare). Low and behold running it for a bit and that one code didn't come back. reconnected the one to my truck and on the first drive the code returned!! go figure lol.

Just be aware that if you do end up replacing the PCM now or anytime in the future, there is a small cover with two screws on the PCM. behind that cover is the programming chip specific to your truck. don't ever let that go even if you buy another reman unit and have to turn in the core.
You can try a junk yard one with it's chip, it might work fine, it might not, depends on what options are programmed into it and emissions the vehicle was equipped with. all the PCM's for your truck (others like it) are the same. it's the chip that tells the PCM what to do and how to act.
 
So I found something that needs to be fixed. Or I should say I made something to be fixed.
The Plan was to remove the VSS's and swap them front to back. Easy Peasy
Things that have been together for 30 years tend to stay together. I think that is one of the laws of physics...
So the front one was being an SOB, so I removed the rear one. Felt the oring compress and it slipped out.
The front, not. It spins in the hole, but I can't get it to budge outward. So I pry a bit. Spin and pry. Pry and spin and snap...
It snapped off flush with the case, the metal retainer came with the plug.
Debating a small chisel to break out the sensor back to the oring area. Then it should slip out.
Or, drill into the sensor, into the steel core, thread it and use a puller of some sort. (maybe too tight of the space for that)
Considering cutting into the good rear one to see what is inside the sensor for clues and ideas.

Back to the pressure question. I found a picture showing a pressure port on the driver side, between the VSS's. Is this the port to use? And found the pressure targets for each gear. But, I was able to shift by manually firing the solenoids, It had a good thump engaging, pressure control was disconnected. (I just answered my own question I was about to ask, the pressure control solenoid ohmed good) I'll get stuff coming for pressure gage, seems like good information to have on an old big rig.

I have also ordered an ALDL cable to usb. Use it with turnerpro software I should be able to see all codes and commands to shift (or lack of). I can also see if the VSS's are working correctly. But it may be better to swap them and check, it will be easy.

Any help is appreciated
 
I won the stuck VSS battle.
I'll post this here as part of my issues, but it may be useful to others for a regular VSS replacement gone wrong.
The drill and tap, no go. No room for the drill.
Used a freshly sharpened small 3/8" wood chisel.
Using a small hammer, cut into the plastic, the steel core is right under the plastic.
Cut away until a good section of the core was exposed. (see below)
I was able to get a corner bite on the core with the vicegrips (man what a tool!!).
Pulled, grunted, pried, and pulled. Pop. It came out.

The sensor broke flush with transmission case. Red arrow.
I cut away with the chisel to expose the core, blue arrow.
The dip from red to blue is the portion chiseled out.
1686406901446.png

Got to go get a new VSS.
 
Wish I had answers for ya. I am late with my answer for the sensor removal as you figure it out before I signed on and read. Be sure you got all of the oring- sometimes it gets hard and causes this- and can break off pieces inside.

If you had a code for bad sensor then swapping location can identify it. But without a code you end up with things like this where now you need to install new parts. New parts are not always good parts so imagine if the problem is something other than one or both of these sensors. Now you install new sensor that is bad, how much worse does it make your diagnostics? Imagine finding the original problem after that, but still now you have the bad sensor you are about to install probably while I write this. Will you even know you solve it when you do? Will you try something and problems still exist so you write off a test that would have otherwise told you the issue ? If you do successfully solve problem #1, but have the bad sensor now installed- how many hours chasing down the rabbit hole before coming back to replacing the sensor(s) again?

Not picking on you, but this happens a lot even in pro shops. I used to do it too.
This is the problem with R&R testing. Factory manuals are specific to say “replace with KNOWN GOOD” nor replace with new part for this reason.

Its been too many years for me to remember details chasing transmissions without all the literature and wasn’t good helping guys in the phone even when I did it. But when I see a potential mistake, I will speak up.

Before you throw a ton of time and parts at it, seriously you need the gm flow chart of diagnostics and follow it perfectly.
 
Buy the GM (NOT Haynes/Chilton) Transmission Service Manual. It is the accompanying manual to the factory 4 Volume C/K Truck Service Manual that many of us own, either as either as the physical books or the CD-ROM that gives you access to GM's on-line database.

The C/K Truck Manual only covers 4L80E (and other transmissions) to the extent of basic diagnostics/service such as changing the VSS sensors, the P/N interlock switch on the selector shaft and transmission filter change. It does not go into transmission shift issues, hydraulic circuit flow or teardown/rebuild. The Transmission Manual does.
 

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You had mentioned the gm flow chart of diagnostics before. I searched for it then and now, but I haven't found it yet. Do you have good source I can get it from?
I found the 4L80E guide for Army Hummer, 2008. (see link in Wed 8:39 post) It has loads of great information, but all the TS is based on OBD2 codes. My OBD1 information states in 1995 the PCMs began capturing the transmission codes. My 95 motorhome is on a 94 chassis. It is a reman PCM, maybe it will catch a transmission code. Strange thing, when I'm driving with the transmission unplugged, it should generate numerous faults, no code or Check Engine Light, so it seems my PCM doesn't catch or notify of transmission errors. Or maybe it is partially fried.
So I have a known good VSS from the rear. It will be installed in the front location. New VSS installed in rear, Speedo working will confirm the new one functions. Did it need replacing? I don't know, but when I replace it tomorrow I will road test it. If that fixes it, I will believe it was the problem. I don't have the guide yet, but as much as possible I do 1 possible fix, then road test.
After tomorrows road test, the TPS will be adjusted. I retested, it is 0.85v at closed throttle. over and over, I must have remembered the value wrong, but 0.85 is still off from the 0.57v spec. Then a road test. If that fixes it, I will believe it was the problem.
With out a better guide, I know of 2 more external tests available and 1 internal repair needed (harness replacement)
I'll install the pressure guage and test according to the Army guide. Nothing available local, ordered, likely just over a week away.
Scan for codes and operation with PCM ALDL cable to USB to Laptop to TunerPro RT (also ordered today). The software can see pretty deep into the PCM. From what I have read it can go deeper than the Tech 2 scanner. Then afterwards I have much more opportunities with this setup.
Then I will go inside the pan and replace the internal harness, even if fixed somewhere above. If there are issues with a component in there, hopefully the software detected it.
 
