Abnormally low transmission temp

[hear]

97 6cyl, factory radiator.
32RH with temp gauge & aux cooler in front of the radiator, plumbed after the OEM trans cooler.
Been running this setup for several years.

It's been cold in TX this week, but last night was in the 50's and I still saw this odd behavior so it's time for a thread. First, my engine has always run with the temp needle on the "1" in 210. Off OBDII it's always between 210-212F. Transmission temp fluxuates a little more based on the outside temp, but it's always between 140-160F, never over 170, but also never has any problem warming up.

Starting this week it is not really even getting to the "2" in 210, but I don't have my reader handy to get the actual temp. The gauge is reading noticeably cooler and the heater seems to work fine. Sure it could be a gauge problem. But at the same time my transmission temp is staying very cool. I've driven multiple 30 minute stints this week, including letting it just idle for 15 minutes after a decent length drive. The gauge won't read below 100F, but for the first 10 min it won't even move. I have yet to drive it long enough to get it over 115F. And when it did get to 115F, the act of driving it cooled it back down to 110F. Sure it could be the transmission temp gauge.

So either both of my gauges went weird at the same time, or I have a different cooling problem. The jeep running cooler could just be a stuck-open t-stat, easy enough to fix. I replaced it with a 195 when I got it back in 2020 and I'm sure I used a cheap part so maybe it failed. But the trans temp is weird. I've driven in colder temps in prior winters and while the trans ran cool, it was still in the 130-140F range. This is just really really really cool for a thing that normally runs 60 degrees warmer.

It's too cold for me to work (I'm soft, I won't apologize) so it'll be a couple weeks before I replace the thermostat. But there is no reason why that would affect the trans temp in such a huge way.

 

[GregBelleville]

I've got just the guy to help you out, he's an expert on this transmission and he's even willing to travel to you (on your dime) to help fix it. I'm sure he'll be along shortly!

 

[KennethS]

The jeep running cooler could just be a stuck-open t-stat, easy enough to fix. I replaced it with a 195 when I got it back in 2020 and I'm sure I used a cheap part so maybe it failed. But the trans temp is weird. I've driven in colder temps in prior winters and while the trans ran cool, it was still in the 130-140F range. This is just really really really cool for a thing that normally runs 60 degrees warmer.

It makes sense to me.

With your external transmission cooler plumbed AFTER the in-radiator cooler, the lower temperature engine coolant is not heating the transmission fluid as much as typical, so the external cooler has lower temp fluid coming in, and is cooling the fluid to a lower temperature than typical.

 

[hear]

I've got just the guy to help you out, he's an expert on this transmission and he's even willing to travel to you (on your dime) to help fix it. I'm sure he'll be along shortly!

lol, this is cooling related, that guy doesn't know shiz from shinola 'bout cooling.

 

[hear]

It makes sense to me.

With your external transmission cooler plumbed AFTER the in-radiator cooler, the lower temperature engine coolant is not heating the transmission fluid as much as typical, so the external cooler has lower temp fluid coming in, and is cooling the fluid to a lower temperature than typical.

Yeah, except that the engine is fairly close to operating temperature after 30 minutes, so I would expect it to behave closer to normal once it reaches that point. I get why the heat exchange doesn't work when the engine coolant is cold, but once it gets to 190 or so (educated guess) it should start bringing the trans temp up, at least somewhat. And then is it not odd that even on a 50F day the engine never gets up to normal operating temp? Even if i just let it idle so there is no 60mph wind blowing through the radiator it still never really gets any higher. A normal running 6cyl wants to be around 210F, the t-stat just helps it get there quicker. A stuck-open t-stat wont make it run cool, or else we've given every "I was overheating so I installed a 180F" bad advice.

