6L80 Heat Related Symptoms

Howdy,

Well now the weather is warming back up, I'm getting the same symptoms as I was before I fixed the missing check ball in my 6L80. Watching the temp in the DIC (scan tool showed within a degree last we checked) and it's hitting over 200ºF in 55mph surface streets with lights. Once it gets over 195º it starts to feel like the vehicle stops faster than the trans can downshift so it surges just before coming to a stop. It also has very lightly lurched into gear from a stop at over 200º.
I think the main culprit is something in this unknown reman transmission is binding up when it gets hot so if I can keep it cool it works fine but considering @NickTransmissions statements on DEX6, this should be well within operating range. I put a Zipkit (minus the pump bits) in after consulting with Sonnax already so I'm wondering if it's transmission hardparts or control complex related. I'm going to put a 40K on it before summer but that's definitely only a bandaid to keep it out of this territory. I might feel fine selling it like this but not towing with it.

Insights or Thoughts?

How long does it take to get the AT up to 200F?

Do you have a factory cooler, or any cooler installed?

Charlie207 said:

How long does it take to get the AT up to 200F?

Do you have a factory cooler, or any cooler installed?

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I was up to 200º within 45min of around town driving at ~85-90º ambient. I only have the factory radiator tank liquid to liquid cooler. The plan is to swap to OEM lines and a Tru-Cool 40K at some point but like I said, I'm quite worried about that just masking a deeper issue with heat generation that could leave me and the Scout trailer stranded.

I think 200F is normal, especially if the ambient outdoor temps are that high, after that amount of time.

My 6L80 always runs 90-100 degrees hotter than whatever the ambient temp is in the warmer months.
It has a smallish trans cooler - rpo code KNP, not really adequate for towing, but I don't tow so all is good.

Do you have factory (before tsb) transmission thermostat still in it? Lookup the tsb on it if you still do. And if you still do have the thermostat a bigger cooler won't do much it will just keep the temperature closer to the temperature the thermostat opens up at.

how old is the radiator? Just wondering if the radiator isn't dropping Temps well and you're getting 200deg on the tranny cooler side?

either way I don't believe 200deg shouldn't effect how the tranny shifts and stuff. but I'm no tranny guy. if a big tranny cooler keeps you from a 5-7k rebuild. mask away?

If I were in Texas I'd put an aux. cooler on the return line, and a good sized one. and do away with that thermo valve on the trans. On my 6l the highest temps I've seen are just above 180 in jams going (stopping) around Atlanta on a warm day.

No thermo valve on my 2013 and yes I have looked. Engine coolant temps are holding 210ºF with no variance. The trans temp worries me most as it will regularly get 15-20º hotter here by the depth of summer so if the 90-100º trend follows I'll be well into the hot end of DEX6 and the worst of the symptoms.
Best I know the radiator is original as it only has 117k miles (yes, it nuked the trans under prior ownership at less than 100k miles). A 40k cooler is in the cards for when I'm done rebuilding my friend's Nissan and can get my commuter car back and sideline the Yukon. The question becomes how deep does the hole go with rad, VB, TECHM, etc... to get this thing up to scratch for it's job or do I put an OEM HD cooler on it and trade this in for a 2007 2500 and skip the 6spd mess with a good old 4L80E.

15burban said:

Do you have factory (before tsb) transmission thermostat still in it? Lookup the tsb on it if you still do. And if you still do have the thermostat a bigger cooler won't do much it will just keep the temperature closer to the temperature the thermostat opens up at.

Click to expand...

GMT900s (up to 2014) with the 6L80 don't have the stupid thermostat in the transmission cooler lines. It was added in the 2015s and up.