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

garageman402

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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?
 
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.
 
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.
 
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.
 
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!!
 
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.
 
I have two cars setting next to each other. One is a totally stock 1978 Cadillac Seville and the other is a modified 1989 GMC S15 Jimmy that someone replaced the stock 4.3 V6 with a 350 ZZ4 V8 engine and the original Jimmy radiator was replaced with an aluminum radiator with the heater return hose going to the cold side of the radiator. The Jimmy was modified before I bought it. See details below:
1) Seville has the original, stock, Oldsmobile 350 engine with a vacuum opperated valve in the heater hose (not sure if it works or not). It has a 180 degree thermistat that does work. Oldsmobile water pumps come in two versions one accepts the heater hose and one has no provision for a heater return hose. This Seville does not accept a hose return to the water pump and routes the return to the cold side of the radiator.
2) Jimmy: SBC water pumps also come with and without heater return capability. This pump can not accept a heater return so the heater hose is routed the the radiator. The Jimmy ,currently, does not have a control valve in the hose so water always flow in the heater. Not sure if GMC originally had a water control valve that was later deleted.

Conclusion: Yes I am quite yappy. Both cars route heater return hoses to the radiator. Before the thermistat opens on both cars the cold side, that contains the trany cooler, gets very hot while the hot side of the radiator remains cold. When the thermistats open both sides of the radiator are working as expected. I would love to put a cut off heater valve in the heater hose so it would prevent the heater core from being hot when I run the air conditioner, what do you think?
 
I have two cars setting next to each other. One is a totally stock 1978 Cadillac Seville and the other is a modified 1989 GMC S15 Jimmy that someone replaced the stock 4.3 V6 with a 350 ZZ4 V8 engine and the original Jimmy radiator was replaced with an aluminum radiator with the heater return hose going to the cold side of the radiator. The Jimmy was modified before I bought it. See details below:
1) Seville has the original, stock, Oldsmobile 350 engine with a vacuum opperated valve in the heater hose (not sure if it works or not). It has a 180 degree thermistat that does work. Oldsmobile water pumps come in two versions one accepts the heater hose and one has no provision for a heater return hose. This Seville does not accept a hose return to the water pump and routes the return to the cold side of the radiator.
2) Jimmy: SBC water pumps also come with and without heater return capability. This pump can not accept a heater return so the heater hose is routed the the radiator. The Jimmy ,currently, does not have a control valve in the hose so water always flow in the heater. Not sure if GMC originally had a water control valve that was later deleted.

Conclusion: Yes I am quite yappy. Both cars route heater return hoses to the radiator. Before the thermistat opens on both cars the cold side, that contains the trany cooler, gets very hot while the hot side of the radiator remains cold. When the thermistats open both sides of the radiator are working as expected. I would love to put a cut off heater valve in the heater hose so it would prevent the heater core from being hot when I run the air conditioner, what do you think?
The original heater system, at least in square body trucks, always allowed coolant flow through the heater core. The cab temperature was controlled through air doors. Maybe this was also used (only?) on A/C equipped vehicles because when the defroster is on, outside air is routed through the evaporator to dry incoming air, then through the heater core to heat the air hitting the windshield. I think use of a heater control valve (for coolant) would have been more problematic in directing variable air temperatures, & slower to react to the temp changes made by the passengers.

Vintage Air systems, at least the earlier versions, use a heater control valve to completely eliminate any hot coolant inside the unit, possibly because of the very compact size. Remember the stock setup had a huge under hood unit with the evaporator & other equipment, & the heater core inside the cab. The entire Vintage Air unit is inside the cab. The early system I have does not use outside air, which I'm not fond of, but I've heard later versions have an outside air source.

As far as you adding a heater control valve (coolant shut off), it may or may not do any good. You can experiment by using vise grips to squeeze off the intake hose to the heater core & see if it make any difference inside the cab. As you mentioned the cool side of the radiator will stay cool until the thermostat opens with the hose squeezed off. My personal opinion is that you want to keep the transmission as cool as possible, especially with the temp sensitive 700R-4 I have in this truck, & I can't see the benefit of dumping hot coolant out of the heater core onto the cooler. Granted, I have not made temperature measurements, but the coolant coming out of the radiator has to be cooler than coolant coming out of the heater core.

All the water pumps I had seen have an extra outlet that would be plugged if the application uses the heater core to radiator setup, or could have a fitting installed for return to water pump. You usually have to transfer your existing fittings to the new pump. One outlet is for the bypass hose.

I still haven't found an answer as to why GM engineers routed the heater core return line to the radiator tank in the first place. As I said earlier in this thread, I suppose hot flow is better than no flow, but on a hot day on a hard pull with the A/C on, hot coolant destined for the radiator is routed through the heater core (which is shut off to air) & directly into the cool side bypassing the radiator entirely. Routing the hose back to the water pump essentially creates another bypass but without bathing the trans cooler in hot coolant. All passenger cars from that era (60s-70s) had that set up.

It has been said that in more modern, compact settings, you need as much cooling area as possible with smaller & smaller radiators due to space restrictions, so the heater core actually contributes to overall cooling. Everyone knows the trick to turn the heater on full blast & roll all the windows down if the engine is overheating, but in normal, non-overheating conditions you won't do that. Most vehicles don't have a transmission temp gauge, so you don't know what's going on with it.
 
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.
I remember the old Chrysler V8 engines feeding the heater core from the rear, possibly due to the routing of coolant flow inside the engine. The coolant coming off the head is the hottest so it makes sense to use the hottest coolant available to run the heater. I don't know why your K1500 routed it that way, but it seems the coolant heading for the radiator would be the hottest. Maybe they wanted to cut down on hose length? Less hose equals less risk of rupture.
 
Thanks Garageman402, you provided good insight. My toy cars are never driven in the rain so heated air for the defroster in never required. During recharge of my A/C the technition stated the air temperature is not as cold as it should be because hot water is constantly being circulated. I do drive the car in the winter so I do not wish to totally bypass the heater core, so anyone know of a manually operated valve? While I don't have any Oldsmobile water pump pictures, these pumps either comes with a boss that has been drilled and taped for a hose return or a boss that has not been drilled, I have had both. I do have pictures for SBC water pumps. One picture shows the common pump with the factory plug installed. The other picture (on a TPI 350 with bad angle for picture) is a non common pump that has a small hole drilled/pluged at the top that is too small to accept a nipple, it is maybe 1/4". Not sure what the purpose of that hole is, maybe for a temprature sending unit. Last comment, I deal mostly with reverse rotation serpentine SBC systems so whenever I buy a rebuilt pump, I always remove the 6 bolt back cover plate to confirm that the impeller has the correct vanes either standard or reverse. I once bought and installed an "oops" rebuilt pump that the rebuilder had installed the wrong impeller or maybe its shipping box was mislabled.

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Vintage Air does sell the valve separately, but I don't know if it's 12v, it plugs into the unit. I had to get another as mine stopped working, it had been installed reverse flow, I don't know if that hastened its failure. It's worth a try to check it out. They have support personnel to assist. You can always install a manual valve on the inlet hose that you can turn off in the summer, & back on in the winter.
 
Heater sprung a leak 25 yrs ago, have the heater hose coming off the intake straight to the radiator.
You have an A/T? Dumping super hot coolant on the trans cooler doesn't sound good, but it's worked for 25 years.........

I'd put the fitting on the water pump & run it there, but, that's just me.
 
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