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Old 03-13-2010, 02:36 PM
glostamann glostamann is offline
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Default New tranny and eng. t-stat, but odd temp behavior

This was posted in the wrong section, so here I go again:

Well, I just recently was given a 1997 Dodge Ram 1500 SLT in lieu of cash for doing construction work. Drove it a month or so and the tranny (A518 4sp AutoM. w/OD) went out. I got an AAMCO rebuilt unit and it seems to be doing well.

However, since I got my truck back from the shop, I put a new waterpump on and decided to omit the viscous clutch w/ steel blade cooling fan. I didn't like the set up and it was dangerous. So I mounted two new 14" electric fans with a custom relay wiring setup with an aftermarket sensor that is inline the output radiator hose. When the 318 hits 195F, it opens up and flows coolant to the radiator, and soaks the new temp sensor and then at 200F, it provides a ground to my two relays that pick up the fans for cooling.

Problem is... I've noticed that from a cold start, the engine warms up and the temp gauge on the instrument cluster is showing the engine temp moves past the 200F mark on the gauge before it starts coming back down. So it is cooling down, it appears that the thermostat is working, but its not opening up until it's about 3/16ths of an inch past the 200F mark on the gauge. It's as if it's around the 215F or 220F mark. It hits that area (past the 200F mark on the gauge), but it does ultimately come down and stay consistantly around 195-200F mark. I'm telling myself that mark where it reaches before it ultimately comes back down and stays consistantly cool has to be bad because the thermostat is supposed to open at 195, not well past 200F.

I don't like the situation because I just paid $1700 for a new transmission and the fact that Dodge incorporated a double core radiator gets on my nerves. I don't see the logic in cooling your tranny through the same fin&loop style core allong with cooling your engine coolant. In other words, it's working like an exchanger system and I'm afraid that the short period of circulating 215F to 220F for up to a minute or so before it comes down and finally rides at 195F-200F will cook the new tranny fluid and burn the new tranny up. Does this sound like a bum thermostat? It's only a couple months old and like I said before, I put a new thermostat in when I changed the waterpump. Or could my gauge be bad? Or a bad sending unit to the instrument cluster gauge?

I do plan on getting an aftermarket stand alone tranny cooling core with a fan. Am I just being to parental with the behavior of this thermostat? Maybe I should drop down to a cooler thermostat? Or will that mess with the way the cpu sees eng. temp and faulter the air/fuel ratio mixture? I just want it to be at the right temp and ultimately sepparate tranny and engine tandem cooling.

By the way, does anybody know what what tempature the transmission is supposed to run at? This will help when designing the new electrical setup for the tranny cooler w/ electric fan. I need to know where to tap into the tranny fluid output to cooling line for the sensor that will pick up the cooling fan motor mounted to the new tranny cooler.
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Old 03-13-2010, 03:08 PM
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trainplanemech trainplanemech is offline
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Thats exactly how my thermostat opens, and its a 195 degree one i just put in last week.My temp runs normaly around 180 im huessing if u divide up the gauge markings.Im pretty sure yours is running how its sposed too.
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