I have an '88' pickup with an efi 22r a/t. after warm up the o/d would
kick in. Now o/d doesn't kick in. I changed fluid, replaced thermostat
and still no o/d. The on/off indicator light works but no o/d. I also
checked all fuses and all good. I've heard a number of possible
causes. Anybody have any info on most common and likely cause, it
would be greatly appreciated.
The ECU delays shifts and prevents shifting into overdrive when the engine
is cold to make the engine run at higher RPM. Running the engine at higher
RPM warms the engine up more quickly so the catalytic converter and O2
sensors reach operating temperature (and go into closed loop mode ) more
quickly, lowering emissions more quickly.
The ECU uses voltage from the coolant temperature sensor to determine
whether the engine is cold or hot. If the coolant temp sensor is bad or
disconnected, no voltage reaches the ECU. The first generation of OBD did
not have a way to monitor the function of the coolant temp sensor so it
wouldn't know if the sensor was working or not.
It is possible that the OP's transmission has internal problems, but the
question was what the most common cause was, which is a problem with the
engine coolant temp sensor and related circuit.
I probably should have added the second most likely cause, which is a bad
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.