Technical Tips > Transmissions - Differentials

C4 1990 Overdrive TCC Auto Transmission

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Vettech:
My 1990 colour glossy Sales Brochure shows the "OD" light lit on the Instrument panel.
But in no situation does it come on, even at start up when the car does an auto bulb check.
By comparison my 87 does and also lights up when in the correct situation the car goes into Over Drive and the TCC locks up.

Several persons have advised different ideas. But on a recent run down to Ulladulla etc - on the long stretches in OD, the car has gone into 4th but the revs dont drop as low as in the 87 when the TCC locks up - it's still doing over 2000rpm at only 100~110 kph.

I only have the GM Service & Electrical Suppliments for 87, but overall these should be OK and applicable for the 90.

In Chart C-8A, it appears I could have the following issues.
1. Brake switch on the pedal box open circuit.(O/C)
2. TCC apply Solenoid  O/C. (mounted inside the transmission).
3. 4th Gear switch inoperative (inside the transmission - signals ECM the box is in 4th).
4. A/T overdrive switch inoperative (mounted in the gear Shifter console).
5. TCC control driver (solid state switch) inoperative (mounted inside the ECM).
6. And at least four (4) sections of the wiring loom which may be O/C.
7. And connector/s down below on the box.
Ummmm puzzles puzzles - where to start, but carefully since this one at least runs, or as some one suggested I should just buy another C4....... ummmm an 88 maybe, in silver grey.........with everything done including the A/C.

OH! and number 8 - could be just a blown globe in the Dash !/?....

MY081:
2000 RPM at 100/110 Ks Absolutely not in overdrive (I would think ) Ray

gtc:

--- Quote from: Vettech on April 15, 2021, 04:06:35 PM ---But in no situation does it come on, even at start up when the car does an auto bulb check.

...

- could be just a blown globe in the Dash !/?....

--- End quote ---

Logical conclusion, methinks.

Despite that, you may also have an OD shift problem.

Vettech:
No GTC we are NOT having a tranny problem.

Vettech:
Um - well after a google and a read of that Chart C-8A 1&2 we found......
By placing a test lamp on "F" and ground lead to "A" of the ALDL our lamp is out untill the ignition is switched to on (dont start).
The Test Lamp them comes on, then with a depress of the brake peddle it goes out.
This confirms power from "IGN 1", "Gages fuse", "cct 39", "Brake Switch N/C". "15way IP Connector". "cct 420 down to transmission".
"thru transmission TCC Solenoid coil". cct 422 up to ECM and the "F" terminal on the ALDL.

So! if we leave the Test Lamp connected and drive it up to op temp, down the Hway into overdrive and the TCC Controll Driver in the ECM operates as it should, the lamp goes out and we should be in 4th gear overdrive with locked TC.

If NOT, why not...... - faulty ECM, 4th Gear switch RS or the actual TCC apply Solenoid is stuck or jambed.
And not to forget the A/T overdrive switch in the console may be faulty.

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