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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| Re: [GMCnet] trans temp sensor [message #153337 is a reply to message #153290] |
Sat, 17 December 2011 17:52  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Dan,
Let me see, brake pads:
EBC Sport Formula Brake Pads - (4-3005) Price: $104.90
Transmission overhaul:
Rebuilt Transmission - (A114) Price: $1,598.00
Seems like a no brainer to me! ;-)
Regards,
Rob M.
-----Original Message-----
From: Dan Gregg
Gary, thanks for the "simple" explaination. Even I can remember to pull it down to 2nd when the rpm drops much below 3 grand. I have
waited until I run below 45 mph. Maybe I need to downshift a little quicker. Never had any trans problems relating to this, but dont
want any either. We tow always and I monitor both the pan and t converter. Have only let the t converter temp scare me once. That
was pulling a hill on the back streets of San Francisco. Changed the fluid at Jim's and it was still pink, even after hitting about
360 degrees for 30 seconds or so.
Dan
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] trans temp sensor [message #153342 is a reply to message #153337] |
Sat, 17 December 2011 18:39 |
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WD0AFQ
Messages: 7111
Registered: November 2004
Location: Dexter, Mo.
Karma: 207
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| Robert Mueller wrote on Sat, 17 December 2011 17:52 |
Dan,
Let me see, brake pads:
EBC Sport Formula Brake Pads - (4-3005) Price: $104.90
Transmission overhaul:
Rebuilt Transmission - (A114) Price: $1,598.00
Seems like a no brainer to me! 
Regards,
Rob M.
-----Original Message-----
From: Dan Gregg
Gary, thanks for the "simple" explaination. Even I can remember to pull it down to 2nd when the rpm drops much below 3 grand. I have
waited until I run below 45 mph. Maybe I need to downshift a little quicker. Never had any trans problems relating to this, but dont
want any either. We tow always and I monitor both the pan and t converter. Have only let the t converter temp scare me once. That
was pulling a hill on the back streets of San Francisco. Changed the fluid at Jim's and it was still pink, even after hitting about
360 degrees for 30 seconds or so.
Dan
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Rob, we are talking about uphill. You are talking about downhill. I am with you. I don't downshift going down. I got some real braking power and like you say, pads are cheap.
Dan
3 In Stainless Exhaust Headers
One Ton All Discs/Reaction Arm
355 FD/Quad Bag/Alum Radiator Manny Tran/New eng.
Holley EFI/10 Tire Air Monitoring System
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| Re: [GMCnet] trans temp sensor [message #153356 is a reply to message #153338] |
Sat, 17 December 2011 19:54 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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""Gary,
I have "heard" that the transmission or torque convertor can "starve" for oil when pulling hard up a hill and you can fry the
transmission or torque convertor in a heartbeat.
Supposedly there is a "hole" that needs to be drilled out or some other mod to prevent that from happening.
Have I got bad hearing or is this info correct?
Regards,
Rob M.
"" I've never heard of that one Rob--very interesting. The highest temps I see are when I'm stuck in a traffic jam. I suspect the convertor is in total slip mode with very little cooling from the heat exchangers. I'm frequently pulling in the mountains here with max speeds WOT at times of 30 in first gear. Keeping the engine revs up keeps the convertor close to "hydraulic" lockup so the heat generated is less.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] trans temp sensor [message #153577 is a reply to message #153576] |
Mon, 19 December 2011 17:30 |
Jiffyjet2
Messages: 86
Registered: November 2008
Karma: 0
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I've heard of the drill a hole in the converter before too, but that was a turbo 400 back in the '70s. Oh yea that's my GMC isn't it? I don't recall what the issue was either but it seems that it had something to do with draining the old fluid out of the converter during a tranny fluid change. A one time event so the next time, it would drain properly with the hole already drilled. Dang, wish I could remember more.
Jess
'74 Canyonlands
"the Great Pumpkin"
Tacoma, Washington
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| Re: [GMCnet] trans temp sensor [message #153579 is a reply to message #153577] |
Mon, 19 December 2011 17:44 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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That sounds like what some have suggested: Drill and tap a 1/8 NPT hole in
the periphery of the TC for draining it. The suggestion was to do it in
situ. IIRC, very early Hydramatics were delivered with such a feature.
Ken H.
On Mon, Dec 19, 2011 at 6:30 PM, Jiffyjet2 wrote:
>
>
> I've heard of the drill a hole in the converter before too, but that was a
> turbo 400 back in the '70s. Oh yea that's my GMC isn't it? I don't recall
> what the issue was either but it seems that it had something to do with
> draining the old fluid out of the converter during a tranny fluid change.
> A one time event so the next time, it would drain properly with the hole
> already drilled. Dang, wish I could remember more.
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] trans temp sensor [message #153597 is a reply to message #153576] |
Mon, 19 December 2011 20:14 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""A converter builder that I talked to said that a small hole can be drilled. He called it "Line to Circuit" He explained what this little hole does, but it was beyond my pea brain comprehension. Maybe Gary Casey can explain in simplified terms?
"" Sort of sounds like a solution waiting for a problem.. Don't get me wrong, this is all very interesting but if I haven't blown my transmission yet it may not happen.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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