transmission cooler question

[TSuter]

OK I just don't get it. In a recent trip from Atlanta to Delaware to St Louis with temps in the mid to upper 90s, truck empty, I had to do around 60 mph to the transmission temps around 165-170. That's with the stock coolers, plus an aftermarket underbed cooler with 1/2 inch tubes. That was a long trip with no AC! Normally, I do around 70, but then the temp would hover around 180-190.



So I decided to put in another cooler. I took out the stock cooler in front of the radiator, and put in a 21" Perma-Cool with with 5/8" tubes (Matched up with the rest of the lines in the system). Took the truck out for a test drive, now the transmission temp sits at 195 at 60 mph.



Bypassed the underbed cooler thinking the smaller tubes were too restrictive. Same results 195 at 60 mph#@$%!.



So what am I missing?



TIA

Tom

 

[Flexiheep]

What has your engine temp been doing lately. My trucks transmission seems to be very sensitive to the engine temp due to the oil to water primary cooler on the pass side of the engine. I just replaced a bad fan clutch and dropped the transmission temp 20 degrees... .

Is your transmission temp probe on the hot side of the converter? It almost has to be to be seeing those temps. I don't know what you are used to, but our trucks run hotter than the newer ones because of the non lockup converters.

Before I had my transmission rebuilt I was used to it running 200 at 60 unloaded on flat ground, and could see 250 climbing the hill to work still unloaded.



Some questions I have would be:

Condition of engine cooling system?

Any mods to transmission?

Miles on transmission?

Is it acting any diffrenly than it was (feels like it is slipping? ok, more than they do normally )

Is the under bed cooler fan assist?

Does the transmission pass the stall speed test in the service manual? (you can create an incredible amout of heat if the converter isn't at stall while cruising)

does it run any cooler in 3rd than in od? That is if you have a 518 not a 727



I just noticed it was a 91, ignore anything that doesn't apply to the 727 if you have that.



Ok, just some things to think about that may lead to an answer. .

 

[TSuter]

Flexiheep,



"What has your engine temp been doing lately"

Engine temp is running at the same spot, the hash mark just to the left of center. I assume that's roughly 180*. The cooling system was flushed @ 20k ago when I replaced the thermostat.



"Is your transmission temp probe on the hot side of the converter?"

Yes, the temp probe is where the temp probe is supposed to be to lock out the O/D.



"I don't know what you are used to, but our trucks run hotter than the newer ones because of the non lockup converters. "

I'm used to seeing temps, unloaded, on the flats of around 150-170* when the outside temp is from the teens to mid 70s. 80s and the under bed cooler can keep the transmission temp @ 170, but the fan is working its butt off. 90s and it will try and sit @180. All of these temps are at 70 mph. In the mountains if the transmission temp gets above 180 I start slowing down. Hottest it's seen has been 195.



"Any mods to transmission?"

No, other then when the transmission was rebuilt 25k ago, I had a TCI torque converter installed,



"Miles on transmission?"

218k total, 25k since rebuild



"Is it acting any diffrenly than it was"

Nope, other then when the fluid gets too hot, but that's expected.



"Is the under bed cooler fan assist?"

Yes,



"Does the transmission pass the stall speed test in the service manual?"

Yes



"does it run any cooler in 3rd than in od?"

Haven't kept it on the road long enough to find out. Once it gets to 195, if it doesn't come down I head for the house. It doesn't start to cool down until I'm under 45mph.



I think that covers all the questions. But now I have some more. I put in a tube and fin style cooler, removing the plate style that is OEM. I figured it was so small it wouldn't matter that much. Are the plate style coolers that much more efficient?



Tonight or tomorrow depending on weather I'm going to try and put the OEM cooler back in with the aux cooler in series and see what happens then.



I'm not a transmission guy by any stretch of the means, but I've always been told that the trans should run about 30* cooler then the engine. Unloaded of course; loading, mountains, and outside temp change the variables a bit. But if the system is set up correctly I should be able to manage the temps without having to slow down to speeds that will get you run over.



Tom

 

[cerberusiam]

So I decided to put in another cooler. I took out the stock cooler in front of the radiator, and put in a 21" Perma-Cool with with 5/8" tubes (Matched up with the rest of the lines in the system). Took the truck out for a test drive, now the transmission temp sits at 195 at 60 mph.



Bypassed the underbed cooler thinking the smaller tubes were too restrictive. Same results 195 at 60 mph#@$%!.



So what am I missing?



TIA

Tom

Well, the obvious of course. :-laf



There is a reason for the line sizes and the restrictions built into the cooling system. Primarily to keep the fluid from circulating too fast and not dumping enough heat. Th 5/8 line coolers are too big and the fluid circulates too fast to adequately dump the heat.



IIRC, the lines are 1/2 OD which means they are somewhere between 3/8 and 9/16 internally. Even going to 1/2 ID lines is enough to drive the temps up. Along with the plate coolers being better at cooling that is probably why you are seeing those temps.



The 195 temps are not even close to being hot so I think you are worrying a bit much. If you haven't yet, switch to ATF+4 as it is rated to short temps in the 320 degree range.



The last thing you need to be careful about when messing with the trans coolers is the actual flow back at the trans. The return flow is used for lube oil so changing that beyond what the stock setup has for restrictions is taking a chance.



What is being recommended now is a stand alone pump to cirulate the fluid thru external coolers. This leaves the factory system inact for lube but adds cooling of needed.

 

[TSuter]

Thanks. As far as changing to ATF +4, how concerned to I have to be with purging all of the ATF +3? Is it reverse compatible?



Tom

 

[cerberusiam]

I don't think there is a problem with mixing it if you get most of the old fluid out. Drop the pan and drain as much as possible there. If your TC doesn't have a drain plug drop the return line into a bucket and run the trans in neutral until the flow slows down refilling in between runs and you will flush most of old fluid out of the TC and cooler.

 

[TSuter]

Thanks for all the help. I probably am overly concerned with the temps, after all the truck worked just fine the worked just fine the first 200k without even knowing what the temps were. I'm trying to get all the preventative maintenance done as well as some extras before I retire from the military in a couple of months. This way I can screw off for a couple of months and sit around driving my wife nuts:-laf



Tom

 

[DBF]

Well, the obvious of course. :-laf

What is being recommended now is a stand alone pump to cirulate the fluid thru external coolers. This leaves the factory system inact for lube but adds cooling of needed.

Who does these pump installations? I'm tired of towing w/OD locked out to get enough flow to keep things cool.

Tx, DBF