av8rdude007

Recently my 85 automatic started stumbling under a load, ie during acceleration, climbing inclines, etc, very much mimicking a possible (TCC)torque converter issue. No SES light so I check for stored codes and got a Code 24. Cleared codes, drove it again and a code 24 reappeared. I am not having any IP cluster, dash or speedometer issues, everything works, it's just the torque converter failure like symptoms. I tested the TCC and tapped the brakes several times at 60mph and got a 200 rpm rise immediately, so I know the TCC is engaging and disengaging thru the brake switch. Am I correct in thinking that this VSS issue could be an issue downstream and after the IP and/ or at the ECM as the speedometer is working? Or could it actually still be the sensor? Can the VSS system thru the ECM affect the TCC to the point of stumbling under load?

Any common issues? Suggestions? Thanks

I have the factory shop manual.
Recently replaced the thermostat to a 180 with new water pump
no vacuum leaks detected
TPS checked and verified

MrWillys

The VSS signal runs to the dash first, and then is converted to a 2 pulse square wave sent to the ecm, and cruise.
See if this helps:
http://www.chevythunder.com/vss_code_24_sd_199092.htm

Cliff Harris

The TCC is controlled by the VSS signal from the dash to the ECM. If the TCC is locking then the VSS signal is getting to the ECM. On my car the ECM locks the TCC when the speed is higher than 40 MPH. It's possible that the signal is intermittent.

You can monitor the TCC lockup by probing pin F on the ALDL connector, referenced to ground (pin A or whatever is convenient). It works "backwards" -- there is 12 volts there when the TCC is OFF.

av8rdude007

Thanks MrW and Cliff.

So, more than likely it's between the dash and the ECM, or the ECM/prom itself? This would be wire 437, the Brown and White off the Instrument Cluster at D12 then Brown at A10 on the ECM, correct? Which connector, 24 or 32?

Any idea how this may affect the Park/Neutral switch as well? I had an intermittant code for the PN Switch this summer, cleared the codes and it hasn't returned. Still just the Code 24.

I'll keep this thread updated as I continue to troublesoot this weekend.

Cliff Harris

It's the brown wire on A10 on the ECM. The two connectors are: A & B are 24 pins and C & D are 32 pins. A is one row and B is the other. Same for the C & D. The pin numbers are molded into the plugs (not the sockets -- those are marked on the PC board).

The Park/Neutral switch position is adjustable. Maybe it's just on the edge of being in the wrong position to work correctly. I had to adjust mine because the backup lights only came on if the shifter was at the rear in Reverse. If I pushed the shifter forward (still in Reverse) the backup lights would go out.

av8rdude007

Just an update, I was washing the car and the underside just the other day. All symptoms of the stumble are gone. Perhaps its a short at the VSS. I cleared the code this am. I'll keep you informed if and when it comes back.