Thread: 2002 Pontiac Grand Prix SE - Idling 3000 RPM

Firstly, I've posted about a 1998 in other threads, but this is about my other car. A 2002 Pontiac Grand Prix SE.

It's now idling at 3000 RPM. When it first started, it was idling around 1500, but as I've tried to fix the problem, it seems like it's only gotten worse!

I've replaced the TPS and IAC. It's currently throwing a P1635 DTC, "5 Volt Reference 1 Ckt". That's similar to "General Failure Error" on PCs, it seems. Of course other DTCs show up as I have something disconnected, so I don't really pay attention to those.

To top it all off, if I disconnect the TPS, the idle is fine, it just has normal shifting issues, etc, that are a result of no TPS. It's very difficult to drive with the TPS connected due to the high RPM.

I've followed some directions I found on the internet about this DTC, check references voltage, etc. I've checked the new TPS and it seems to be doing fine. I've disconnected the PCM, and checked 5V reference for a short to ground, nope. Checked for shorting with the Low and High reference voltages on the EGR valve, nothing. I've disconnected the MAP sensor, and checked there for any voltage change when disconnecting anything that uses a 5V reference voltage. Everything seems fine.

The last resort is getting the PCM checked and/or replaced, and I know that's gonna be fun because the new one has to be reprogrammed, and I'm not really sure how to go about doing that.

Does anyone have any other ideas of things I should check?

Many thanks,
Robert

Did you check the voltage readings at the TPS? One wire should be 5 volts with the key on, one should be ground, the other (usually the center one) will be a variable voltage depending on throttle position. It should read very close to .5 volts at closed throttle and 4.5 volts at full throttle. The voltage needs to be a smooth change from idle to full throttle.

Have you checked for a vacuum leak? A common vacuum leak is the grommet where the PCV valve hose goes into the intake plenum just behind the throttle body.

I followed the directions on easyautodiagnostics.com. I was getting 12V from the ground wire you mentioned, and the signal wire was getting 0.5 to 4.5 smoothly throughout the range of the throttle. According to those directions, I also tapped on the TPS throughout the range of motion and didn't see any variances in the voltage.

I haven't really checked for a vacuum leak, I wasn't aware that it could cause the engine to rev to 3000 RPM when the TPS is connected, and when disconnected, would drop the RPM down to normal. I'll check for one and throw a vacuum gauge and see if the needle is steady.

Thank you for taking your time to give me suggestions,
Robert

You should not have 12 volts at the TPS. If you have 12 volts that is your problem. The 5 volt reference signal goes to the TPS, EGR, MAP, and fuel tank pressure sensor. Here is the factory diagnostic flow chart for P1635: p1635-1.gifp1635-2.jpg

Here is the wiring diagram for the TPS: TPS.gif

The only parts I buy from Autozone are name brand parts like fel-pro. Anything that says duralast I stay away from. They are not known to sell quality parts. You would be much better off with Napa or Carquest parts. Most professional technicians will tell you the same thing.

Just because a part is new, it does not mean it is good.

Ok, just so I know I'm doing this right, a couple questions about the diagnostic flow charts you provided.

Freeze Frame/Failure Records. Is this from an expensive piece of equipment that only professional techs have?

In step 9, it says to disconnect the PCM, turn on the ignition, with the engine off. Step 4 under that step says to check for short to voltage. What does that mean? I'm thinking checking the resistance between the 5V reference and ground or what?

When checking for a short, I'm essentially checking for non infinite resistance between, something and ground, indicating there's some sort of connection, correct?

These questions may seem simple, but I'm just trying to make sure I know what I'm doing, and not wasting my time.

Thanks again for your help,
Robert

Freeze frame and failure records are what you can view only with a diagnostic scan tool, like a tech 2.

Checking for a short to voltage is checking for battery voltage on that circuit. In this case the circuits you are checking for battery voltage are the 5 volt reference (PCM connector C2 pin 33 Gray wire), MAP (PCM connector C2 pin 25 light green wire), and FTP (PCM connector C2 pin 55 dark green wire). So you connect the black lead of your volt meter to ground, and the red lead to each of these circuits one at a time. If you have battery voltage on any of them you need to find where that wire has melted or rubbed to make contact with a wire that has battery voltage.

A short is not always to ground. A short is where 2 circuits that should not make contact and making contact. So any 2 wires that has rubbed or melted together are shorted.

Here are pictures of the PCM connector:
PCM C2-1.jpgPCM C2-2.jpg

Ok, I followed your directions. I found no voltage on any of those pins with the ignition turned on, and the PCM disconnected.

I then went to follow the diagnostic flow chart. Not having the Tech 2 tool, I skipped that step. I checked the PCM and engine grounds, fine. I checked the gray wire at the EGR valve, 5V (all of them were within 0.02V of 5V) I checked the TPS, same voltage. The flow chart says go to intermittent conditions, I have no idea what to do with that, so I just went ahead to the next step. I disconnected all of the devices, MAP, EGR, TPS and FTP and the voltage never changed. The next step was essentially what you told me how to do, verify that the 5 volt reference didn't have any voltage when disconnected to the PCM. The next step, according to the flow chart is to replace the PCM. I'm hesitant to do that because following the flowchart I should have gone to intermittent conditions.

