Auto Parts: for a friend, vehicle speed sensor, th thm


Question
he has a 1987 gmc 1500 2wd pickup it has 6000 original miles he was driving one day and he just lost drive and overdrive i got under the truck to look for the vehicle speed sensor which is supposed to be located near the tailshaft section of the trans its not their so i started looking around and noticed a transmission tag it said hydramatic their is supposed to be a 700r4 in the truck i was wondering if u had any idea as to what could cause this or if their was anyway to run the numbers i found on the transmission tag to determine what is actually in the truck please help cause im mechanically inclined but dont know much about transmissions thank you

Answer
Hi Nick,

A TH700r4 is a hydramatic transmission also (TH/THM= Turbo Hydra-Matic). That only means that it uses standard ATF under pressure.

There were two transmissions put into that truck that were not ECM controlled, a TH400 and a TH700r4. The TH400 is a heavy duty 3-speed and the TH700r4 is a light duty 3-speed with overdrive. Your shifter should read: P-R-N-(O)-D-2-1.

Onto the issue. Unless your trans is ECM controlled, the speed sensor in that transmission should be mechanical and only operates the speedometer.

As for the no shift, if first, reverse and second gears are operating and shifting fine, I'd guess that the drive (third) band actuator is bad or stuck/sticking. If that is not the issue, check the valve body for blockages.

Thanks for the question, I hope this information was helpful to you.
Fred
Knucklebusters Used Auto & Truck Parts, Inc.
www.knucklebusters.us
570-453-0910
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