91 Gtp

In my 91 GP GTP 3.4l V6 i switched the motor and transmission (automatic) and with the new transmission it will not downshift when the pedal is floored. when the selector in in the overdrive position from a standstill if the gas pedal is floored the car takes off in about third gear and is very sluggish. if the selector is on the 1 the car takes off normally in first gear as i should. when at a cruising speed of say 35mph with the gear selector in the overdrive position if the gas pedal is floored the car does not downshift and simply increases rpms very slowly in 3rd gear.

Im wondering if this is an electrical problem? the new transmission is out of a 95 grand prix and the colour coding of the wires was completely diffrent compared to the cars stock wiring.

There is also no diffrence with the vaccum hose going to the transmission pluged or unpluged. there is also no diffrence with the electrical plug on the transmission in or out.

First, we need a bit more information.

1. Does your car have a floor shifter or column shifter (it makes a difference on the neutral safety switch)?
2. Did it originally have the 4T60 (ME9 non-electronic) or 4T60-E (M13 electronic) trans (check the SPID label in the trunk for the trans ID codes above)?

It has the floor shifter and i will check the number in the trunk shortly and post a reply

i just checked and both are the 4T60-E

First, so we're not speculating, what was the issue that led to the trans swap in the first place? Is it still doing the same thing, or is this a new/different problem?

2nd, Did you transfer the neutral safety switch from your old trans to the new one (as they are significantly different)?

Lastly, both the 91 and 95 versions of the 4T60E use the same TCC and PWM solenoids, so I don't see an incompatability issue there.

Let us know what the original issue was, and we will go from there.

The problem with the original transmission was that it was simply cooked and was slipping when shifting gears.

And yes i transferred the neutral safety switch from the orignal tranny to the new one.

Did you try adjusting throttle pressure? Are these tranny's adjustible? I swapped a tranny in a 1990 Buick with a 3800 and the trottle pressure had to be properly adjusted in order for it to shift at the correct speed/throttle position.

Have we done the simple checks like for pinched wires, disconnected / bad / melted harnesses, etc...?

As far as i can see all the wiring is good when idleing with a circuit tester there is two wire which have power going though them at the plug in.

the electrical ends are very slightly different between the original and new transmission both electrical ends are the same shape but the old one has two webs which new the transmission did not have so to make it fit i cut out the two webs. they are both the same pattern there was just a little extra plastic.

Sorry for the bump, but is there any other suggestions out there?

Did you check out the throttle boost cable that goes to the t-body?

the throttle boost cable is the one beside the cable which runs to the gas pedal correct? and it goes tot he hvac thing? if so as far as i can see the cables are all connected but the vaccum thing doesnt seem to do anything?

Your tranny might or might not have a t-boost cable. I dont think 4t60e's do to tell you the truth. But it is the cable thatruns from the top of the tranny to the throttle body (if its there).

I will see if I can find the trans wiring diagrams from a 91 4t60e and a 95 4t60e, to me it sounds like you have an electrical problem.

oh ok i know which cable your talking about and no there isnt one on this car. and much appreciated you looking up the wiring diagrams.

Did you check the final drive and most people forget to do this.If it is the wrong gear ratio,it will do this and the final drive has to be changed,the final drive from the old transmission to the replacement.There is a metal tag near the final drive cover with two big letters showing and and the third in the smaller letters and numbers.

where on the transmission should this tag be roughly?

I meant to say where on the final drive is the tag located but my edit button is not working. Also how difficult is switching final drives? is it just a matter of unbolting the final drive from the transaxle?

I am also curious as to how the final drive gear would cause this? Is it a compatibility issue with the computer?

The wrong gear ratio,it will not work.It is real easy to remaove,remove the cover and the final drive is held in with a c clip using two flat bladed screwdrivers being careful not losing the clip or using the special tool made by Kent Moore which is not cheap.Look for a white tag with 2 big letters near the final drive cover near the back

Is it the wrong gear ratio for the computer? im sorta confused but anyways i looked for the tag and i remember seeing it before but it appears to have been removed :confused: but the swap sounds easy so i think i will go ahead and do it. Swapping them out can only be for the better right? because im either going from the wrong final drive to the right final drive or the right drive to the right drive again right?

Yeah, but you will have to disasemble the trans to get the final drive out.

Hang tight on the wiring diagrams, i'll get them asap for you.

Here are the diagrams I promised you. There were two different ones for a 1995 GP, one with 3.4L and the other with a 3.1L. I got both of them because I didnt know if your tranny came from a donar car with a 3.1 or a 3.4. I also got a diagram from a 1991 so you can compare and splice accordingly. Like I said, to me it sounds like a wiring problem because your car is defaulting to "limp-in" gear when in drive.

1991 Pontiac Grand Prix GTP 3.4L V6 4T60e Trans Wiring Diagram (http://img399.imageshack.us/img399/7898/1991gp34ltranswiringdiasw5.jpg)
1995 Pontiac Grand Prix GTP 3.4L V6 4T60e Trans Wiring Diagram (http://img135.imageshack.us/img135/6204/1995gp34ltranswiringdiaeo1.jpg)
1995 Pontiac Grand Prix GTP 3.1L V6 4T60e Trans Wiring Diagram (http://img399.imageshack.us/img399/7780/1995gp31ltranswiringdiaql5.jpg)

BTW, after you click on the image, if you right click and click on Save image, you can save the picture to you computer and enlarge it to make it easier to read.

Good Luck! Let us know how it goes!

Thanks a lot for the diagrams!

Both transmissions are from 3.4L engines and the tranny actually came with a motor so i just put the 95 motor in aswell as the tranny.

I checked out the diagrams and it turns out both tranny actually have totally diffrent wires going into them:eek7:. Here is the list of the wiring someone please help me out here. Is it possible to cut and splice the wires to get everything to work?

91 transmission

A- Stop LT SW
B- 4th gear signal
C- 1st gear signal
D- Tcc signal
E- Ign fuse
F- Shift sol A
G- Shift sol B

95 transmission

A- 1-2 shift sol
B- 2-3 shift sol
C- Tcc pwm sol
D- Tcc sol
E- Tcc fuse (hot in run)
F- TFT sensor in.
G- Engine harness & OM from under hood electrical center breakout.


If anyone could tell me how to cut and splice the wires or if its even possible that would be amazing!

...I checked out the diagrams and it turns out both tranny actually have totally diffrent wires going into them:eek7:

Thats what I was afraid of.

I'm no expert at wiring, especially wiring modifications. Maybe some one else can help you out. Another possible solution might be to get the engine wiring harness and PCM from the 95 doner car.

the guy i bought the car and motor/tranny off of took the motor nd tranny out 2 years ago and scrapped the car so the harness is long gone =(

ahhh man!

Send a PM to Troy1, he's the GP tranny expert. He might be able to help you out.

I sent a PM to troy1 but he never responded =( i dont suppose anyone else would know of a way to splice the wires so i can at least get overdrive working?