Transmission Thermostat Question

[GoToGuy]

You sound like your doing alot of guessing on your actual weights. Your concern on transmission life is valid. Yet without knowing what your actual weights are is just wishfull thinking. Go to scales get your tow vehicles and trailer weight, then go back with just your tow vehicle. And get the weight of what you got on the trailer. You may be shocked at how quickly it all adds up. Those numbers on glove box are real. Your H2 is built on an HD chassis. Are you sure about 4L65, not a 4L80 or 85 ? Thats what the RPO code is? Unless it's 100 % synthetic, regular mineral oil trans fluid begins to breakdown at and over 210 to 220 degrees F. What does the Owners handbook say about trans temp gauge monitoring when towing? Good luck.

 

[Curt]

You sound like your doing alot of guessing on your actual weights. Your concern on transmission life is valid. Yet without knowing what your actual weights are is just wishfull thinking. Go to scales get your tow vehicles and trailer weight, then go back with just your tow vehicle. And get the weight of what you got on the trailer. You may be shocked at how quickly it all adds up. Those numbers on glove box are real. Your H2 is built on an HD chassis. Are you sure about 4L65, not a 4L80 or 85 ? Thats what the RPO code is? Unless it's 100 % synthetic, regular mineral oil trans fluid begins to breakdown at and over 210 to 220 degrees F. What does the Owners handbook say about trans temp gauge monitoring when towing? Good luck.

Guessing? No. Estimating? Sure. My numbers are based off the added equipment on the H2 paired with the weight my ski boat + trailer says on it. The truck is an overestimation and the trailer, while knowing the exact weight, I'm rounding it up to 5000lbs which is 430 lbs more than I should be counting. Better to be overprepared. I mentioned 4L65e previously, but I stand corrected. It is equipped with a 4L60e, I got the transmission mixed up with the one in my truck. How I know this? While the RPO codes in the glovebox would confirm the transmission, the pan shape is the more accurate way to tell. And yes, it is a 60e/65e pan. Either way you skin it, they're not the best transmission.

Funny part about the owners manual is it makes zero mention about transmission temp monitoring while towing. It's like the people writing the book completely forgot the gauge was there. On top of that it also states that when driving on grades it is OK to leave the truck in D, which is just laughable in my opinion.
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In general I'm noticing we're getting off-track from the original question. This thread wasn't to start a discussion on transmission coolers or the truck set-up. I appreciate the feedback on both but I was just curious to know if a cold side bypass is necessary if I am retaining the OEM transmission cooler routing (through the rad, to the ext. cooler, back to the trans). To give a little background, the truck will see occasional (occasional as in maybe 1-2 months a year) sub 30*F temperatures and I just want to ensure the transmission would properly heat up to 180*F without the bypass. I appreciate the feedback from @Frank Enstein and @Schurkey regarding this matter. Any other feedback on the cold bypass is welcome.

 

[Pinger]

Any other feedback on the cold bypass is welcome.

My set-up on a C2500 Suburban with the 4L 80E uses a cold bypass thermostat and no saddle cooler (coolant rad cooler) and my typical ambient temps are 14 to 80F.

My saddle cooler was bypassed before I got the truck. I didn't recommission it because, contrary to popular opinion I don't see it contributes much to warming the trans fluid from cold because there is no coolant flowing until the coolant thermostat opens. Until then, the trans fluid is heating the coolant. And, I don't see the slight temp difference when the engine is running at 200-210F is enough to cool the trans fluid when everything is getting hot.

So without the saddle cooler what I have then is the trans fluid bypassing the cooler when cold and returning straight to the gearbox and only when the trans fluid is at 176F at the bypass thermostat (located close to the cooler) does it circulate through the cooler. When the bypass thermostat was first fitted it took a long climb (in winter, maybe 40F ambient) for it to open.

This set-up is frowned upon by some but to me it saves needlessly overcooling the trans fluid (through the cooler) but still allows it be cooled when its temp starts to get too hot. If - in that latter scenario - the fluid first went through the saddle cooler at 176F it would be borderline if it passed any heat to it and just as likely it absorbed heat giving the cooler more to do.

 

[Curt]

My set-up on a C2500 Suburban with the 4L 80E uses a cold bypass thermostat and no saddle cooler (coolant rad cooler) and my typical ambient temps are 14 to 80F.

My saddle cooler was bypassed before I got the truck. I didn't recommission it because, contrary to popular opinion I don't see it contributes much to warming the trans fluid from cold because there is no coolant flowing until the coolant thermostat opens. Until then, the trans fluid is heating the coolant. And, I don't see the slight temp difference when the engine is running at 200-210F is enough to cool the trans fluid when everything is getting hot.

So without the saddle cooler what I have then is the trans fluid bypassing the cooler when cold and returning straight to the gearbox and only when the trans fluid is at 176F at the bypass thermostat (located close to the cooler) does it circulate through the cooler. When the bypass thermostat was first fitted it took a long climb (in winter, maybe 40F ambient) for it to open.

This set-up is frowned upon by some but to me it saves needlessly overcooling the trans fluid (through the cooler) but still allows it be cooled when its temp starts to get too hot. If - in that latter scenario - the fluid first went through the saddle cooler at 176F it would be borderline if it passed any heat to it and just as likely it absorbed heat giving the cooler more to do.

Thank you for the input. I completely forgot there is no coolant flowing through the radiator until that thermostat opens. I'm leaning towards throwing a cold weather bypass on the H2, just like I did with mine. I'd rather just spend a little more time doing some fab work and having that peace of mind.