 

[rouxbicon]

Yeah, except that the engine is fairly close to operating temperature after 30 minutes, so I would expect it to behave closer to normal once it reaches that point. I get why the heat exchange doesn't work when the engine coolant is cold, but once it gets to 190 or so (educated guess) it should start bringing the trans temp up, at least somewhat. And then is it not odd that even on a 50F day the engine never gets up to normal operating temp? Even if i just let it idle so there is no 60mph wind blowing through the radiator it still never really gets any higher. A normal running 6cyl wants to be around 210F, the t-stat just helps it get there quicker. A stuck-open t-stat wont make it run cool, or else we've given every "I was overheating so I installed a 180F" bad advice.

A stuck-open thermostat will absolutely make the engine run too cool, except perhaps in extreme conditions. At any "normal" ambient temperature (lets leave normal undefined for this conversation) a stuck-open thermostat or a 160 or 180 rated thermostat will cause the engine to fail to reach proper operating temperature, assuming the cooling system is otherwise in good condition.

And the transmission cooler is in the lower radiator tank, i.e. the "cool" tank. The coolant temperature in the lower tank will be a good bit cooler than the coolant temp in the upper tank or thermostat housing where the temp sensor is.

 

[macleanflood]

It's been 27 degrees here in Fall Creek and I'm a needle width or two south of 210...just a touch lower.

I'm watching my oil temperatures and it takes a long time to bring the oil up to temperature...and with my oil heater (not even bothering to call it a cooler any more) dragging down the coolant temperature it takes another 20 minutes to get the oil up to temp.

You could bypass your extra cooler or add additional temperature sensors to try and isolate the issue.

-Mac

 

[KennethS]

A stuck-open t-stat wont make it run cool

Yes, it definitely will. I removed the thermostat when doing a cooling system flush and the temperature gauge stayed way down to the left while driving around town.

On my "other" vehicle that has a mechanical temperature gauge, I once removed the thermostat and drove around in hot summer temps with the AC on and the coolant stayed around 130F.

The cooling system HAS to have more cooling capability than the engine has heat generating capability, and the thermostat is used to maintain the desired operating temperature under varying (and often times adverse) conditions. Otherwise, severe overheating would occur on a regular basis.

 

[hear]

There are probably a thousand threads here where a dude put in a lower value t-stat to solve an overheating problem and people can't jump on him fast enough.

I guess I'm only as good as the threads I've read, but your post seems counter to the conventional wisdom, for whatever that's worth.

 

[mrblaine]

There are probably a thousand threads here where a dude put in a lower value t-stat to solve an overheating problem and people can't jump on him fast enough.

I guess I'm only as good as the threads I've read, but your post seems counter to the conventional wisdom, for whatever that's worth.

You have to understand the basics of the cooling system starting with how wrong your statement (it wants to run at 210) is. No, it doesn't want to run at any temp. It is made to run at 210 by the thermostat. The radiator has excess cooling capacity by a factor of about 1.5 to 2, or roughly twice as much as it needs to cool the engine under all high demand situations. If you pull the thermostat and let it work on just the radiator, it will way overcool below normal operating temps. The way that is prevented is when it gets below the thermostat "set point", the thermostat starts restricting flow through the radiator.

 

[KennethS]

There are probably a thousand threads here where a dude put in a lower value t-stat to solve an overheating problem and people can't jump on him fast enough.

I guess I'm only as good as the threads I've read, but your post seems counter to the conventional wisdom, for whatever that's worth.

Are you referring to my post? I had a bit of an ongoing “discussion” with a fella on the AZVJC site about coolant temperatures with no thermostat. He was adamant that no thermostat = overheating, while I maintained my position (from actual experience) as posted above.

For anyone who really wants to know, it’s easy to remove the thermostat and see for yourself.

 

[KennethS]

Follow up: I was thinking about it, and if there is an overheating issue, a lower temperature thermostat will only delay the overheating event (assuming it was not due to a stuck thermostat).

On my “other” vehicle, I once replaced the radiator and had running hot issues. Removing the thermostat and it just took longer to get to the elevated temperatures. Replaced that radiator and resolved the issue. Having more cooling capability than the engine’s heating capability is key.