That's funny. I'm a carpenter by hobby, in my 12th year of building a huge 50 x 90 workshop with two basements and a tunnel. I had three careers; tv / vcr repairman for 37 years, mechanic for 16 of those years, (electrical and suspension and alignment specialist), and automotive teacher for 9 years.
I help a buddy who has two careers. He owns a body shop, and he builds houses. I'm way better than the average do-it-yourselfer, but what you can build in a day, no, ..., In an hour, I have to think about for at least a week. I'd go broke if I had to do it for a living.
As for your throttle position sensor, the same story applies as with the MAP sensor. As long as it reads between approximately.5 and 4.5 volts, no fault code will be set. However, the Engine Computer does more than just take each sensor's readings individually. It reconciles them with each other under various conditions. For example, the TPS could legitimately read over 4 volts indicating wide-open-throttle, and the MAF sensor could legitimately read very low air flow, by weight, entering the engine, indicating idle, but they better not happen at the same time. The TPS is one of the smaller players in the fuel metering calculation but it can cause a stumble. Usually the MAF is so quick to respond to changing air flow conditions, a TPS problem might go unnoticed. Nevertheless, disconnect him and see how the engine responds.
Here's where a clinker comes in. When an intermittent contact occurs in the TPS, the voltage on the signal wire will get "pulled up" to 5 volts by the Engine Computer. (To prove that to yourself, unplug it and look at the reading on the scanner or measure the voltage with a digital voltmeter. You'll find a 5.0 supply wire, a 0.2 volt ground wire, and 5.0 volts on the signal wire). That 5.0 volts is what sets the fault code and turns on the Check Engine light. GM used to have a lot of trouble with the light coming on for every little glitch, so now the computers are programmed to sit and watch the problem first. When the problem occurs again or for a certain number of times within a specified time period, it will finally set the code and turn on the light. Some glitches might not last long enough to see on a scanner. The vehicle's Engine Computer responds faster at analyzing glitches than the scanners do. That's where the snapshot, or recording mode, is helpful. To make matters worse, there are some GM systems that display a different reading on the scanner than what you will measure at the sensor. When a problem with a sensor is detected, the computer knows it can't count on its reading, so it disregards it and injects an approximate value, from memory, based on the readings from other sensors. In the TPS story, when it is unplugged, the signal voltage will default to 5 volts, but the computer knows the engine is at idle, so it may inject a 0.5 volt signal and run off of that. The 0.5 is what would be displayed on the scanner. You would see the related TPS fault code and wonder why it is setting that one when the signal voltage is normal, Not all GM vehicles display readings on the scanner that way, but just be aware that could happen if your numbers don't make sense. I was told once that Chryslers do that too, but I've never seen it myself. If a number doesn't seem right, double-check it with a voltmeter right at the sensor's connector.
Sunday, April 4th, 2010 AT 9:59 PM