 

[tayto]

Pinger, maybe this works for your climate and usage, but I would not recommend it. have you done any datalogging to see what you trans temps are? fluid that is cooler than 160 can be a baf thing too. I recently added an 8"x11"x1.5" cooler to a 92 gmc for a friend in 15*c weather it ran at a consistent 160-165*f. should be good to go when pulling his landscaping trailer in the summer time (usually 30 to 35*c here).

i hate to be "that" guy but the factory did things a certain way for a reason. talk to any transmission guy in the know and they would also not advocate your method. water cools better than air, full stop. you have to also consider your terrain. hills and mountains + towing + cargo + hotter climate will be very taxing on an air only cooler. guys have gotten away with air cooler only. great. they also never seem to have any data to support their transmissions stay cool in all conditions. i've also never seen an evidence to support that "hot engine coolant" doesn't cool transmission oil. if it can't you have a serious problem with your cooling system.

 

[Pinger]

Pinger, maybe this works for your climate and usage, but I would not recommend it. have you done any datalogging to see what you trans temps are? fluid that is cooler than 160 can be a baf thing too.

Mine will get to temp though. Other than a 4 mile round trip to the village every 7-12 days, when it runs it never does fewer than 120-170 miles that day and is never stopped for long - an hour max.

i hate to be "that" guy but the factory did things a certain way for a reason. talk to any transmission guy in the know and they would also not advocate your method. water cools better than air, full stop.

Cold water does....

you have to also consider your terrain. hills and mountains + towing + cargo + hotter climate will be very taxing on an air only cooler. guys have gotten away with air cooler only. great. they also never seem to have any data to support their transmissions stay cool in all conditions. i've also never seen an evidence to support that "hot engine coolant" doesn't cool transmission oil. if it can't you have a serious problem with your cooling system.

What we don't know is the temp of the coolant vs the temp of the trans fluid. If it is 200-210F in the cylinder head then it won't be that much cooler in the saddle cooler and if we are trying to cool trans fluid that is around 200F there isn't that much of a temp gradient. Also, whatever heat leaves the trans fluid and enters the coolant still has to be dissipated to air.

In my defence - given my approach is controversial - I don't tow or do anything that really taxes the transmission. If I did and I had any doubts I'd dig deeper into actual temps but for the light road running I do I'm happy I've got a reliable set-up and, one devoid of any risk of coolant trans fluid cross contamination - an absolute trans killer.

 

[thegawd]

my 97 K1500 Burbs internal rad transmission cooler has been bypassed for about 8 years now. this was done by a transmission shop that's been in the business for over 60 years. obviously they did not recommend it as a permanent solution and just a temporary one until I could afford to replace the Rad.

I recently measured the pan with an infrared thermometer after an hours drive and to my surprise it was not a bad temperature, but I cant remember exactly what that temp was, It's on the site somewhere though.

Where I live is pretty darn flat and I dont tow very heavy loads all that often. So I am not concerned about the internal bypassed tranny cooler.

The transmission shop found coolant in the transmission fluid by a fluid anylsis. It was not visible to the naked eye and the fluid only had 20 000 kms on it before the used transmission that I installed gernaded. man I was pissed.

The shop rebuilt my tranny and provided a written warranty that they stood by even with that internal cooler bypassed. they said none of the race trucks that they build have radiators with internal coolers and were not phased by it. just said replace the rad as soon as I can. Well I pressure check it a couple times a year and it's still holding pressure.

Man I cant believe GM put the 4l60E in that H2! that's crazy. the shop rebuilt my 4L60E like a 4L65E but even stronger. it's their stage 3 HD and all the internals were replaced.

I cant offer anything to the topic though. Obviously I'm breaking the rules. but am very interested in this subject as I have more than one truck.

 

[618 Syndicate]

An H2 is essentially a Tahoe in a different dress. Completely different animal than the H1 which is a civilian humvee. An H2 has the same drivetrain as the Esky/Denali twins, 6.0/4L6*E

 

[Pinger]

my 97 K1500 Burbs internal rad transmission cooler has been bypassed for about 8 years now. this was done by a transmission shop that's been in the business for over 60 years. obviously they did not recommend it as a permanent solution and just a temporary one until I could afford to replace the Rad.

I recently measured the pan with an infrared thermometer after an hours drive and to my surprise it was not a bad temperature, but I cant remember exactly what that temp was, It's on the site somewhere though.

Where I live is pretty darn flat and I dont tow very heavy loads all that often. So I am not concerned about the internal bypassed tranny cooler.

The transmission shop found coolant in the transmission fluid by a fluid anylsis. It was not visible to the naked eye and the fluid only had 20 000 kms on it before the used transmission that I installed gernaded. man I was pissed.

The shop rebuilt my tranny and provided a written warranty that they stood by even with that internal cooler bypassed. they said none of the race trucks that they build have radiators with internal coolers and were not phased by it. just said replace the rad as soon as I can. Well I pressure check it a couple times a year and it's still holding pressure.

Man I cant believe GM put the 4l60E in that H2! that's crazy. the shop rebuilt my 4L60E like a 4L65E but even stronger. it's their stage 3 HD and all the internals were replaced.

I cant offer anything to the topic though. Obviously I'm breaking the rules. but am very interested in this subject as I have more than one truck.

But you have the auxiliary cooler - yes?

 

[thegawd]

yes sorry I never specified. the stock tranny cooler was never hooked up and still had the caps on it. It was hooked up when the internal rad cooler was bypassed. the tranny cooler lines were slightly diverted and the rad inlet and outlet were capped closed.