Not achieving 5 volts at sensors (TPS/MAP/IAT), is this an issue?
 

I have been having a sticking idle problem lately. When the car comes to a stop, once in a while (more often lately) the car settles at 1500 rpms for 30 or more seconds and the finally settles down to a normal idle of 950. The throttle body mechanics and throttle cable were fine, but the TPS is not within spec. With the throttle closed, it is perfect, but it would only get a max of 3.8-3.9 volts at WOT.

After removing it, I tested the harness connection and only saw a max of 4 volts. The manual (section F32) states that I should see 5 volts. I would think that the TPS would never get up to 4.8-5.0 volts at WOT when the supplied voltage is only 4 volts.

Then I tested other sensor connections that are supposed to get 5 volts (MAP, IAT) and I saw 4 volts on both of them as well. The car only has about 7k on a brand new factory harness.

Is this an issue? Has anyone seen this before?

Thanks.

I have seen very close to 5 volts on mine.

What kind of ECU are you running? If it's stock, I'd trade for another stock ECU to see if the problem goes away.

I am getting 4.00-4.02 consistently. Running a PFC.

The grounds and the harness all look good and clean... is it really an ECU issue?

You should NEVER see 5 volts. when operation, lets say 1-5v. Normal voltage you should see if 1.1-4.9v (example) But if spec is 4.8v-5v. And you have 4.01v and you made sure your grounds are good, then your ecu or SOMEwhere within the circuit there is a draw.

-AzEKnightz

There is a 5v precision reference bus sourced by the ECU. It is used by the TPS, MAP, and a few other things. It should be very, very close to 5.00V on this rail when referenced to its associated ground. If it is not, then you either have a problem with the ECU, the harness wiring or something loading it [the 5v bus] down. One way to isolate this is to unplug the harness and see if you achieve the +5v at its corresponding ECU pin. If you don't have the correct 5V reference, then you will definitely have problems. (Also check the harness at the ECU connections for corroded pins where the wire crimps are, paying special attention to the grounds.)

As mentioned above, the variable outputs on the TPS may not get to the full 5v depending on its adjustment; however, there should be 5v present at all times on one leg of the sensor.

Be sure the voltmeter you're using is sufficiently accurate to make this 5v measurement. Many inexpensive testers are not. If you're using a Fluke, Beckman, or similar professional meter, you should be OK.

Thanks for the help.

Can you guys reference me to a good schematic of the ECU pins? I looked in the manual, but may have missed it.

The wiring diagrams should have all the info you need. All the pins have a letter/number combination assigned to them, and you can look at the pinouts of the connectors that plug into the ECU to determine the position of the pin. As I recall, these letter/number codes are also on the big wiring diagrams as well in the square representing the ECU.

Just remember that the illustration of the connector (front view) is different from the orientation as it plugs into the ECU (rear view).

your WOT TPS voltage is not going to cause a sticking idle, whether you have a reference voltage problem or not. Further reducing TB air (air adjust screw under TB elbow, throttle plate adjuster screw) and performing an idle learn procedure is your best bet for the idle problem.

So the drawing on the bottom of the page is supposed to be the ECU connection? I feel pretty dumb after looking for 1/2 hour through the electronic manual :).

I agree, but maybe the sensor is off at lower rpms as well. I don't know if it's good because I can't really test it. Once I confirm it's ok, then I'll try the learn procedure and TB adjustment. Thanks :icon_tup:

Yes, it is. It's a picture of the terminals if you're looking at the ECU with the wiring coming at you.

Don't worry - it took an hour of poring over the "how to use the wiring diagram" section to really get it at all.

David

So after diconnecting my ECU and reconnecting, I got 5.01 volts. I don't even want to know why :). I adjusted the TPS and it's now reading the correct volages. I'll play around with the idle tomorrow.

Maybe a slightly bad or corroded connection. It takes very little resistance to lose a volt.

Dave

The connectors looked real clean, but there may have been some dirt in there or something. I'm just glad this was one of the easier problems. Thanks for your help.

One thing that surprised me was that when checking the connectors on the PFC I was getting about 9.18 volts relative to the ground connecting the ECU. I was expecting 5...