I have a 2003 Chevy Silverado 2500hd, 6.0l gas and the 4l80e trans.For the last 15 or so years it's had a 16 pass stacked plate trans cooler with a fan on it to keep the temps in check.about 15 months ago, my radiator started to leak and I replaced the radiator with an aluminum 3 row desert radiator - and I thought things were great - except my trans started to run warmer, a lot warmer.In the past, it would always run about 175 to 180 on flat level ground and on a hill climb, it would bet up to 200 or 220 the absolute highest and only for a short period of time.After the swap, it started to run 190-200 most of the time and last summer, it was up to 250. I Just moved to denver, so I figured that It must have been the internal radiator was restricting the flow of the fluid so I bypassed the internal transcooler and used just 16 pass cooler (fan on) and hoped that would resolve it. Leaving LA, it was up to 250 on the simple climbs out of the city.I spent the night in Palm Springs and the follwing morning, I did some testing to see if the pump wasn't moving enough liquid - I disconnected the hose before and after the cooler and it had plenty of flow. It's not the pump. No slipping on the gears, and lots of volume through the external trans cooler.I checked the fluid level (flat level ground and when it was hot, idling after stepping through all the gears and I'm right on target for fluid).I had the Trans fluid swapped out a month or so ago, thinking new fluid would help (I have, for the last 18 years, had the trans oil pan dropped, filter swapped, and a flush after the fact just to keep up the maintenance (and had the rear differential fluid swapped as well). It's not a lack of maintenance.If I tow on flat level ground in Overdrive, the temps are normal (175-190), but as soon as I drop out of OD, temps begin to climb.Any ideas as to what's going on? Thanks in advance!Josh

It absolutely shouldn't get hotter if you drop a gear. Possible cause? Is the tq converter not locking up in 3rd gear?

The common denominator is the radiator swap. When all is said and done air flow is extremely important. I'm thinking the fins are tighter together and not allowing as much air flow. It's correct that running it in 3rd shouldn't increase the trans temp, not by much anyway. Dropping it to 3rd increases the RPMs and puts more demand on the cooling system. A working system simply keeps up with the temperature regulation. My Sierra had a custom grill cover on it when I bought it. Pretty cool looking. A few years ago we went to Gatlinburg. Very hot out. Around Indianapolis my trans temps were way higher than I was used to seeing, 220-230. It finally dawned on me about air flow. I removed the grill shell and it was back to 180 on flat land.

Campfire Time wrote:The common denominator is the radiator swap. When all is said and done air flow is extremely important. I'm thinking the fins are tighter together and not allowing as much air flow.I was thinking close to the same thing, except I was suspect of the in-tank cooler. There's a big difference in some aftermarket radiators. OEM heavy-duty grade coolers are stacked-plate style.Light-duty grade in tank-coolers are a tank-style cooler:The lowest-cost in-tank cooler is just a loop of copper tubing. But, assuming the triple pass radiator is from a reputable vendor, they probably still use the stacked-plate cooler (I always checked before installing any aftermarket radiator).The triple-pass dessert radiators are not 3 rows, but have 3 coolant paths. This is good for coolant temps, but cuts the area of cool-coolant by 1/3rd, so only the section of trans oil in the "last-pass" is getting cooled effectively. Also, this setup won't work hardly at all if the in & out lines are swapped. The out-line has to be on the 3rd pass side of the cooler.IMO - This style radiator should only be used with a transmission cooler large enough to effectively cool on its own. Something like the Tru-Cool max.

Except higher rpms (lower gear, same speed) apples to apples should run cooler Fan runs faster if it’s belt driven, water pump pumps more volume, heat generated is close to the same, especially in newer engines that are fuel injected. Agree it could be the radiator and if something different is allowing less airflow to the trans cooler, e fan on it or not, that could be a cause, but then it would run hotter in OD too. My vote is old truck, trans issues.

Sounds like the cooler in the radiator might not be as efficient. Do you run through the radiator than cooler?I had the same truck just an 02. I put a tru-cool 40k on mine with the stock radiator and it dropped my overall towing temps from 180-200 to 160-180. How many miles on the truck?

Hot Running Trans Temp - Ideas? | Good Sam Community

1320Fastback
Explorer
Sep 28, 2019
time2roll wrote:
I would reluctantly go back to the oem radiator. I don't like patch on patch to make things mostly work.

