Vehicle speed sensors

[Daver94K2500]

I am in the process of resolving speed sensor issues and have determined that there are no wires to the speed sensor on the side of the 4L80. It does not look like there have ever been wires to the sensor and I don't see any signs that the forward driveshaft may have ripped it off during entanglement with brush. The dealer says the wires should be there. I have not been able to find a '94 K2500 with AT (4L80) that I can verify this with in the area. The speed sensor located on the top of the transfer case on it's backside is there and was the first sensor I changed in the repair process. If anyone has a model like this one can I ask that you look at the sensor on the side of the transmission; behind the selector sensor, to see if you have wiring and what the wire colors are so I may trace them back to repair.

[Bobbie Martin]

You should have one sensor in the front of the transmission and one in the transfer case. The rear transmission sensor is only used in 2WD models on DS4 equipped trucks. The transfer case sensor takes the place of the rear transmission sensor. If the wires to the sensors are missing, you should be setting all sorts of codes.

[Daver94K2500]

This problem started intermittantly and has deteriorated with time; over a year to get to the point the cruise quits after about 20 miles. I get code "9" from the RWAL almost everytime I get into the truck now. According to the parts manuals the truck possesses two sensors so I bought them both not knowing which could be the culprit. The transfer case sensor was the easiest to get to and change so I did it first. I had difficulty locating the other; has a bracket clamped onto the sensor sold as a unit. Once I discovered it's location; on the side of the transmission above the left rear corner of the transmission drain pan, I noticed there was NO wire plugged into the sensor. Upon clser inspection, it appears there has been NO wire connected for some time due to road grime build up inside the sensor connector. I considered that the drive shaft may have removed the connector tail from the harness but could find no signs that it had been jerked through a loom as everything appears untouched. Outside of the cruise dropping out and the code "9" everything works; the speedo never fails, the torque converter clutch locks, etc. Everything will restore to normal after stopping and restarting the truck, at least for another 20 miles. My logic tells me that if my problems were related to the sensor wire being ripped out that these would have manifested themselves instantly and not slowly degraded over the course of a year. I am going to get the truck into the air and peel back the looms to see if I can find loose wire but with out the wire colors and how far back the wire could have been stripped it is going to be a shot in the dark.

[a5150nut]

If you have the manual there is a test procedure to check the entire speed sensor system. I had to do this on my 94. Turned out it is the Drac or the little computer in the dash. Its on the left side of the glove box opening with the box removed. Spendy, and dealer only item.

Do all the checks in order in the book to find the problem. Might even have some info to find your missing wires. Mine only has the sender at the rear of the transfer case. Was yur truck origanly a stick?

[Daver94K2500]

Update:

After getting the truck in the air and removing the forward driveshaft, I discovered that there are TWO identical sensors on the side of the transmission; one directly above the range selector shaft and the other above the left rear corner of the drain pan. The sensor above the selector shaft has a wire connected to it and I replaced it. I can only assume the second sensor is used to plug a sensor location used by a differently equipped vehicle. I haven't had it repaired long enough to determine if this will end my problem but will post an update as soon as I can. The truck is rock stock with no modifications beyond the remotely mounted FSD cooler.

[Bobbie Martin]

Originally posted by Dave Roland:
Update:

I can only assume the second sensor is used to plug a sensor location used by a differently equipped vehicle. This sensor is used for 2WD trucks as the transfer case sensor does the same thing in your truck. Since you need the sensor in the TC to give the correct speedometer reading in low range, my guess is GM programmed the PCM to compensate for the speed difference in low range and eliminated the trans sensor. You will find in mechanically injected diesel engine trucks with 4X4 & 4L80E transmissions, all 3 sensors were used. Is this trans the original or has it been replaced? Most have a plug where the rear (trans) sensor is. IF the reluctor ring is installed for the rear sensor, it may be very useful should you ever want to convert to a DB2 pump.

[Daver94K2500]

Yes, the 4L80E was replaced by the dealer with a GM reman unit at 114K before I bought it. Since then, I am the only wrench it has had. The cruise held up on a road trip to Austin yesterday but I still got a code "9" near the end of the return. When did GM switch from the DRAC to the VSSB and where is the VSSB located on the '94?

[Bobbie Martin]

That makes more sense. My guess is the reman trans has every sensor so it can be used in most any application. Not sure when the switch to VSSB was. It should be behind the glove box by the PMD. Are there any DTC codes stored?