So, what's this intermittent conditions part? I know what intermittent means, and I know that's a pain to track down.


Also, I checked the computer, there aren't any other DTCs other than the P1635, and another about the TPS being disconnected. What I found strange, and maybe this is relevant, and I just didn't know it, is that I had a code in the computer that when I first diagnosed this, that indicated the after cat oxy sensor was bad, but it hasn't thrown that code since this problem started. I'd go ahead and change that sensor, but with this other problem it didn't seem to be a big deal. Was I wrong?

I also, and this blew my mind, had a DTC for the light bulb for the SES light being out. I had NO idea that there even WAS a DTC for that. I went ahead and fixed that one.

Thanks for your help,
Robert

Do you have 5 volts or 12 volts on the grey wire at the TPS? One of your post said you had 12 volts, now you say you have 5 volts? Does it idle high all the time? I would check the readings at the PCM with the engine running when it is idling high. C2 pin 33 should be 5 volts, pin 66 should be about .50 volts at idle.

I believe i was testing one of the other wires on the TPS, and got the 12v, today when I tested it, I only received 5V. Not sure what I was doing at the time when I got 12V, it may have been an invalid test.


It idles high all the time. If I put it in drive, the car can do quite a bit of speed without me pressing the gas pedal. While this is going on, the coolant temperature gets really high due to this high idle(normal, I assume).


I've checked the voltage at the TPS, and it went up and down with the throttle like it's supposed to. That was at the TPS. I also checked the TPS percentage on the scan tool and saw it go up and down without any "dead" spots.

As far as I can tell the TPS is working correctly. Yes, I also bought that at AutoZone and not NAPA.

Can the car run with the PCM disconnected? How would I test this voltage you suggested at the PCM otherwise? This sounds like a good idea, just not sure how to go about doing this.

Robert

You need to back probe the wires at the PCM. I use sewing "T" pins to do this, but you can use any small thin metal like a straight pin. I use small alligator clips on the leads of my volt meter to clip to the back probe pins at the PCM.

Ok, I went out to do as you instructed, but it also seemed like a good idea to check the values from the TPS again at the TPS. It was hot out, and before I roasted under the sun, I figured I'd check these out first, really quick. What I found seems really really odd.

At idle, everything plugged in, the TPS sensor signal was at 1.6V at idle. It was actually idling a little under 1K RPM, like normal. I revved the engine, and the value climbed like it should. At this point, the 5V was at 7V.

When I disconnected everything from the 5V reference, I measured 10V at the 5V reference. That seems odd.

I connected things slowly:

TPS & EGR = 9V
TPS & EGR & MAP = 7.4V
TPS & EGR & MAP & FTP = 7.1V


To me, that looks bad. When I checked without the car running, (Key on, engine off) everything was 5V.


Is this normal, and I should continue with the test you suggested? Or does this point to something more... diabolical?

Robert

Those readings are not normal. Normally the 5 volt signal is always within .2 volts of 5.00 volts key on engine off and engine running. Did you check the 5 volt reference circuit at the MAP, EGR, FTP, and PCM? Either the PCM is defective or something is sending extra voltage to that 5 volt reference circuit. You might want to try unplugging the alternator and recheck just for grins.
Here is a wiring diagram for the 5 volt reference circuit:
5 volt.gif

Ok, I was thinking those readings weren't normal. I got those values only at the TPS, while it was connected to the TPS. It would make sense to check the voltage at the others. I'll do that and disconnect the alternator and see if there's any differences.

Robert

I've given it a few days, and this is just plain wierd. The problem disappeared. Currently everything is plugged in, but the cover to the fuel pump/fuel pressure sensor is still off, but I can't imagine this fixed the problem. I was all ready to do as you suggested, check the voltage with the Alternator disconnected, but it hasn't occurred again. I cleared out the codes, and it's been a couple days. The original one, P0440 is in there again, but that's the one I had when I took it to emissions a month or so ago.

Does this something that could be the PCM or simply a wiring issue?

Robert

The rest of the story....

All of the sudden, the car wouldn't start. I had to have it towed, to of all places, Pep Boys. They said fuel pump. 700$ please. I said go ahead. They called back and said that didn't fix it, so they put the old one back, and said I had to take it to the Stealer. I had it towed to the dealer. They wanted $1000+, and it was DEFINITELY the fuel pump. I said, forget it, I'll do it. I changed the fuel pump, still didn't work. I connected the pump directly to the battery, it ran like it's supposed to. I checked for resistance between the fuel pump relay and the gray pump wire at the pump, it had a lot of resistance. So, I found the issue. I started to see where the wires went from the pump, found it connects to something in the floor of the trunk. I pulled it and noted a bit of water, then I saw the connector....

In the picture, you can see that the one tab is missing.. it came completely off and still in the other side....

Robert

Did you ever figure out the high idle issue? Was it the fuel pump issue? I am having the exact same issue with mine with the high idle. Thanks!