I was going to post this in the tranny section but there is no section for the 700R4 tranny there. I just put a rebuilt 700R4 transmission in my 1991 GMC K2500 with a 6.2 diesel and the torque converter won't lock up. I recently had a code 21 (TPS voltage to high) so i changed the TPS and got rid of the code hoping that it would solve my the problem with my torque converter not locking up but it hasn't. The service engine soon light is still on but the only code that comes up is code 12, which from what i have read means nothing. How can i go about testing to see if there is a signal going to the tranny telling it to lock up the converter? Any other information or ideas on this issue would be greatly appreciate.

Thanks
Will

Not many 3/4 tons have the 700-R4, '91-later usually have the
4L80E.

One of the pins on the ALDL connector is tied to the TCC solenoid. Can't remember right off, I'll dig around for it.

Code 12 indicates the ECM/TCM isn't getting an engine RPM signal.
This code will set whenever you pull codes as the engine isn't turning but the engine speed sensor could still be bad.
Edit: This would cause the TCC to not engage.

Thanks for the info,
If the speed sensor is bad than wouldn't my tach. not work?
Where abouts is the speed sensor on the engine?

Thanks
Will

IIRC the tach operates on a signal from the alternator. The ESS is on the rear of the engine, where a distributor would go on a gasser.

I changed the engine last summer and the orignal engine never had a speed sensor in that spot but the engine i put in did but i just replaced it with the piece that was on the orignal? Is there possibly another spot where it could be? The service engine soon light has been on since i changed the engine but the truck has never had a problem with the torque converter not locking up. Is there any other possible problems that would stop the torque converter from locking up?

The transmissions can be wired differently internally. The wiring can involve a gear switch and a pressure switch. Supply 12V to the wiring on the transmission, and it "decides" internally if it can lock the TCC or not. The TPS and the brake switch can cut the 12V supply and force it to unlock.

I don't know in which way the PCM is involved in locking the TCC for a 700R4. Maybe you're being mislead and trying to track down an electronical problem because you're seeing an error code --- while it's a totally different issue, to be found inside the transmission? Or do you have a 4l60E?

Can you confrim weither it is a 700R4 or a 4l80E??

it is a 700r4 tranny for sure, i just rebuilt it.

how can i check to see if the voltage is getting to the tranny , before i drop the pan and replace the pressure switch and everything else involed with the wiring.

0lee

do you have a diagram of the manual TCC setup you put in your truck. It would be greatly appreciated.

Check to make sure your brake switch is still working, sometimes they fail, so that the brake lights aren't on, but it still interrupts the TCC.

is there a way to by pass the brake switch to see if that is what is causing the problem

You could always go the toggle switch route, sorry I can't help you with that I haven't done it myself yet.

4L60E transmissions didn't debut until 1992.
4L80E transmissions debuted in 1991.
I can't think of any 3/4 tons that came with a 700-R4 but GM did do some silly things back then.

IIRC, the 700-R4 has a four position connector on the side. Functions vary, (thanks GM) but pin A is usually the TCC solenoid pin. As Lee said, internal wiring varies as well and some have a 3rd+4th gear pressure switch or fluid temperature switch in series with the solenoid.

how can i check to see if the voltage is getting to the tranny , before i drop the pan and replace the pressure switch and everything else involed with the wiring.

You could check the wires that go into the transmission to find out under which conditions they get voltage --- but before you know how it's wired internally, that won't tell you much as long as it's not working as it should.

0lee

do you have a diagram of the manual TCC setup you put in your truck. It would be greatly appreciated.

I only have a crude diagram on paper somewhere I made for wiring up some relays so that the TCC is unlocked and stays unlocked when I apply the brakes.

I have only two wires going into the transmission. With the lever in 4th gear and 12V on one wire (blue one), the TCC locks up. With the lever in 3rd and 12V on the blue wire and ground on the other wire (white), the TCC locks up. Switch off either ground or 12V, and it unlocks in 3rd. Switch off 12V, and it unlocks in 4th. I don't have a TPS.

Unfortunately, it will shift up or down while it's still locked. I'm always "balancing" on the pedal to prevent it from shifting down from 4th to 3rd while locked, but nonetheless it sometimes happens ...

I want a manual ...

I have a 1983 K20 Silverado that has a 700R4 from the factory, at least that is what I was told. If it's not a 700R4, what else would it be?

You could look it up on the glovebox sticker, if it's still there. Maybe you have a TH400 --- but the only way to be sure is to look under the truck and identify it by the shape of the transmission pan.

The label listing the option codes is still in the glove box and the code for the 4-spd automatic is "MX0". How can I translate that and the other codes? The trans pan is rectangle but almost square, if that makes sense.

Your key word is "RPO code": http://home.fuse.net/corvette/rpo.htm:

MX0 Merchandised Transmission, Automatic O/D Provisions

That's not very helpful ... Maybe it's for Corvettes only, but you could find a better site. A TH400 doesn't have an overdrive. And there are charts showing the different pan shapes like this one:

http://www.transmissioncenter.net/PanShapes.htm