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.