Is this the manual?
1686417071122.png

I also found
1686417351649.png
I have a manual from this group for a Ford. It is fairly comprehensive. The diagnostic information isn't in a flow chart format.
They are both about the same price.
 
since you have a 94 motor home chassis, I am wondering if there is a slight chance you have a mix-breed where the engine and the trans have it's own PCM controller. Many in the pre 95 era used a stand alone controller for the trans.. look under seats and around other inconspicious areas for a second controller. if this is the case you won't get any codes for the trans unless there is another connector somewhere to read data. Plus only the GM Tech 2 will be able to read this data. no other scanner would be able to connect.
 
No, this has the single PCM for engine and transmission. Wires from transmission to pcm checked for signals and shorts. All ok.
The 94/95 mix I think is the version that doesn’t capture transmission codes.

I was able to make the VVS swap/install.
Test drove. No change.
Then adjusted the TPS.
Test drove. No change.

Waiting for PCM link cable, pressure gauge and GM 4L80E Manual. A little more than a week out.
Then internal harness replacement.

I found a “4L80E Stand Alone” kit. EBay or Amazon search it. It is a module plugged into the transmission PCM port. It converts it to a manual shift. Not by switches like I rigged up. But it is tied to the shifter position. There is an electrical module that I figure is interpreting the pressure valve switches signals to put the solenoids in the right sequence. Seriously considering, not as the fix for my current problem. As an alternative. The thing looks almost plug and play. And it can always be put back to original…considering…
 
I have a PCM from a 94 in my 95 now and can get trans readings. I use my laptop with GMTDscan software, but that software was specifically designed for the 6.2/6.5 diesels. I have a thread here somewhere with a bit of info on how to build your own usb cable to communicate with the truck. you could try that just to read trans codes but IDK if it will work being it's for diesels. the free version will read codes and live data.

the actual credit goes to someone on the DP forum (Diesel Place) for the info on making the cable. just google search "Scan your OBD1 6.5 for less than $10" youll see the forum post from DP. I talked about it here in a few places and linked which item you need from amazon along with the needed drivers for it. But again the software might not work.

in an earlier post you mentioned using tunerpro. for that you will need specific files for the software to read and communicate with the truck. I think there is also a version of the tech 2 also made to run on a pc, but that one I have not tried to use. I have played with tunerpro and made some custom tuning on my truck, but that's for another discussion lol.
 
Short Recap
With PCM connected to the 4L80E transmission it only has 1st and Rev.
Unplug and it has 2nd and Rev.
Attach switches and can shift all gears smoothly.
No check engine light, no codes when short AB
Small amount of Trans fluid in plug housing, (cleaned transmission plug, will replace) replaced external plug.
Checked all I knew to check with multi meter, checked good. TPS was adjusted.
1 speed sensor replaced and swapped front to back.
I believe that covers it.

Today I hooked the laptop thru TunerProRT to the PCM.
Went for a drive. 1st only fwd gear. Recorded the data.
No error codes set.

Review of the data:
Sensor Data Section:
All conditions are met to shift from 1 to 2, but it doesn't shift. (at the engine temp, Shifter, TPS and Speed)
Filtered vehicle speed displays and matches speedometer.
There is a "Raw Output Speed" displays 0 the entire time, regardless of speed. (again, the speedometer works)
Input speed is very close to the engine tach speed.
The ratio is a constant 4.0
But there is no error code set.
Available codes are, Transmission Slipping, Transmission Output Speed Loss, plus many others.

In the "Bitmasks" section:
This section shows the gear selected by the shifter, feed back from the transmission pressure switches.
It shows correctly to the shifter position.
Furter down the list is a group called "Discrete Fault Flags"
Here there is a "Yes" by "Quad Driver Module 2 Fault Detected".

Reading via the inner web, many comment that the PCM is bad and should be replaced.
This backs up my conclusion from earlier testing. But there are a few comments that the PCM may have gone bad, and others that something caused it. So, the troubleshooting for that suggested to test the items that the Quad 2 drives. I haven't found any information for the inside of the PCM 16197427 to know which pins go with Quad 2. Does anyone have the information or know a good source? I have also searched for "Raw Output Speed" to see if this is part of the problem.
 
since you have tunerpro rt working, try using the GMTDscan software and look at the live data. make sure the PCM is actually commanding the trans to shift. even though all the conditions are met using tunerpro doesn't mean it's actually commanding the trans to change gears. you can also read the trans codes live and see what might be an indicator to first look at.

Tunerpro isn't really meant to be used as a scan tool, but made to map live data and change PCM parameters. if you don't have the correct PID files or if the ones you have were custom built by someone, you might not be seeing everything or reading what is really happening with the PCM.
 
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