Trans Temp Important for Fuel Economy?

[oil pan 4]

Fuel economy was definitely down after I installed my grossly over sized transmission cooler with thermostat.
The thermostat didn't work as advertised and it made my transmission run real cold.
I basically added 3 transmission coolers, one with a fan to my suburban. Then shortly after the T/C lock up just quit. That's why I have only ran 2 tanks of diesel in it since December (and its still nearly full).

Question: How to hook up a transmission cooler - Diesel Place : Chevrolet and GMC Diesel Truck Forums

[redpoint5]

Originally Posted by jeff88

So what I'm interested in is first, does the cooler make the trans take longer to warm up or does it not 'open' until the ATF gets to a certain temp threshold? Second, if sized too large can the cooler actually over cool the ATF after it gets to operating temp.?

1. I have never heard of a transmission thermostat, but that doesn't mean they don't exist.

2. Over-cool isn't a practical concern as far as wear is concerned, but fuel economy could take a slight dip if the trans fluid temp never gets much above ambient.

[aerohead]

An SAE paper on fuel economy,published long ago,showed the relationship between temp and mpg.
Basically,they said that you wouldn't achieve your highest mpg until driving continuously for 22-miles at 50 mph,after which viscous shearing forces would bring the fluid up to its equilibrium temp and viscosity for whatever outside temp you were at at the time.
At Bonneville,many teams pre-warm all the lubricants before a run.
Full synthetic fluids can help,and are standard for many contemporary cars.

[silvermazda]

wow. I hardly drive 22 miles at 50mph. But it makes sense since there is no engine to heat up the oil, the oil only heats up when you are moving and when you're moving there is cooling (kinda like driving without a thermostat, the engine never heats up/looses heat while moving).

On another topic, the suburban probs has 13quarts of transmission fluid, meaning 3 gallons and pumps at least 1 gallon at a time and on top its cold & thick. You can do the math on that, but I'd say an alternator or two of drag or 10-20%.

[XYZ]

Originally Posted by matt36415

Many auto boxes will not shift into the higher gears untill they reach a set temp, heat is the most common killer of auto transmissions, the fluid breaks down, gets burnt... everything goes nasty.

Absolutely. But many here might ignore that good advice, seeking to save a few pennies in fuel economy by heating transmission fluid.

If insufficient fluid cooling results in a ruined transmission it becomes a case of "penny wise, pound foolish".

[jeff88]

What about a thermostat like this? It bypasses the cooler until temps reach 180 at which point it opens, allowing the fluid to run through the cooler.

Derale 13011 Fluid Control Thermostat Kit

Is 180* too hot for the trans?

[Miller88]

My Taurus was programmed to not lock the torque converter until the transmission fluid came up to temperature.

I had a giant transmission cooler on it. Sometimes, when it was below 0F, the torque converter would take 20 miles to lock up ... or if it was cold enough, it would never lock up.

[jeff88]

Great! Do you guys think that thermostat would work?

[oil pan 4]

Originally Posted by jeff88

What about a thermostat like this? It bypasses the cooler until temps reach 180 at which point it opens, allowing the fluid to run through the cooler.

Derale 13011 Fluid Control Thermostat Kit

Is 180* too hot for the trans?

That is precisely what I used and it did not work. Save your money.

I figure the best way to keep the transmission at the ideal temperature is to run 2 systems. A factory system (at least for GM) where the cooler acts more like a heater or heat exchanger with the engine coolant and a straight up air to oil cooler that puts a large amount of fluid surface area in contact with air, for use when towing or when its Texas/New mexico hot out.

Then the 2 systems would be manually opened or closed or both opened.
An electronic system would work by opening a normally closed heat exchanger/heater when the trans is cold. Then as it warms the cooler side, which is normally open and is energized and held closed during warm up opens. During normal operation in summer the heater constantly flows and the A/O cooler occasionally opens up.
In winter the heater always flows and the A/O cooler almost never opens.
Then when you hook up a 10k GTW trailer in the middle of summer and really warm up the transmission the "transmission heater" actually closes and everything runs through the A/O cooler. Or you can manually flip it to cool only.
If the system fails and goes non functional the "transmission heater" section closes (the reason for the normally closed valve) and the Air/Oil cooler fails open (the reason the A/O cooler has a normally open valve) and the transmission runs really cold.

[oil pan 4]

Originally Posted by jeff88

Great! Do you guys think that thermostat would work?

No.
In theory a thermostat would work, just not that one.