Home » Public Forums » GMCnet » Re: [GMCnet] trans temp sensor
| Re: [GMCnet] trans temp sensor [message #153286] |
Sat, 17 December 2011 12:00  |
Gary Casey
Messages: 448
Registered: September 2009
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Just to add a little to Ken's excellent post, while working at Hydramatic when the THM-425 was still "news", we did a lot of instrumentation to measure temperature throughout the transmission. A couple of tidbits: The temperature rise in a clutch during a full-throttle upshift can be 100 degrees. If the clutch is asked to do another high-energy shift before it has a chance to cool down, the internal temperature can get frighteningly high. I've seen people with a twitchy foot upshift and downshift a number of times climbing even a short hill. The third gear clutch, also used in reverse will burn up quickly with this kind of abuse - and you might not even see a change in pan temperature. When the temperature of the oil rises above 300 or so it starts to oxidize or degrade, limiting its life. When the temperature inside a clutch gets over 400 the clutch material, which is essentially a glorified cardboard, will start to burn. Once it has
burnt surface the coefficient goes down, lengthening the shifts and further increasing the heat input. Gone. The clutches are designed for infinite life if they don't get too hot, but overheat them and they can be fried in a couple of miles (ask me how I know). The weakest "clutch" in the 425 is the second overrun band. That's the one that pulls the engine speed up when you manually downshift into second coasting down a hill. This band won't tolerate frequent downshifts. And finally, as Ken said, the torque converter is certainly the source of the most steady-state heat. The heat input is pretty much inversely proportional to the SQUARE of the rpm. Put it under high load at low rpm and the temperature will go up rapidly. The best way to save it is to reduce torque and increase rpm, both of which can be accomplished by downshifting. It's much easier on the transmission to go up a hill at 3,000 rpm in second than 2,000 rpm in third.
Actually, I've found that once I'm down to about 2,000 rpm in third I can pull the same hill at the same speed in second at about 2,600 rpm at less than full throttle - a win-win.
An anecdote or two: Once I looked at the inside of a transmission, totally fried, that hadn't even got out of the guy's driveway. Temperature was about zero and he got stuck in the snow. Proceeded to rock the car back and forth a bunch of times. Another time I saw a driveshaft snap when a guy did just one rock cycle on the ice - ran it so fast forward the transmission upshifted to third when he dropped it into reverse. Not something that will likely happen with a GMC!
The more you know about the transmission and how to operate it the longer it will last.
Gary Casey
It's important to read every sentence in that section carefully: While
they state the normal location of the temperature sensor as at the output
from the converter, most of the information, including the explanations of
the temperature ranges, and the fluid life table are base on IN PAN
temperatures. IIRC, the only place the converter output temperature is
really emphasized is in expressing how rapidly the temperature there can
rise during "stuck" conditions. I personally wouldn't worry about the
temperature until it rises in the pan. I've run with both locations
monitored, and usually the converter output is about 50*F above the pan; on
a HARD pull, that might rise to +100*F -- without the pan temperature
reaching anywhere near that level. Remember that the converter output goes
directly to the cooler, and the hotter the fluid, the more efficient that
cooler.
The information was probably written to cover most transmissions, and
merely repeated in the P-30 manual about the 4L80, which is VERY similar to
the TH400.
Ken H.
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