Here's an update on the TB check out. The TB has a potentiometer that's attached to the air restricter plate. It has three leads. One is ground, one is B+ (a regulated voltage) and third lead is the wiper arm. If you unplug the jack and look that the harnessplug with the clip in the up position. The ground is top left pin, B+ is the right top pin, and the wiper arm is the bottom single pin.


B+/\/\/\/\/\/\/\/\/\/\/\/\ ground 5wiper arm

 

Steve,


Now that is some good information! I am suggesting you for teacher of the year. Honestly, I think you have nailed my problem and I cannot wait to check the linearity of my throttle body restrictor plate wiper arm. Steve, let me know what you find out too. I won't get at my Saturn until my wife gets off shift Sunday at 7:00. The lag is now well defined and an explanation exists. In addition, a troubleshooting methodology has been identified.

 

Steve,


I checked the potentiometer on the restrictor plate. It is a 1/4 turn pot. The outcome is exactly what you theroized; on the low end-what would be a high idle- the pot what intermittently steady at best. In fact, the pot would not post a reading every time for the first 1/32 turn or so. Further into the resistor, it was very hard to hold a steady resistance. Up into fast idle and then higher speeds the pot was easy to hold any value of resistance. I checked this numerous times with the meter aligator clipped to the plug prongs and the meter situated where I could read it from inside the car so all readings were from foot adjustment rather than trying to hold steady with my hand. I am convinced that the situation would be exacerbated at higher temps. The engine was cold for this test. I will replace the pot and reply. Thanks again for the excellent insight.

 

Ken, sounds good but you might want to check one more thing before you shell out any more money on parts that you don't need. On the B+ side of the pot, make sure that the voltage is not bouncing up and down. I'm not sure if Saturn does it this way but if the B+ sidevoltage where controlled by the other sensors (ECTS, MAP,etc) it would be a way of changing the fuel mixture. All of the numbersthat I'm giving are just made up for an example, this may or may not be true. You need to check it out for yourself. Let us saywhen the engine is cold the B+ is 9.25v and the wiper is reading 2.56v at certain position. As the engine warms up the B+ is dropping. Now lets say at 1/4 on the temp gauge the B+ is at 8.75v, the wiper arm voltage would drop to 2.40v at the same position. This would effectively give you less gas for the same amount of air. Again I'm not surethat they're doing it this way. The wiper voltage may being adjusted on the wiper side. The PCM is a mystery as to how each sensor is tied into each other to give the finale injector drive voltage.


I'm working on cooling fan "on" control voltages. Here's my method of plotting the control voltages. With the engine cold, write down what each sensor voltage is reading. Do it again at 1/8, 1/4, 3/8 and 1/2. The two wire sensors are thermistors (ECTS) Theydrop their resistance as they get warmer. The one wire sensors (O2) are also thermistors that use chassis ground as one end of the thermistor. Here's where it can get screwy. The PCM has an eprom that has all of the sensor voltages programed in. The epromtells the PCM, is sensor should be reading this when this other sensor is reading that and ifit's not, make up a default reading. It's a house of cards, pull this card out nothing happens. Pull that card out and the whole thing collapses. What you need to do is go down toRadio Shack and pick-up some pots to substitute them into the circuit where the sensors are. 1k and 10k would give you a good range of resistances.

 

Your words underline the reason I hate the auto underworld. This underworld controls the good information. Every voltage at the PCM should be known for a given set of conditions. This way, someone such as myself would not have to go fishing for answers by reproducing the actual running scenario. The problem with replicating voltage levels with radioshack parts is twofold: One, it's just as likely I will get a bad new part and not be able to hold the value of voltage precicely at the inputs; and two, I have no assurance the voltage levels I produce would be required at the PCM. I am happy to know that the pot is not linear in the most used region. I am not above installing a psuedo resistor in parallel with the existing one in order to increase resolution just to see, but likely I will just install a new trimmer and watch. At least I will be replacing a known bad part even if it's not the final solution.

 

Yes, I agree with you. Just get it fixed. But if the time comes that it is necessary to do further trouble shooting, the sub pots come in handy to hold sensor voltages at a constant level. If you where to take a resistance reading of the ECTS cold (282 ohms), dial up 282 on the 1k pot and sub it into the circuit, you can hold the PCM temp sensor stable to check other signal voltage changes. I'm an electronics tech by profession and sometimes it becomes necessary to break down a system into sub-systems. Normally the sub-systems will have feed-back loops to other sub-systems. One component can appear to to bad when it's an other's feed back causing theproblem. One time I had a Jagthat had a problem with the delayed entry system.After 3 days of having all of the under dash electronic pulled out, I noticedthat the right rear stereo speaker wasn't working.I pulled the back seat out to find the ground on the speaker was shorting to chassis ground. I fixed the spk problem and that took care of the security systems failure.Um, now that makes sense, a bad spk ground will cause the delayed entry not to activate.I'm guessing that the DE cpu was doing a sweep of all protected devices and not getting a green light to activate. The reason it's a guess is because manufactures won't release detailed info on their electronic packages. Your lucky to get a block diagram. At one time I was able to get the firmware for certain devices that I was making programed changes to. Now when I call a manufacture for firmware, the response is "when hell freezes over you'll get it"

 

sounds like one of a few things. plugged exhause but it shouldn't matter too much hot or cold. coil packs are bad for corroding (white powder on the teminals under the plug boot)and plug wires are bad for causing a major lack of power and a bucking feel under acceleration hot. Also the egr valves can give a rough run at cruse but normally not to the point where the car is undrivable.keep me posted

 

Well, enough time has passed to report that the throttle position sensor was indeed the problem in my '98 sl-2. The problem was that there was about a 75% decrease in power at low speeds (around town, etc.) but highway travel was normal. The TPS was not sending the appropriate voltage levels to the computer at low speeds which was attributed to TPS failure at the low end of the potentiometer which is also the most used area.

 

Its been a while but I finally found out that my computer was reading my ECTS wrong. I tried everything that was suggested but it didn't help. So i took it to a mechanic and they found it was my computer. PCM. I replaced it and it works great. Thanks for the info everyone. I appreciate it.


~Scott

 

are you still using the original computer? if so put in a dohc computer