No torque converter lock up

[Hovenator]

Need a little help isolating the cause of this problem. On my usual trip home from work the other night my T/C did not lock up when on the highway as it usually does.

I have no CEL light or flashing O/D light. So it seems that as far as the computer is concerned everything is fine.

After driving it today I did notice that the RPM's seemed a little different. Also the idle seems a little low; around the 400 mark.

I know that the TPS has an input to the ECM concerning lock up. As far as I've read it will cause the T/C to unlock if it thinks that the throttle is fully closed.

Would it be possible for the TPS to be going bad but not trip a code yet?

Any experience or advice would be welcomed!

 

[BlackLS1]

if the tps is out of adjustment maybe?

 

[Hovenator]

I'll have to look up how to adjust it and give that a try before buying any parts. If I had a code at least then I'd have a starting point for this problem!

 

[rove_mcanus]

Get the FSM and a multimeter. There are ways of testing all the bits to find the problem.

It could be the TPS, a problem in the brake circuit, the lockup solenoid, valve body problem, or the lockup mechanism itself.

 

[Sarmajor]

It could also be the transmission temperature sensor. From memory the torque converter will not lock up until the transmission is up to 60 degrees C.

 

[Hovenator]

I have the FSM and have been reading as much as I can on the various interfacing systems. The fact that I don't have a CEL or a flashing O/D for the trans is making me wonder if it is actually something "unmonitored"

As for the tranny temp. I removed the sensor from the tranny about 2 years ago when I put the temp sender in for my pillar gauge. The sensor has just lived at the end of the connector zap strapped out of the way.

As far as I know the sensor is only there to put the "idiot" light on when you overheat your tranny. Others on this forum have done this mod and not had an issue afterward. Mines been good for 2 years!

I haven't read anywhere in the FSM that the tranny temp sensor has an input on the lock up sequence.

I know the engine coolant sensor does determine when the tranny is able to go into O/D based on temp.

 

[Hovenator]

Well, I've check my truck for codes using a friends reader. No faults and no impending faults.

So on the way home I tried a little test while on the highway. Once I was up to speed I kept my foot steady on the throttle and then I pushed the brake pedal a little. This made the RPM jump up by about 200... hence a T/C un locking

I think something screwy is going on with my tach. It reads low at idle by about 200 and high at cruise by about 200. So the normal point where I was seeing the T/C lock up before is now out by about 200 RPM.

I'll have to drive the truck some more to see if my theory proves out. I sure hope it does!

If this proves out to be all it is I will post that. I hate threads that end unsolved!

 

[Hovenator]

Conclusion: After driving several more times all is well except the tach is "off" occasionally.

I can un-lock the converter by pressing the brake pedal while holding the throttle steady. If the tach is not acting up then everything happens at the right RPM.

It's funny how well you get to know a machine that you use every day! The slightest thing different and the alarm bells go off