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TCC Lock-up

bmxdkj

bmxdkj

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Question for anyone with a 3500HD truck and tranny knowledge.



I own a 1999 Chev 3500 HD, 4L80, 19.5 tires, and a 5.13 gear. At 65 mph I'm running around 2500 RPM. Pretty high I think. The TCC doesn't seem to be locking up to me, however I dropped it off at a tranny shop and the guys said it was reading it was locking up on the scanner. For some reason it just seems like the RPM's are way to high. However, I am aware the truck has very low gears, so if thats the case and how it is supposed to be, then I'm fine with it. But if it's off then I'd like to figure out whats wrong. There are no codes truck runs great etc. If the TCC isn't locking up, what should I check. I know its a long shot. I checked the FAQ's and it said the engine temp, brake switch, and speed. The truck warms right up to where it belongs (195 or so), obviously I'm getting to speed and the truck's brake lights are working. Also I've heard about PWM's in these transmissions??? Any input is appreciated. Even a point in the right direction in what to check is appreciated.



Also, this tranny shifts perfectly, no shudders, slips, grinds, burps nothing, shifts like its brand new. The truck also is far from abused and for a landscape construction truck it see's a pretty easy life. Thanks again!
 
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Engine RPM is correct on the scanner. Gasser pulley?? What pulley could be on the truck that belongs on a gasser?? The motor and tranny are all factory, nothings been changed that doesn't belong on there.
 
Quite often when an alt. is changed they have a pulley from a gas motor on it. The tach signal comes from the alt.

My guess is those are the correct rpms for the speed. Someone with a HD will chime in or you could PM "Brooklyn Tow", he has a HD
 
You know, the alternator was changed years ago. So are you saying that TC is engaging and the tach is reading wrong because of the alternator. Or are you saying that the tach is reading wrong causing the TC to not lock up. That is interesting, do you happen to know what size the pulley is supposed to be on the diesel. I can obviously measure the one I have to see what it is supposed to be. That is real interesting news to me....you learn something new every day!
 
The alt. pulley is about 2 5/8 in I think. If you have one from a gas motor 2 3/8 in your tach will read high.
 
NVW said:
The alt. pulley is about 2 5/8 in I think. If you have one from a gas motor 2 3/8 in your tach will read high.
Click to expand...

X2

That is the outer diameter of the lip, what you can easily measure. When getting the right pulley, it is basically a 62mm pulley, thats the diameter of the actual belt part of the pulley.

It will not mess with anything else, it only runs the tach. the PCM uses the Crank Position Sensor (CPS) in the timing cover to determine RPMs. With those gears its going to be high RPMs.
 
The ECM gets it's rpm signal from the optical sensor in the IP and the crankshaft position sensor. The alternator pulley size only effects the gauge in the dash.
The rpm you stated sounded about right for those gears and tires. To check if the lock-up is working tap the brake with your left foot while holding the throttle steady going over 50 mph. The brake will release the lock-up and rpm will shoot up at least 500 rpm. When you let off the brake the rpm should drop back down and feel like a manual transmission in that the rpm won't just flair up when given more throttle. Count the shifts when taking off from a stop to 55mph. There should feel like 4 shifts. Lock-up should happen in 3rd gear. The last shift should be overdrive unless you have it floored.
 
The truck def has 1-4, shifts all the way up no problems at all. The RPMs don't drop off once it goes into 4th. When I tap/hold the brake then let off, nothing happens at all, when I let off and get back into it nothing happens at all. I would obviously assume that I would feel the TC lock up, but I'm definitley not.
 
Bison, the truck is equiped with 5.13 gears. There is no codes stored or in the history on the truck. Is it possible to have a bad switch somewhere, that wouldn't throw a code and cause the TC to not lock up?
 
What size, height are the tires? The RPMs really arent off for those gears if the tires arent very tall.

You wont feel the TCC lock once you get into 4th, you will most likely be able to feel it in 3rd. So put tranny in 3rd and accelerate a fair amount. The shift to 3rd should come around 45-50mph and then it will lock, and it might feel like one long slipping feeling into the 3rd, but it will start shift and maybe 2 seconds later drop into lock. Then its already locked when you get into 4th.

A lot of people just think its the tranny slipping.
 
The tires are 225/70/19.5's. Tomorrow I'll take the truck and check it out. I'll put it in 3rd and check it that way. The only thing that concerns me is that when I step on the brake in 4th the TC doesn't unlock like it should be. Or as per bk95td had stated earlier in the thread.
 
Another quick way to see if you are locked up: Around 60mph (to insure you should be locked up) and light throttle input (cruising on level ground)... "goose" the throttle about 1/2". You should see NO increase in RPM. RPM should only increase as your speed increases.

I feel the lockup when in 4th gear if I accelerate gradually as I would be in 4th gear by the time I get to the 50mph lockup zone.

How's your fluid? Maybe that doesn't matter - just a thought.
 
The TCC valve bore wears in these transmissions due to lack of fluid changes and a PWM 99% duty cycle vs. just 100% lockup. The TCC slipping or not locking at all should set a trans component slipping code. A missing gear like the OD sprag failed would set a ratio error code. I digress and stupid computers may not code due to other reasons. Like posted above if you are not warmed up, for example, due to a bad sensor TCC lock may be locked out.

A ride with the trans shop showing each gear and TCC lock with RPM drops for each would be useful.
 
Update....I drove the truck...no lock up in 3rd, nothing in 4th as well. There are no codes at all. I'm really stumped at what to look at. The truck is warming up fine and shifts like theres nothing wrong at all. No shudders or anything. I don't know where to start looking. It's hard finding a decent transmission shop around here that wants to diagnose and fix the problem, most just want to rebuild it and thats all. I'd really hate to rebuild it if it could just be a switch somewhere....
 
NVW said:
Read this thread, in post #6 there is a chart for checking the coolant sender in the cross over.
Click to expand...

Leo, your link didn't show.
There is also a internal transmission temp sensor in the harness inside the transmission. It will inhibit lock up if it is sensing too hot or too cold. It should be able to be read by the scan tool . A scan tool should be able to command lock-up and shifts. Honest transmission shops are hard to come by. What brand of scan tool did the tranny shop have? Not all scan tools will work on a 6.5 diesel.
 
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