Engine/Transmission (1994 - 1998) Mystery Switch on my 1995 PLEASE read

[Howard Durand]

ABorchard,



That is an interestling circuit. I like the idea of a warning buzzer.

I think I'll hook up something similar to warn me if the switch is in the locked postiion when the truck is started.



Joe is right, I have noticed the LED's tell me that the TC is electrically locked but nothing has happened in the transmission yet.



I normally don't use the switch except for the exhaust brake and when I am towing I keep the OD locked out until 45 mph and hold the TC unlocked until 55 mph. After that, I let the PCM do it's thing.

 

[Joe G.]

I think you can use lockup in 2nd gear with the factory valve body.

It will lock in 2nd with the stock VB but it won't hold if you lift regardless of the switch position. You need an after market VB for it to hold the lock in 2nd.

 

[LDPetersen]

I too put in a mystry switch because of the lame TPS. I pull a 12k to 16k tralier all the time. Use the switch always. Have had several TPS's, I have a routine of upshift and downshift , the switch is just part of it. I go by the sound and seat of the pants. Also the tach. I wish there was a good fix for that TPS. They never seem to last more than 25,000 miles. The only thing I dont like about this truck. 95'CTD. Larry.

 

[CHardin]

I incorporated a cutoff switch activated by the brake pedal into the lockup circuit. This avoids the transmission shake if you forget to unlock BUT it will lock up again when you hit second gear (automatic transmission). For the most part, I have found the lockup switch to be of little value in ordinary driving, and it really does shake up the driveline when you forget. Charlie.

 

[Joe G.]

I would think that a cutoff switch activated by the brake pedal is not what you want if you are towing. On a down hill stretch you may help the e-brake with the service brakes. In that case you sure don't want the TCC to let go. I have a push button mounted on the floor to break the ground to the switch any time I want just by stepping on it.

 

[Howard Durand]

Quick question for Joe G,

Does your DTT transmisssion TC stay locked up when you release the accelerator, allowing the exhaust brake to work?

When I first got mine, it did for a while and now it is not. I have been messing with the TPS and think that is a factor. Have you done any experimenting with the TPS voltage? I know that as you increase the voltage, it will shift into OD later but am not sure that it has an effect on keeping the TC locked.



Howard

 

[Joe G.]

Howard,



I really dunno. I always have my TCC locked with my switch if I want it to stay locked when I lift. Never tried to see if it will stay locked for a while in normal operation mode.



Joe

 

[Howard Durand]

Joe,

Yesterday I raised the idle voltage on the TPS to 1. 2 volts and now the TC will stay locked when I release the accelerater. It stays locked until the rpm gets down to a little over 1000 or if the foot brake is applied. Works great with the exhaust brake.



Larry Petersen,

I am curious what your symptoms are concerning the TPS. I have the same one that came with my truck and in over 100K miles have not had any real problems other than early shifting, which I don't think is a TPS problem as much as it is a PCM problem. I tried to get the dealer to re-flash my PCM but it turns out that mine is one that can't be re-programmed. Have you ever checked on yours?

I do have a spare that I have put in a couple of times just to see if there is any difference and find they both work the same.

I made some radical changes to one of them so I could adjust the voltage to where it really should be. The max I could get with either one of them was about 0. 5 volt which is way too low. The one that I modified can now get up to 1. 5 volts.

Did you keep any of the ones that you replaced or just pitch them. If you kept them, I would buy a couple of them to play with.

 

[LDPetersen]

95' mystry switch and tps.

I went through the PCM routine 2 yrs ago at about 120-130k miles. Almost bought one and found bad wiring harness to tps. Soon after found tps bad. Dealer I had fix the 1st time replaced it free. Worked fine for about 20-25k miles. Then same thing again. Was 400miles from home and called local dealer and they called dealer where I was on a trip , they replaced it free. On a Saturday no less !! Worked ok till now. Same as before. I put in the mystry switch so I could always get home. The TC would lock and unlock every few seconds. I have run several thousand miles with tps being over ridden by switch. Just a little inconvinent but I will soon call dealer and try to go to the well 1 more time. Maybe I can get another one. If not I do have the last one they changed and will try to clean this one. I forgot how to PM so feel free to PM me . Thanks Larry.

 

[ndurbin]

Wish I could get my local dealer to replace the $300 TPS for free 2 or 3 times on a Saturday. Instead I just take my tube of Vasoline along when I need a new one.



Nathan

 

[LDPetersen]

TPS and mystery switch

Hi ndurbin; I called dealer again for 1 more try on free TPS. Dealership changed owners and said 1yr only on past repairs. Oh well was worth a call. I guess the well went dry. Larry.

 

[Jesselee]

just wondering i might have done something wrong mine will only lock up from a stop i thought that was the way it worked but you guys are talkiing about locking up after your moving. i used a switch with on off and acc

 

[Joe G.]

Jesselee,



The original idea was to provide a way to keep the TCC locked when decellerating so that an exhaust brake would work. The switch was turned on after the TCC had locked up normally to keep it that way. Using it for forced locking for drag racing is a good way to tear up a transmission, especially a stock one. If your setup only locks when it is stopped something is wrong with the way you wired it or the switch you used, or both. If it actually locks up when the truck is stopped it should kill the engine. I don't think a 47RH will do that. It will lockup in 2nd gear if there is a ground on the lockup solenoid.



You may not be driving it properly to cause it to lockup if you turn the switch on after it has shifted to 3rd. A 47RH does not work the same as a 47RE for lockup.