HELP! PCM not getting Vehicle Speed Data (sort of....)

[Daveo91]

My 4L80e will not shift out of 1st gear. I'm sure it's not 2nd, and I'm sure it's not in limp mode. No codes. Using a trans-specific .adx, I see 0 mph vehicle speed while driving. Everything else seems to register. But the weird thing is, and the reason I say "sort of" in the title, is if I use my typical engine .adx, speed registers normally. I can't figure this out. Why would one .adx show speed and the other won't? I have previous data logs using the trans .adx and they DO show speed and of course trans shifted normally.

Here's what I've done so far:

- made sure VSSB is getting key-on 12v and ground to it is good.
- tested continuity on twisted pair wires that go from t-case VSS to VSSB. All good
- tested continuity on both wires that go between VSSB and 7427 PCM. C11 on VSSB to F13 on PCM - good. C13 on VSSB to F12 on PCM - good.
- Swapped to a backup 7427 PCM - no change. Note that I only have one MEMCAL, however. Had to use this same MEMCAL in both PCMs.

Possible suspects:

- VSSB is bad.
- Bad connection somewhere between VSS, VSSB, and PCM.
- MEMCAL is bad.
- Bad ground somewhere
- Something else I'm not thinking of

Backstory and disclaimers

- This came after valve lifter and intake manifold replacement. Of course I had everything on top of the engine off and disconnected. I was going to swap to Vortec CSFI multiport, but I ran into too many challenges and put that project on hold. Went back with everything same as before. (46 mm TBI, 454 injectors. Engine is a 383, vortec heads)

- I have both the original 1991 DRAC and a VSSB from a '94 truck. DRAC runs the speedo only. (speedo working fine) VSSB provides both outputs to the PCM. (40 PPR and 2K PPM). There are two twisted pairs connected at the t-case VSS - one goes to DRAC, the other goes to VSSB. This set up worked fine for months before the lifter replacement. One thing I'm considering is eliminating the original DRAC and using the VSSB to run the speedo. Eliminate the second feed from the VSS and use the original feed - rerouting from DRAC to VSS.

- '93 and older 4wd with 4L80e used 3 VSS: trans input, trans output, and t-case. When I switched to 7427, this eliminated the need for the trans output sensor. The sensor and wiring is still there, just not connected to the PCM. Again, everything worked fine before the lifter replacement.

- I've also been messing around with the original vacuum modulated cruise control. As in eliminating it and swapping to electric cruise from a GMT 400. I've been tracing wires under the dash, but haven't cut into anything. I can't see how this would cause my problem, but need to mention it since I've been under there in the time frame from when everything worked to now.

[1project2many]

Often, complex and challenging symptoms trace back to very simple issues caused by an oversight or two during reassembly. It's possible that a visual inspection could reveal the problem without spending time in diagnosis. A thorough visual inspection can reveal pinched wires, disconnected wires, connectors plugged into wrong places, damaged terminals or wires. A thorough visual inspection should never be skipped or rushed.

My 4L80e will not shift out of 1st gear.

You should work to narrow the possibilities. Determining whether or not the problem is in the transmission can be very helpful.

Does your adx show calculated gear ratio?
Have you validated commanded shift solenoid state against the transmission gear?
See P. 48 of https://gear-report.com/wp-content/u...leshooting.pdf
1st gear should show solenoid 1 (or A) "On" and solenoid 2 (or B) "Off." Any other combination should not result in 1st gear. If the PCM is commanding the correct gear and the transmission is not responding appropriately then you may not need to chase the lack of speed data in your transmission adx.

I'm sure it's not 2nd, and I'm sure it's not in limp mode.

Disconnecting the large connector at the transmission will force the transmission into 2nd gear. Removing power to the transmission will force the transmission into 2nd gear. If you cannot force the transmission into second gear by removing power from the solenoids then you have a problem inside the transmission. Conversely, if you remove power and the transmission remains in the same gear that it was in before you removed power, it would be very wise IMO to confirm (again, if necessary) the transmission is not actually in 2nd gear.

Why would one .adx show speed and the other won't?

That's a good question. Check to see where the two files are getting data from by opening up the adx for editing. Solving this issue may not resolve the shifting issue but it could help you resolve this question so you can move to the next step.

I have previous data logs using the trans .adx and they DO show speed and of course trans shifted normally.

So what has changed in the vehicle configuration between those logs and present day? Was anything changed in the calibration?

I have both the original 1991 DRAC and a VSSB from a '94 truck. DRAC runs the speedo only.

This seems unnecessary. If you could simplify this system it would reduce some unknowns now and in the future.

I've been tracing wires under the dash, but haven't cut into anything. I can't see how this would cause my problem, but need to mention it since I've been under there in the time frame from when everything worked to now.

