my experience with trans temps and gauges

[brods]

I showed this post to Bill & he asked me to put up the following for you guys to have as a reference. We kind of take it for granted that everyone knows this because we do this day in and out. Here is a little info that may help you if you don’t already know the functions of the ports. When we install & recommend temp gauges for customers we only like to see them done in the transmission out line and if necessary in the pan. We prefer the outline as it is the most accurate but if a customer wishes the pan is the 2nd place. . With the exception of the 1995 and earlier model trucks. Since this is a 3rd gen. it is very simple.



The transmission cooler temp line we use for the 3rd gen is the custom line we get from Mass Diesel 1-508-660-7093. The second choice is the pan. However we do not recommend the pan for guys towing heavy or off road guys



Very briefly I will go over the test ports so you guys can get an idea of their functions and it may explain our recommendations and will also give you some tools when considering advice.



On the passenger side of the transmission there are 4 pressure ports and the driver side there is one port.



1st. or Front Port is the Front Servo Port; I have seen guys ruin their transmissions by putting a temp sensor in here. If your temp sensor probe is long enough it will actually make contact with the spring in the front servo, causing problems in 2nd gear apply cycle. This pressure port only has oil in it when you are in 3rd gear, & 4th gear. Since your transmission will build its max heat in 1st, 2nd, or reverse this pressure port is pretty much useless for accurate readings.



2nd or Middle port is your mainline pressure or what is referred to as your accumulator port, this pressure port has oil in it in 1st, 2nd, 3rd, & 4th but not in reverse. If you do not have an RV trailer chances are you are not going to overheat it when trying to back up your truck but if you do have a trailer again or want to monitor accurately it is a useless port.



3rd port is your low & reverse servo port. It is the one that faces towards the rear end. This pressure port has oil in it only during reverse & manual low applications. Making this pressure port pretty much useless to measure from as it has no oil in it during the 1st, 2nd, 3rd or 4th gear.



4th pressure port is the governor port. It is located about 4" above the 3rd pressure port. Basically this pressure port gets energized with vehicle speed. But it has no oil in it during reverse application.



# 5 pressure port is the 4th gear pressure port. It is located on the driver’s side of the transmission above the rear cooler line going into the transmission case.

It only has oil in it during overdrive apply. Hopefully after reading this you guys can see why we do not like customers using pressure ports to take readings from. A lot of customers use these ports because it is convenient; a lot of shops use these ports because it is a profitable & quick install.



If you measure your transmission oil from the transmission out line that is the hottest point to measure from . It also will give you a good indication of when to service your transmission. The best example I can give you is with reference to our own personal drag car. Most of time when we launch our drag car the temps are approx 275 degrees, but there have been times when we have exceeded 375 degrees, we know that it is time to change the oil as it is no longer any good. It has broken down and we can prevent damage by servicing. You guys may be surprised how high these trans can get to if you are trying to back up a big 5th wheel trailer. While it doesn’t mean you have wrecked your transmission it does mean the oil has been broken down. So from our point of view, don’t use the ports as they are not the most accurate point to do so from.



Hopefully some of this can help whoever got the advice to measure the oil from the reverse port. All of the above is verifiable by your service manuals.

Stefan's post should go in the FAQ!



Question: if the transmission cooler out line reads, say 250 degrees, what is the temp at the source of the heat? I assume this would be the shearing action in the torque converter? Would the temps at the point of shearing be significantly higher and therefore the oil would break down from the heat?

 

[Matt400]

It is my understanding that our trucks have a transmission temperature sending unit from the factory. Where is it located? Brian

In the upper right corner of the transmission pan, the computer uses it to keep O/D locked out if the fluid is too cold and also will lock the converter when fluid gets too hot along with turning on a dash warning led at 275 deg.

Hopefully after reading this you guys can see why we do not like customers using pressure ports to take readings from.

That was an excellent post Stefan and I can now see why a couple years ago my pan and port measurements were so close as I never tested reverse.

Thanks for sharing with us about the ports.

 

[WHazelwood]

If there was no fluid there I doubt I would see that. I also see big differences in temp when towing, I am up to 195 in less than 10 miles towing 5K in traffic without ever backing up, once on the highway the temp drops about 20 degrees with the TC locked up. When I do back up (only with trailer attached) the temp increases very quickly sometimes 20 degrees in a couple minutes. These temp readings seem close to what others are seeing with pan mounted probes and line probes. How do you explain the temps when there should not be any fluid there? Here's a picture of the probe incase I am incorrect on which one you are calling



What you may be seeing is the affect of the exhaust temperatures. If you're in reverse or slow in traffic there is not much air movement to take the exhaust heat out from under the truck and this will influence the temp probes on the passenger side of the transmission.

 

[Bertram65]

If there was no fluid there I doubt I would see that. I also see big differences in temp when towing, I am up to 195 in less than 10 miles towing 5K in traffic without ever backing up, once on the highway the temp drops about 20 degrees with the TC locked up. When I do back up (only with trailer attached) the temp increases very quickly sometimes 20 degrees in a couple minutes. These temp readings seem close to what others are seeing with pan mounted probes and line probes. How do you explain the temps when there should not be any fluid there? Here's a picture of the probe incase I am incorrect on which one you are calling



What you may be seeing is the affect of the exhaust temperatures. If you're in reverse or slow in traffic there is not much air movement to take the exhaust heat out from under the truck and this will influence the temp probes on the passenger side of the transmission.

Yeah right, it's the exhaust

I do not know why you guys can't accept that there is fluid in those ports, there have been a number of people who have confirmed the readings are the same with a probe in the port in in the pan or in the line adapter.

Maybe my thrust washer will give out next, didn't some transmisssion "expert" say that there was huge flaw in the 48RE with those? IF so where are all the failures??

If you are going to quote me use the quote button in my post.

 

[WHazelwood]

Yeah right, it's the exhaust

I do not know why you guys can't accept that there is fluid in those ports, there have been a number of people who have confirmed the readings are the same with a probe in the port in in the pan or in the line adapter.

Maybe my thrust washer will give out next, didn't some transmisssion "expert" say that there was huge flaw in the 48RE with those? IF so where are all the failures??

If you are going to quote me use the quote button in my post.

Sorry, I'll try to do that from now on. I did have the temp probe in the 2nd port passenger side but noticed that every time I drove thourgh a puddle the temp dropped to about 120 deg. or would go to about 200 deg. in slow traffic. I changed to the Mass Diesel line and the temp only goes to about 155 in slow traffic and puddles don't make a dfference. That's why it seems to me the probe was picking up a lot of the exhaust temp.

 

[Bertram65]

Sorry, I'll try to do that from now on. I did have the temp probe in the 2nd port passenger side but noticed that every time I drove thourgh a puddle the temp dropped to about 120 deg. or would go to about 200 deg. in slow traffic. I changed to the Mass Diesel line and the temp only goes to about 155 in slow traffic and puddles don't make a dfference. That's why it seems to me the probe was picking up a lot of the exhaust temp.

Interesting, I have driven through some pretty heavy rain storms and puddles and never seen any change in temp, what brand of probe was it? was it in the same port as the one in my picture?

 

[WHazelwood]

No, it's the one that Geno's sells with the gauges. has two wires and goes to an electronic controller. Since I moved the location to the cooler line, I don't see any fluxation in reading like I did before.