I'm thinking the same, OEM. I know things have changed over the years but us 1st Gen fans never get aftermarket radiators. The parts house replacements weigh about 7 pounds less than factory for our trucks and that is heat absorbing and heat displacing metal. We have our rodded out by local radiator shops and put back in whenever possible.

I know modern radiators have plastic tanks and I want nothing to do with those.
time2roll
Nomad
Sep 27, 2019
I would reluctantly go back to the oem radiator. I don't like patch on patch to make things mostly work.
Krusty
Nomad II
Sep 27, 2019
Was thinking that too. Maybe the cooler has a thermostatic bypass built in and its sticking.
1320Fastback
Explorer
Sep 27, 2019
Does the aftermarket cooler have a thermostat in it?
jodeb720
Explorer
Sep 27, 2019
To answer all your questions:

175,000 miles
I'll be the in-radiator Trans Cooler isn't a stacked plate design.
External is mounted up in front of the AC and Radiator with a push design electric fan. 16 pass stacked plate design (Derale I believe is the manufacturer).
I've had the in-radiator transcooler as part of the circuit and removed it to see a difference.
Trans has always run hotter when doing long sustained hill climbs out of OD - but never to this level (about 220).
The Water temp inside the radiator never budges above 190 - even on sustained hill climbs - which is great, but if the internal Trans Cooler isn't as efficient as the OEM, then that's where my problem lies (Water to Oil is more efficient at removing heat than Air to Oil)

I think my next steps are to reconnect the Internal Trans cooler, check the volume coming out (it should be the same) and contact the seller of the Radiator to see if the internal is stacked plate like the OEM version.

thanks to all for your advice and guidance. I really appreciate all your insight!

Josh
Campfire_Time
Explorer
Sep 26, 2019
carringb wrote:

I was thinking close to the same thing, except I was suspect of the in-tank cooler. There's a big difference in some aftermarket radiators. OEM heavy-duty grade coolers are stacked-plate style.

I think you're on to something. I hadn't thought about the trans cooler differences.
ls1mike
Explorer II
Sep 26, 2019
Sounds like the cooler in the radiator might not be as efficient. Do you run through the radiator than cooler?
I had the same truck just an 02. I put a tru-cool 40k on mine with the stock radiator and it dropped my overall towing temps from 180-200 to 160-180.

How many miles on the truck?
Grit_dog
Navigator
Sep 26, 2019
Except higher rpms (lower gear, same speed) apples to apples should run cooler
Fan runs faster if it’s belt driven, water pump pumps more volume, heat generated is close to the same, especially in newer engines that are fuel injected.
Agree it could be the radiator and if something different is allowing less airflow to the trans cooler, e fan on it or not, that could be a cause, but then it would run hotter in OD too.
My vote is old truck, trans issues.
carringb
Explorer
Sep 26, 2019
Campfire Time wrote:
The common denominator is the radiator swap. When all is said and done air flow is extremely important. I'm thinking the fins are tighter together and not allowing as much air flow.

I was thinking close to the same thing, except I was suspect of the in-tank cooler. There's a big difference in some aftermarket radiators. OEM heavy-duty grade coolers are stacked-plate style.

Light-duty grade in tank-coolers are a tank-style cooler:

The lowest-cost in-tank cooler is just a loop of copper tubing. But, assuming the triple pass radiator is from a reputable vendor, they probably still use the stacked-plate cooler (I always checked before installing any aftermarket radiator).

The triple-pass dessert radiators are not 3 rows, but have 3 coolant paths. This is good for coolant temps, but cuts the area of cool-coolant by 1/3rd, so only the section of trans oil in the "last-pass" is getting cooled effectively. Also, this setup won't work hardly at all if the in & out lines are swapped. The out-line has to be on the 3rd pass side of the cooler.

IMO - This style radiator should only be used with a transmission cooler large enough to effectively cool on its own. Something like the Tru-Cool max.
Campfire_Time
Explorer
Sep 26, 2019
The common denominator is the radiator swap. When all is said and done air flow is extremely important. I'm thinking the fins are tighter together and not allowing as much air flow. It's correct that running it in 3rd shouldn't increase the trans temp, not by much anyway. Dropping it to 3rd increases the RPMs and puts more demand on the cooling system. A working system simply keeps up with the temperature regulation.

My Sierra had a custom grill cover on it when I bought it. Pretty cool looking. A few years ago we went to Gatlinburg. Very hot out. Around Indianapolis my trans temps were way higher than I was used to seeing, 220-230. It finally dawned on me about air flow. I removed the grill shell and it was back to 180 on flat land.