Stock 1992 c1500 700r4 TCC lockup conditions? (Fixed)

[OleGreen93]

I bought my 92 c1500 with 100k miles last month in early September. It has the 700r4 transmission and shifted perfectly while it was still warm last month. Now that outside temps are cooling down I've noticed that it takes around 10-15 minutes of driving for the TCC lockup to occur at speeds 45+ mph. When the TCC lockup does occur, it will stay locked for a little while and then unlock without anything changing such as road conditions or throttle position. It then takes a little while before it locks again. I swapped the brake light switch from my other gmt400 and nothing changed.

My main question is what are the conditions for the truck to engage the TCC lockup at highway speeds? Does engine temperature have an effect on TCC lockup? The reason I ask is because after my drive to work this morning in mid 50s weather which is only about 4 miles but truck was running for around 25 minutes, the temp gauge in the cluster is barely showing over 100 degrees. I brought my digital thermometer with me and the temp switch in the driver side cylinder head was showing around 165 degrees. The temp sensor in the intake for the computer was around 145 degrees. The thermostat housing was also at 145 degrees and upper radiator hose was at about 122 degrees.

My concern is that maybe this truck has a lower rated thermostat installed and if the TCC lockup is tied to engine temps that this might be why it is taking the TCC lockup longer to engage and not staying engaged at highway speeds. The fluid is at the correct level and looks to be in good shape although i do not know if the transmission has been serviced before.

 

[PlayingWithTBI]

There are a lot of variables for TCC, one of which is min CTS. Here's a screen shot of my 1227747 ECU with AMUS (not stock) bin. Others include min speed and %TPS position (the bottom 4 tables upper and lower limits). It sounds like you're running too cold, you need a 195* T-Stat. If yours is anything like mine your CTS in the head will always read hotter than the one next to the T-Stat (which feeds the ECM). I'd change it out and start from there.

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[OleGreen93]

There are a lot of variables for TCC, one of which is min CTS. Here's a screen shot of my 1227747 ECU with AMUS (not stock) bin. Others include min speed and %TPS position (the bottom 4 tables upper and lower limits). It sounds like you're running too cold, you need a 195* T-Stat. If yours is anything like mine your CTS in the head will always read hotter than the one next to the T-Stat (which feeds the ECM). I'd change it out and start from there.

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Thanks for this information. I think i'm running too cold as well. Outside temps now are in the mid 60s so when i head out for lunch in a bit ill take some more temp readings with my thermometer when i get back and see if there is a noticeable difference.

 

[OleGreen93]

Quick update-

Drove the truck on a 10 mile round trip to get a new 195* thermostat and only on the way back to work did it finally lock the TCC. Stayed locked as it should the whole way back driving 45 mph. Got back to work and checked temps on the various sensors with my digital thermometer:

Coolant sensor in cylinder head: 185*
Coolant sensor in intake manifold: 160*
Thermostat housing: 160*
Upper radiator hose: 145*
Lower radiator hose: 122*

I'm leaning to the thought that the truck must have a 160* or 180* thermostat in it or no thermostat at all. I'll be replacing the thermostat tonight once I get home.

 

[L31MaxExpress]

The TCC locks well under the coolant temps you are running at. No more than 100°F on any GM I have seen. My guess is that your lockup solenoid is going out.

 

[OleGreen93]

There are a lot of variables for TCC, one of which is min CTS. Here's a screen shot of my 1227747 ECU with AMUS (not stock) bin. Others include min speed and %TPS position (the bottom 4 tables upper and lower limits). It sounds like you're running too cold, you need a 195* T-Stat. If yours is anything like mine your CTS in the head will always read hotter than the one next to the T-Stat (which feeds the ECM). I'd change it out and start from there.

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You were absolutely right with the thermostat being the issue. I changed it this weekend. Turned out the truck had a 195* acdelco thermostat installed but it looked like it was pretty old and the spring pressure felt weak when opening it by hand compared to the new thermostat. My guess is that it was opening too early and not allowing the engine to get to full operating temp. With the new thermostat installed my temp gauge reads correctly now and the torque converter locks like it should when driving. Thanks so much for the help

 

[PlayingWithTBI]

Good to hear!