Question: I have a 1995 GMC Sierra truck with a 4.3-litre engine and 168,000 km. I recently had the transmission rebuilt. Everything seemed to work fine for a couple of weeks until it was shifted into passing gear, redlined and would not shift until the pedal was released. It never went over 4,000 r.p.m. before this.
The truck went back to the shop on Monday and they have been trying to find the problem. They keep saying it’s not the transmission and now they’re looking under the dashboard. They stated the transmission would blow if driven. I paid $2,000 for the work and can’t drive it. I’ve owned it for 20 years, I’m 82, and we are both still in good shape. I read your column and can really use your help.
Answer: Your 1995 GMC Sierra uses a four-speed automatic transmission that has the shifting controlled by computer-operated solenoids. The shifting problem with your transmission may be an electrical fault or it could be a hydraulic fault. Reading your email, I would assume the transmission went into passing gear by itself and was not caused by you flooring the accelerator pedal. If that is the case, then I suspect the problem is electrical.
There are two shift solenoids inside the transmission that operate in conjunction with the hydraulic shift valves. When the solenoid is energized electrically, it blocks the flow of transmission oil out a relief passage and this pressurizes oil on the end of a mechanical shift valve which directs oil to the correct clutches and serves to engage the desired gear. One solenoid can be off while the other is on. However, in third gear, both solenoids are turned off. This would cause the transmission to shift from fourth gear down to third gear and make it feel as if you were in passing gear.
When both solenoids are off and the vehicle is not moving, it would start out in third gear and the response would feel very sluggish. You can still manually select first and second gears with the shift lever, but putting it back in drive position would never give you fourth gear. It would remain in third gear until you manually selected a lower gear again.
From your description, it would seem the electrical connection to these solenoids has been broken somewhere. This could be in the wiring harness inside the transmission, it could be one of the solenoids themselves (although these can be tested with an ohm meter for the correct resistance), or it could be external wiring between the powertrain computer and the transmission. I would have the repair shop test the resistance of the solenoids when the transmission is at operating temperature. Sometimes these solenoids will test OK at room temperature but will have too high a resistance at operating temperature. This indicates a fault with that solenoid and it should be replaced.
The shop can also operate the transmission through the gears with a diagnostic scan tool to test external electrical connections. They will want to jiggle the wiring harness and connectors while doing this to see if there is a bad connection or broken wire. Usually a wire breaks beneath the vehicle because of the harsh environment, but a bad connection at the computer could also cause the problem.
Finally, check to see if the vehicle has had an aftermarket remote start installed. These are tied into the transmission power wires and if the remote start is acting up, it can cause the transmission fault you describe.
Question: I bought a 1998 Pontiac Firebird in good shape but it only has one ignition key. Do I have to go to the dealer or can I get a key in other places? I have heard the keys are very expensive.
Answer: The key for your Firebird has a small oval pellet inserted into the metal portion of the key that is a resistor. When you put the key into the ignition, contacts touch this resistor and the theft module compares the resistance to its programmed value. There are only 15 different resistance values and fortunately, these keys are not very expensive. Some locksmith shops can measure your original key resistance and cut you another key, or you can get one from the dealer, who also has the ability to cut it to fit your vehicle.