Transmission not getting to temperature

[wilbelectric]

I have a 21 JTRD 26,000 miles. I have noticed my fuel ecconomy getting worse than normal and the trans shifting harder than normal well after the truck is warmed up. Even for winter temps and fuel. So, I was monitoring my temperatures today while on a 25 minute trip or so around town. Approx. 30 degrees F outside. Engine and oil hovering around 200 degrees after a few minutes of normal driving. However my trans temp didn't get above 135 for the entire trip. This should be around 180-210 normal operating temps at least from what I've normally seen.

So the bad fuel ecconomy kind of makes sense because the trans will upshift later when its cold. I'm guessing some sort of temp sensor in the trans is bad but wanted to post on here to see if anybody had any kind of similar issues. There's no lights on the dash. I really don't want to take it to the dealer. I have a banks tuner, taser, lift, 37's. I'm sure they will try to blame me for modding it.

Any help on diagonsing this would be much appreciated!

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[WesternJeep401]

The transmission utilizes a thermostatic bypass valve mounted in series with the transmission cooler lines. It is possible that it has stuck open or partially open.

 

[CrazyCooter]

25 mins really isn't enough time to fully warm the trans up?

 

[Hootbro]

My 2 cents, given the ambient weather temps and time driving you stated, that sounds about right.

There is a thermostatically controlled transmission heat exchanger that opens up around 140 degrees trans fluid temp and allows full flow of fluid through cooler. Below that, the trans fluid is in a smaller capacity closed loop.

@WesternJeep401 has a point that it may have a stuck thermostat but I would suggest driving it longer than 25 minutes. In as cold or colder outside temps, I have observed my transmission temp take at least 45 minutes to get into the 140+ degree range.

I will also ad that I have yet to find a dedicated transmission fluid temp sensor in any diagram. It does not show up with diagnostic scanners as a active PID with generic OBD2 data but does when the scan tool is in manufacture selected data. My speculation is that any transmission fluid temp displayed on the cluster is an algorithmic estimation taken from other data like the engine coolant and/or engine oil temp to estimate what the transmission temp is.

 

[NCJL]

The transmission has a very good cooler set up. With temperatures around freezing mine takes what seems like forever to get warmed up. 20 plus miles on the highway with some grades to pull. Haven’t noticed shifting differences or lower fuel mileage.
Oil and coolant temps are the same/very similar regardless of ambient temperature.

 

[Lunentucker]

It's 32 here now.
As soon as I finish this Scotch I'll go out and test mine to compare (JK)

 

[ShadowsPapa]

25 mins really isn't enough time to fully warm the trans up?

Naw, I can drive quite a bit when it's 30 out and not get the transmission up to full heat.

 

[ShadowsPapa]

My 2 cents, given the ambient weather temps and time driving you stated, that sounds about right.

There is a thermostatically controlled transmission heat exchanger that opens up around 140 degrees trans fluid temp and allows full flow of fluid through cooler. Below that, the trans fluid is in a smaller capacity closed loop.

@WesternJeep401 has a point that it may have a stuck thermostat but I would suggest driving it longer than 25 minutes. In as cold or colder outside temps, I have observed my transmission temp take at least 45 minutes to get into the 140+ degree range.

I will also ad that I have yet to find a dedicated transmission fluid temp sensor in any diagram. It does not show up with diagnostic scanners as a active PID with generic OBD2 data but does when the scan tool is in manufacture selected data. My speculation is that any transmission fluid temp displayed on the cluster is an algorithmic estimation taken from other data like the engine coolant and/or engine oil temp to estimate what the transmission temp is.

Could it sense in the "pan" ? Often engine oil temps are taken there and that means cooler than other oil.
At least some are taken from the sump.

 

[IPRoutes]

I have a 21 JTRD 26,000 miles. I have noticed my fuel ecconomy getting worse than normal and the trans shifting harder than normal well after the truck is warmed up. Even for winter temps and fuel. So, I was monitoring my temperatures today while on a 25 minute trip or so around town. Approx. 30 degrees F outside. Engine and oil hovering around 200 degrees after a few minutes of normal driving. However my trans temp didn't get above 135 for the entire trip. This should be around 180-210 normal operating temps at least from what I've normally seen.

So the bad fuel ecconomy kind of makes sense because the trans will upshift later when its cold. I'm guessing some sort of temp sensor in the trans is bad but wanted to post on here to see if anybody had any kind of similar issues. There's no lights on the dash. I really don't want to take it to the dealer. I have a banks tuner, taser, lift, 37's. I'm sure they will try to blame me for modding it.

Any help on diagonsing this would be much appreciated!

Tried uphill or in stop n go traffic? But I think its pretty normal On my previous automatic, in regular traffic or on flat highway driving, the load on the transmission is pretty low and temps take a long time to reach near 200F. Long after engine and oil reach optimum temps.

 

[ShadowsPapa]

It's 32 here now.
As soon as I finish this Scotch I'll go out and test mine to compare (JK)

Keep your toes out.

 

[Hootbro]

Could it sense in the "pan" ? Often engine oil temps are taken there and that means cooler than other oil.
At least some are taken from the sump.

Maybe, might be part of the TCM which is in the valve body. Description and operation tech data for the transmission is vague and just refers to the scan tool to monitor fluid temp.

 

[Hootbro]

Just realized this is in the Diesel section so my previous posts may not be applicable for the 8HP75 transmission but I would suspect them to be pretty similar.

 

[Almost]

I think you need to drive longer. Mine takes awhile to get to temp when it’s cold out.

 

[Rusty PW]

I think you need to drive longer. Mine takes awhile to get to temp when it’s cold out.

Mine too.

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