This is another area where a good visual inspection could be very important. And it's an area where it's challenging to see. I believe that it would be beneficial to see if the pcm is commanding 2nd gear. That is likely the fastest and least invasive test you can perform.

[Daveo91]

Often, complex and challenging symptoms trace back to very simple issues caused by an oversight or two during reassembly. It's possible that a visual inspection could reveal the problem without spending time in diagnosis. A thorough visual inspection can reveal pinched wires, disconnected wires, connectors plugged into wrong places, damaged terminals or wires. A thorough visual inspection should never be skipped or rushed.


You should work to narrow the possibilities. Determining whether or not the problem is in the transmission can be very helpful.

Does your adx show calculated gear ratio?
Have you validated commanded shift solenoid state against the transmission gear?
See P. 48 of https://gear-report.com/wp-content/u...leshooting.pdf
1st gear should show solenoid 1 (or A) "On" and solenoid 2 (or B) "Off." Any other combination should not result in 1st gear. If the PCM is commanding the correct gear and the transmission is not responding appropriately then you may not need to chase the lack of speed data in your transmission adx.


Disconnecting the large connector at the transmission will force the transmission into 2nd gear. Removing power to the transmission will force the transmission into 2nd gear. If you cannot force the transmission into second gear by removing power from the solenoids then you have a problem inside the transmission. Conversely, if you remove power and the transmission remains in the same gear that it was in before you removed power, it would be very wise IMO to confirm (again, if necessary) the transmission is not actually in 2nd gear.


That's a good question. Check to see where the two files are getting data from by opening up the adx for editing. Solving this issue may not resolve the shifting issue but it could help you resolve this question so you can move to the next step.


So what has changed in the vehicle configuration between those logs and present day? Was anything changed in the calibration?


This seems unnecessary. If you could simplify this system it would reduce some unknowns now and in the future.


This is another area where a good visual inspection could be very important. And it's an area where it's challenging to see. I believe that it would be beneficial to see if the pcm is commanding 2nd gear. That is likely the fastest and least invasive test you can perform.

Thanks much. I've spent hours visually inspecting, both before the diagnostics and after. That was the first thing I did - should have mentioned that. I've looked at the harness in back of and on top of engine, harness at the trans and up firewall, under the dash, and at T-case. Next step here will be taking off the TB, brackets for MAP, and maybe even dist and coil - just so I can get a better look at the harness back there.

Calculated gear ratio: an export of the log file to Excel shows two "ratio" columns. In the log I'm looking at, both columns are populated with "4" all the way down.
Trans solenoids: the log files shows A as ON and B as OFF all the way down. Doesn't seems like the PCM is commanding a different gear?
Unplugging the trans connector: I didn't get a chance to do any physical work on the truck today. I will try this tomorrow. Very curious to see if it will go into 2nd.

Vehicle speed in two different ADX files: this is the confusing part to me. Once we start talking about addresses on the chip, I'm out of my element. Trans ADX export has two vehicle speed columns: one is called vehicle speed, the other is "MPH". MPH has appropriate data in it, whereas vehicle speed is zero all the way down. Engine ADX has just one vehicle speed column, with data populated. All three seem to refer to different addresses (packet offset?) by looking at those items in the ADX editor. I also did an export of the log from before tear down (mentioned previously) that used the same trans ADX. In this one BOTH speed columns show data, but the weird thing is they don't match. At low speeds they differ by a few mph, and high speeds (40 mph and above) they differ by 10 or more.

What changed in the calibration from before tear down to now? Nothing. I parked it for tear down, trans working fine. No changes to the .bin after that point.

Parallel DRAC and VSSB. I agree this is unnecessary. I thought it was necessary at the time based on various posts on this site and due to the oddities of my 1991 V2500, but another member here (@Tayto) pointed out to me I could just use the new VSSB to run the speedo. But I was already mostly done with the conversion so I stuck with both. I think eliminating the old DRAC (and extra twisted pair from T-case VSS) will be my next step here. Based on my data analysis described above, it really does seem like the PCM is not getting a vehicle speed value it uses to determine shift points.

Thanks again for walking me through some stuff.

[Daveo91]

SOLVED! It was a bad connection on one of those two wires between the VSSB and the computer. Which is weird because I DID check those wires visually and I'm positive I tested continuity on both, but it was definitely the problem. I used the splice connectors that have the ring of solder in the middle - I think it didn't melt when I installed it originally. This time I made sure the strands intertwined and made sure the solder melted. All four forward gears again!

[In-Tech]

Great job on your diligence :) That volt/ohm meter only checks with the slightest amount of current so it cannot possibly show anything but a short or open circuit. Some times we have to go back to logic and with wiring there are many times where you just replace. Chasing an electrical gremlin is chasing what you cannot see at times :)
Make sure you do a "twisted wire" on any sine wave connection.