I have had this issue for around 6 months or so.

 

I will start from the beginning...

 

Rebuilt transmission 1 year ago.

5-6 months later no lock up.

Tested alternator

Cleaned grounds

Monitored transmission via snap on scanner

Wired in lock up switch 

Added noise isolator

Tested continuity on lock up cable to PCM. 

Tested apps

Changed VB

Changed solenoids

Changed OSS 

Tested brake light switch

All came back good

Trans doesn't slip

TC when it was locking didnt slip at all

 

 

TC would lock up only if coasting whilst feathering throttle around 5% tps input. it would lock flick the lock up switch and it would stay locked

 

Flick the switch and coast it would eventually lock up and stay locked 

 

We had a our first frost friday night saturday TC wont lock at all no matter what.. 

Done the ATS apps re wire today made no difference other than throwing p0121 code tonight. I'm at a complete loss the only issue I can find is the 8 pin connector on the trans doesn't seat all the way down but this has been like this for the last 2 years no issue.. I have now ordered a connector to replace that.

 

Does anyone have any further suggestions? 

Anyone give me a list of problems that will cause a no lock up situation. 

Does this sound like the TC may be toast even though when it did lock it wouldn't slip at all?

Do I buy a standalone controller if so what one is recommended. 

 

 

Thank you

 

 

Swap the transmission relay in the PDC. Test pressures at ports I use an oil pressure gauge with extended hose and run it up in through the passenger door. If I recall the connector is kind of flimsy and the valve body has to be lowered to replace it. Check wires they often get damaged or pinched by the bell housing and the bolts / brackets / dipstick tube. You did not mention if the overdrive works. That too shares some common wires. 

 

Rare but could happen, the flow through the torque converter reverses when locked. If the cooling lines or check valve at the heat exchanger clog that can cause lots of problems. 

 

Internally the torque converter. Sticky leaky valves in the valve body, incorrect input shaft alignment with the oil ports caused by incorrect parts stack-up shims, bushings end play measurement.

Edited January 20, 20205 yr by Great work!

Had same problem last summer.  No lockup except coasting down hills with mystery switch on.  Transmission ran warmer when towing on the flats by 10-20°F and over heated, up to 280°F going up hills.  Did all the tests and replaced relay and solenoids the same as you have.  Replacing the torque convertor, with a Dynamic overhaul, fixed the problem.    

PCM? Wiring between PCM and Trans? Wire connector on trans? Other than that I'd say the converter itself.

 

7 hours ago, Ronniemx3 said:

TC would lock up only if coasting whilst feathering throttle around 5% tps input. it would lock flick the lock up switch and it would stay locked

 

Flick the switch and coast it would eventually lock up and stay locked 

 

This makes me think it's the converter.

@kzimmer I've checked wiring between trans and PCM. PCM is commanding LU according to scanner... yes someone has posted similar symptoms above to me and it turned out to be converter. I was never happy with the converter when it was delivered to the UK looked pretty beat up. Looks like a 2k bill coming up ? 

 

@IBMobile would your converter lock up occasionally without the switch? ive just exhausted pretty much everything I can think of now. Only issue I can see is trans connector not seating fully but it has been like this for around 2 years and I never had this problem previously. 

2 hours ago, Ronniemx3 said:

@IBMobile would your converter lock up occasionally without the switch?

No, only going down hill with the switch.

 

Ohm test between pin 87 of the transmission control relay in the PDC and the orang /black wire at the middle connector ( connector C2, pin 11) at the PDC.  Should have around 80 ohms, the amount for the winding of the solenoid.  If you have that reading then the connection at the connector is good and so is the solenoid. 

Changed the converter, LU is perfect again! Had to go with a DPC converter through firepunk as no one could match the shipping price.. 310USD was delivered in a week to the UK ????