Trans in limp mode, rear trans speed sensor?

[j_cuz]

I've been looking around for a while, and I'm trying to diagnose the problems on a truck I just picked up. It is a 94 Chevy 2500 4x4, with the 6.5TD and the 4L80E trans, with about 300k miles on it. There are no check engine lights, but if I jumper the connector it gives code 74. As far as I can tell, this puts the transmission into limp mode, and gives the error as "transmission input sensor circuit". There are 2 transmission sensors that are identical... one to the rear and one to the front of the transmission, like are seen in this picture:




The left (front) one is plugged in with a blue/white and a gray/red set of wires. The right (rear) one has nothing going to it... I assume this is my problem. I tore all the sleeves off the wiring near the transmission and can't find where the plug should have been going to. The entire wiring harness side of the sensor is missing (plug, wires, anything). Can someone help me out? A link to somewhere that would tell me where this sensor leads to or anything would help. Also, I have replaced the front speed senor and made sure that it is plugged in, and disconnected the batteries afterwards to clear the codes. Any help or suggestions would be appreciated. Let me know if you need more information, pictures, or anything.


PS... Tachometer has no reading to it, but speedometer works. Top speed of truck is ~25 mph.
Thanks,
Jason

[GenBiltstein]

You will find it on your transfer. You were close on guessing where it could be. Some transmissions will have the Vehicle speed sensor where you mention. 4X4s will have them on the transfer.

[j_cuz]

I know the vehicle speed sensor is on the transfer case, but what is the sensor on the rear of the transmission? Is that for a 2wd option and not wired up in 4wd? Would that mean that the error I'm getting could be the VSS and not the trans input sensor?

[GenBiltstein]

The sensor on the left of the transmission is for the transmission input speed. (TISS) for short.
The sensor on the right is referred to as the transmission output speed sensor. (TOSS)
If your computer is not configured to run on that TOSS then you must run the VSS on the transfer.
The sensor on the left is transmission input speed.

If you have the input speed wiring hooked up to the output speed sensor then the computer will definitely think something is very amiss and throw you into limp mode.

[j_cuz]

There are 2 sensors on the left side of my transmission, like in the picture. Only the one towards the front is hooked up. The rear exists, but isn't wired into the harness.

[GenBiltstein]

What type of vehicle are you driving? List the type.

[j_cuz]

I listed it in the first post... 94 K2500 with the 6.5TD. I can get some pictures of the sensors tomorrow, but don't have any now.

[GenBiltstein]

It seems to me that the transmission you have may not be original. Most of the time the transmissions output speed sensors will be blocked off.
you should have a vehicle speed sensor located in the transfer.
Back to your original posting.
My apologies for not reading it thoroughly. The input speed sensor might be going out on you what can be the case Dirty with metal filings.
Take it out and inspect the transmission input speed sensor (Tiss).
You can elect to interchange it with the transmission output speed sensor (Toss) since it is not used. Or clean it with a white rag and alchohol and try again.
Both are the same part and perform the same function.

[j_cuz]

I replaced both sensors (TISS and VSS) and it went away. I think I have 2 unrelated problems, next step is posted here: Engine won't rev, top speed 25, checklist enclosed.

[GenBiltstein]

I am glad to see you were able to solve the problem. The eyes and ears of the computer were kinda blind. Input speed is needed to tell the computer how fast the rpms in the transmission. Vehicle speed sensor tells the computer how fast or slow the truck is moving down the road.
As far as the other posting on why the truck is not revving. While it's true you might have excess backup pressure go ahead and check your fuel return at the fuel return line at the injection pump for volume. Fuel filter restriction can pose a problem. Fuel restriction returning to the tank can also restrict power. this is a simple test that is frequently overlooked. Those injectors need to recirculate fuel continously freely and when fuel return is restricted there is loss of power.