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Discussion Starter #1
As some of you know, I recently bought my 1998 C2500 knowing it needed transmission work.( it would stall when you put it in reverse like it was going into drive and reverse at the same time) I had the transmission rebuilt including having the valve body replaced as it appears some one had drilled the old valve body to boost shift pressure. The transmission seems to shift fine although it will not shift into fourth until it reaches approximately 48 MPH. I assume this is normal? I have driven it for three or four weeks now and I have noticed that the TCC is not going into lock-up. I read on an earlier post that some trucks will not go into lock-up until operating temps come up.? I have noted that my temp never goes above 170. Could this be the issue? Also I am getting no check engine light codes. Will continued driving with out TCC lock-up do any harm to my transmission or will the fuel mileage just suffer? (I'm getting about 17.8 MPG).
 

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at about 140 degrees, tranny temp, TCC starts to engage. Get ahold of someone with a good scanner and have them check for codes. Sometimes tranny codes will set, but wont turn the light on.

If its just a something dumb not allowing the converter to lock then the only thing that will be affected is you fuel mileage. Have you looked around on the tranny to make sure everything is plugged in tight, no frayed wires, and such?

Thats all I can offer, Mike L seems to chime in here a lot, hopefully he'll see this and give you some more advice.
 

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A scanner with realtime data will show if the computer is requesting lockup or not, and if the trans actually has locked up or not. You need to rule out electricals outside the trans, such as a brake switch out of adjustment or bad. This will also inhibit lockup.
 

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Discussion Starter #4
Thanks guys, I'll try to find someone with a good scanner. I did my own test by tapping the brake pedal while driving at 55 MPH but the RPM's do not change at all. The brake lights are working when I tap them so I assume the brake light switch is not the cause.
 

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Not true. There are two different brake switches, one for the lights and the other for the trans. Sometimes they are both contained in the same switch, and sometimes they are separate. The LU switch is normally closed, the brake light switch normally open.
 

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Discussion Starter #6
Good info, thanks.
 

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Hey Marc, could a bad brake light switch cause me to lose overdrive? I just remembered having some trouble with my brake light switch shortly before the tranny problem (no OD) happened. It was getting stuck in park, only way to release it was to jiggle the wires under the dash near the column, was going to replace the brake light switch but it stopped sticking in park so I forgot about it.
 

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Nope. Only affects lockup. With a scanner, you can see if the computer is sending a lockup request or not. You can also see if the brake switch is working correctly.
 

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Just replaced my brake light switch and ECT. Still does the same thing, although I found if I don't manually downshift it into third I can manipulate the throttle and get it to re-engage and continue driving in 4th, but it feels like no lockup.
I needed to change the brake light switch anyway. Next thing is to go after the TCC solenoid tomorrow. I've decided I'm not putting a new tranny in until I've exhausted all options.
The fluid temp point makes me wonder. My gauge always reads pretty low, even working it I can't get it hot. I think I may have overfilled the tranny, would that possibly keep it too cool to get into TCC, or could overfilling cause some other kind of problem?
 
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