[Daver94K2500]

There were no codes in memory for the ABS but I retreived 39 and 68 from the engine codes. According to out DTC tables on TDP, these indicate a "stuck off" torque converter clutch and a slipping componant in the transmission. However, when I referenced the code tables in the SunPro code scanner manual, these codes indicate that 39 is a TCC circuit fault (similar) and 68 is one of three possible conditions: 1) Cruise control circuit problems, 2) air conditioner clutch circuit shorted, or 3) Transmission overdrive ratio error; engine RPM greater than input speed, which is similar to TDP code translation. Which brings me to my replacement alternator: since the alternator is the source of RPM for the tach and potentially other systems, it reads about 300-400 RPM's faster than the original which tells me that it is not the right alternator for my truck as equipped, but I haven't been able to get the parts store from which it was purchased to change it out. So as a result of this maybe the code 68 is related to the alternator signal not matching other sensor input.

[JohnC]

The TCC code can cause the slipping code. Deal with it first. The Alternator has no bearing on the speeds sensed by the PCM, so ignore it for now, too.

[Daver94K2500]

I have learned that if holding the throttle at a steady pace and slightly touching the brake pedal will release the TCC and you will see an increase in RPM. It has never dailed to pass that test and I am concerned as to how to determine a failure of this nature without actually finding something wrong.

[Daver94K2500]

Update:

I have now verified that the TCC is slipping and am in the process of finding alternate transportation before the rest of the transmission fails. This will not fix the cruise control problem as I believe I have narrowed it down to a tired VSSB ($400, dealer only item). It is old and has lots of miles on it of which have been relatively trouble free. This will be my first big ticket expense in operating this truck. It is time to pay the piper.

[JohnC]

Originally posted by Dave Roland:
Update:

I have now verified that the TCC is slipping and am in the process of finding alternate transportation before the rest of the transmission fails. How did you make this determination? (Just curious)


This will not fix the cruise control problem as I believe I have narrowed it down to a tired VSSB ($400, dealer only item). This would surprise me, but maybe it's the case. On the other hand, if the output shaft speed sensor was flakey, wouldn't that affect both the TCC lockup AND the cruise? Anyhow, if you need one, I've got a used one you can have for a lot less than $400...

[Daver94K2500]

As stated in my Aug 26 posting, the Torque Converter Clutch is the last item engaged by the transmission and at a steady highway speed you can lightly touch the brake pedal (brake signal is sent to the ECM to disengage the TCClutch) and the engine RPM will increase demonstrating the release of the TCC. Recently I took a road trip and watched the RPM increase while climbing on a steady hill at highway speed (increased torque demand on the clutch pack) and after depressing the brake pedal discovered no increase in RPM indicating the clutch pack was slipping and the torque converter was driving the truck without benefit of lockup. On the level part of the road (decreased torque demand as compared to going uphill) the RPM would increase if the brake pedal were touched (indicating that under a lower demand the TCC clutch packs were able to hold). This is a trick I learned from a buddy that worked on GM transmissions at a local GM dealer. As for the item you offered for sale less than $400, are you referring to a transmission or the VSSB? Also, I have already replaced both speed sensors and the cruise issue was not resolved.
Tks, DR.

[JohnC]

Originally posted by Dave Roland:
As stated in my Aug 26 posting, the Torque Converter Clutch is the last item engaged by the transmission Maybe, maybe not. the TCC lockup is a function of road speed and throttle position. It can lock up in third if you are going fast enough, and then again in 4th (OD) after the shift.


Recently I took a road trip and watched the RPM increase while climbing on a steady hill at highway speed (increased torque demand on the clutch pack) and after depressing the brake pedal discovered no increase in RPM indicating the clutch pack was slipping and the torque converter was driving the truck without benefit of lockup.Again, maybe, maybe not. Under some circumstances the TCC will unlock under load. On the other hand, if it is doing what you say, it will (definitely) set one or more DTC's.


As for the item you offered for sale less than $400, are you referring to a transmission or the VSSB?I have an extra VSSB.

On further review, I see you do have codes indicating slip. Once the TCC slips, the PCM unlocks it, which could explain what you're seeing. Also, I'll bet that the PCM disables cruise if it's slipping, too. Is the trans still on warranty?

[ 09-09-2005, 06:08 AM: Message edited by: JohnC ]

[Daver94K2500]

No chance of warranty at this age and miles. The original trans was replaced by the previous owner at about 100K.