Why Did GM Route the Heater Return Hose to the Radiator on AT Equipped Trucks in the 70s-80s?

[garageman402]

Does anyone know why GM routed the heater hose return line to the cold tank of the radiator? These trucks didn’t have heater control valves so the flow is constant. It seems to me that dumping hot coolant on to the transmission cooler is counterproductive.

I rerouted the hose to the water pump as passenger cars were in those days & plugged the nipple. Now I’m wondering since the cooler is a tube with hollow walls with ATF running though it, they intended flow through the center, even with hot coolant. Without this flow, the center of the tube would be stagnant & get overheated?

Any thoughts?

 

[Cujet]

Interesting question. Anybody who is a fan of American cars will sometimes look at an item in wonderment. Wish I had an answer for you, other than to say, it may have been for reasons other than ATF temperature control. Guessing it's for adequate heat.

 

[DriveHard]

To help warm the tranny in cold weather?

 

[garageman402]

Interesting question. Anybody who is a fan of American cars will sometimes look at an item in wonderment. Wish I had an answer for you, other than to say, it may have been for reasons other than ATF temperature control. Guessing it's for adequate heat.

Every other application outside of trucks with AT routed the return hose back to the water pump, certainly creating more flow through the heater core.

 

[garageman402]

To help warm the tranny in cold weather?

In cold weather I would assume the heater was being used, substantially cooling the coolant. Maybe if the thermostat never opens at least it gets some coolant flow?

 

[marc1]

Any thoughts?

Ok, yes.

1 - That was done in the mid 70's on the truck line on automatic equipped trucks only as far as I recall.
2 - Functionally there is no difference in returning to the radiator nipple or the water pump. Many times replacement radiators did not have the nipple so you just routed the traditional way to the pump (you needed to replace the plug on the pump with the usual water pump nipple in that case). Air in the heater core system is still successfully purged in either routing. This assumes, of course, that you have the hoses on correctly at the heater core. Air is purged from the bottom of the core then returned to the cooling system either at the water pump or that radiator nipple.
3 - Your AT cooler inside the radiator is immersed in coolant 100% of the time (if the coolant level is correct) so whether you return via the radiator nipple or the water pump nipple won't matter to your AT cooler. Millions of automatic transmission Chevrolets/GMCs return via the water pump, no radiator nipple. Chevrolet cars of earlier and of that era are all water pump return as far as I know, I don't know any that have that nipple. I know some guys have installed a nipple radiator and then changed to that radiator nipple return and plugged the water pump return. However most people in that situation put a cap on the radiator nipple and stay with the "OG" return via pump system. I was never keen on that, I always was worried the cap would pop off.
4 - Why the factory change? That is a great question and I honestly do not know and have never been told definitively by a GM engineer / Tech in the day etc. My 100% pure guess is it was a routing issue. To return via the water pump is a little tight in that area. If there is an AC compress there, that further complicates things. It might have been to facilitate assembly, guys were tired of trying to attach the return heater hose behind brackets and an AC compressor. Finally... heat... maybe they wanted to get the heat away from the fuel system (the fuel line runs up that side of carbureted SBC/BBC engines) so running a heater hose straight to the passenger side of the radiator along the inner fender keeps the heat away from the engine / fuel lines.

 

[garageman402]

1 - That was done in the mid 70's on the truck line on automatic equipped trucks only as far as I recall.

Right. Only on trucks that could tow with AT. Maybe that was intended as additional cooling for the ATF?

3 - Your AT cooler inside the radiator is immersed in coolant 100% of the time (if the coolant level is correct) so whether you return via the radiator nipple or the water pump nipple won't matter to your AT cooler. Millions of automatic transmission Chevrolets/GMCs return via the water pump, no radiator nipple. Chevrolet cars of earlier and of that era are all water pump return as far as I know, I don't know any that have that nipple.

Yes it is immersed in coolant, but there is no coolant flow when the thermostat is closed. Is it possible on a cold morning while towing, before the thermostat opens the fluid could get hot? Also, if you look in through the cap, you can see the cooler has a passage inside for coolant to travel. The coolant in a cross flow radiator will only hit that from the side, the column of coolant inside will be stagnant to a point. So on a heavy pull, the coolant inside the cooler tube could overheat?

Finally... heat... maybe they wanted to get the heat away from the fuel system (the fuel line runs up that side of carbureted SBC/BBC engines) so running a heater hose straight to the passenger side of the radiator along the inner fender keeps the heat away from the engine / fuel lines.

The heater input hose parallels it & would be hotter.

4 - Why the factory change? That is a great question and I honestly do not know and have never been told definitively by a GM engineer / Tech in the day etc. My 100% pure guess is it was a routing issue. To return via the water pump is a little tight in that area. If there is an AC compress there, that further complicates things. It might have been to facilitate assembly, guys were tired of trying to attach the return heater hose behind brackets and an AC compressor.

They would specify a different part for trucks with automatics just to cut down on assembly line time? I guess by 73 practically everything was automatic, so it would be the rule rather than the exception.

I suspect it was there to keep a constant flow of coolant around & through the ATF cooler. Even though on a hot day with the A/C on the coolant hitting the cooler is almost as hot as the coolant coming out of the engine. With a Vintage Air A/C there is a heater control valve that shuts off the flow when the A/C is on.

To flow, or not to flow, that is the question!!

 

[dfarr67]

I've asked this question in the past with no answers forthcoming. I have a 1989 K1500 that originally pulled hot water off of a problematic fitting at the rear of the intake and dumped into the cool side of the rad. I now pull hot coolant from the front of the intake and dump into the water pump. There is a difference in cooling performance- but it's been so long since I looked at it that I no longer remember what's what.