June 2003 Articles & Features
Humans and machines alike use what is known to fathom what is unknown. When the known becomes suspect, the unknown is likely to remain so.
For Your Reference
I am working on a customer's 1991 Pontiac Grand Prix, which has the 3.1L engine. The Check Engine light is on and Code 22 is stored in the PCM's memory. This code indicates a problem with the throttle position sensor (TPS).
With the ignition on, I tested the reference voltage at the appropriate TPS pin with a digital multimeter (DMM) and found only 1.37 volts. After disconnecting the harness pin for TPS reference voltage at the ECU, the DMM read the full 5 volts. I've checked the TPS for a bad ground or a TPS signal wire that's shorted to ground and found no problems. The MAP sensor also receives a 5-volt reference signal from the PCM, but the PCM hasn't stored any codes for the MAP. Do these symptoms indicate a bad computer? Some ideas would be appreciated.
Fort Pierce, FL
On your customer's Grand Prix, Code 22 will set if the engine is running and the PCM sees a TPS voltage of less than 250 millivolts (mV) for three seconds or more. The TPS provides a voltage signal to the PCM that changes relative to the throttle opening. Under normal conditions, the TPS signal voltage ranges between 290 and 980mV at idle to about 4.8 volts at wide-open throttle.
The first thing is to make sure you're actually measuring the TPS reference voltage at the proper terminal on the TPS. On this sensor, it's the A terminal. The wire color from the PCM is gray. With the ignition on, you should read 5 volts on your DMM at this terminal. If you don't, check for a voltage drop in the reference voltage wire between the PCM and the TPS. Here's how: Use one of your DMM probes to backprobe the reference voltage pin at the PCM (pin C12). Use the other DMM probe to backprobe terminal A at the TPS. Turn the ignition on. The DMM will now indicate the total voltage drop between these two points.
Ideally, the voltage drop should be very low (less than .1 volt).
If the 1.37 volts you saw at the TPS was measured at the correct terminal, that means there's a 3.63-volt drop in the reference voltage wiring between PCM and the TPS. It's certainly possible that there's a nearly complete break in the wire, corrosion or other damage that's causing this voltage drop. Follow the wire from the PCM to the TPS, and pay special attention to any harness connectors or splices along the way.
If the reference voltage wire were shorted directly to ground somewhere in the harness, the entire 5 volts would be dropped before it ever reached the TPS. So you probably wouldn't have seen the 1.37 volts at TPS terminal A, unless it's only a partial, relatively poor short to ground. Likewise, if there were an open in the reference voltage wire to the TPS, not even the measly 1.37 volts you recorded would have reached it during your test. As you mentioned, the MAP sensor shares the same buffered 5-volt reference signal from the PCM, although the MAP sensor has a separate terminal and wire connected to the PCM (terminal C7). If any of the circuits that depends on reference voltage is shorted to ground, reference voltage to all will be affected and Code 22 will set.
There's another possibility I'd also like you to consider. Perhaps your 1.37-volt TPS reading was taken at the TPS signal terminal (terminal C), rather than the 5-volt reference voltage terminal (terminal A). The TPS is a three-terminal potentiometer that modifies the 5-volt reference to tell the PCM the engine's throttle position. With a slightly open throttle, the TPS could read in the neighborhood of 1.37 volts.
The PCM has an auto-zeroing ability built into its evaluation of the TPS signal. As long as the closed throttle reading is within the range of 290 to 980mV, the PCM will use the lowest recorded reading as its closed-throttle indicator. This allows the PCM to adjust to minor changes in the operation of the throttle plate and TPS, without the need for a TPS adjustment. Problems can arise if there's a carbon buildup in the throttle housing that keeps the throttle plate from closing completely. When this happens, the TPS signal to the PCM may not be within the range stated above, even though the throttle has closed as far as it can. If your customer's throttle housing is really gummed up, it's possible the TPS signal could read as high as 1.37 volts at idle.
Before you go down that road, verify the 5-volt reference signal at the TPS. If it's out of spec, check the TPS wiring for opens or shorts to ground. If everything looks good with the TPS and its wiring, isolate and check each of the other sensor circuits that rely on a 5-volt reference signal from the PCM. There are at least two others-the MAP sensor and the a/c pressure transducer. Good luck!