Service engine soon

[todesengel]

light just came on, it is throwing a code #21, throttle position sensor circuit high.

Anyone had this issue before? I can only wonder if this may have something to do with my idle issue :icon_scra

[tomtom72]

I never had a 21 but the FSM has a diagnostic path to use to check it. I guess it could cause a higher than normal idle if the ECM thinks that the blades are open and they actually are not.

The simplest way to do one check is with a VOM. KOEO, throttle closed, probe TPS with VOM at dark blue ( ckt 417 ) to read V output.

closed throttle V range = .46 to .62 volts. If it's out side the range you could try to adjust it back into range using the mounting screws. They go into slots which allows for some adjustment. I would guess if it's high & you can't get it back down then the next check I would do is on the Black wire ( ckt 452 ) that is the sensor ground. If it's open it will set a 21. Or maybe check the 452 wire first for a bad ground, then check the output V?

I hope this helps some?
:cheers:
Tom

[todesengel]

Thanks Tom, that made perfect sense. The light just went off the next time I started it. Seems to be running quite well, as the little c6 vert can attest to :sign10:I never had a 21 but the FSM has a diagnostic path to use to check it. I guess it could cause a higher than normal idle if the ECM thinks that the blades are open and they actually are not.

The simplest way to do one check is with a VOM. KOEO, throttle closed, probe TPS with VOM at dark blue ( ckt 417 ) to read V output.

closed throttle V range = .46 to .62 volts. If it's out side the range you could try to adjust it back into range using the mounting screws. They go into slots which allows for some adjustment. I would guess if it's high & you can't get it back down then the next check I would do is on the Black wire ( ckt 452 ) that is the sensor ground. If it's open it will set a 21. Or maybe check the 452 wire first for a bad ground, then check the output V?

I hope this helps some?
:cheers:
Tom

[tomtom72]

I know that I'm not fluent in electronic engine control technology...so I have to ask did the SES just not reappear on the next start cycle?

If yes. I would offer this from the peanut gallery. I used to get a lot of Cam Sensor codes when I first got my car. I had read up on "how to start an LT5" and knew all about the holding the key over to avoid the back fire deal. However, I was getting tired of this constant cam sensor code so I changed my "starting procedure" to putting the key over to run, stopping there long enough for all the DIC lights to come on, and then going to the "start" position. I haven't had a cam sensor code since.:dontknow:

I did the scanner thing & found nothing. I took apart every engine room connector and used some dielectric on the gaskets and the pins, took apart all the grounds I could find and did the star washer + dielectric thing.....still until I changed my "Starting" procedure I was still getting the cam sensor code. :icon_scra

[todesengel]

Tom, thanks for the reply. Yes, it just did not reappear inthe next starting sequence. I am not sure if these things are prone to phantom codes, but it was certainly stored in memory for me to extract.

I know that I'm not fluent in electronic engine control technology...so I have to ask did the SES just not reappear on the next start cycle?

If yes. I would offer this from the peanut gallery. I used to get a lot of Cam Sensor codes when I first got my car. I had read up on "how to start an LT5" and knew all about the holding the key over to avoid the back fire deal. However, I was getting tired of this constant cam sensor code so I changed my "starting procedure" to putting the key over to run, stopping there long enough for all the DIC lights to come on, and then going to the "start" position. I haven't had a cam sensor code since.:dontknow:

I did the scanner thing & found nothing. I took apart every engine room connector and used some dielectric on the gaskets and the pins, took apart all the grounds I could find and did the star washer + dielectric thing.....still until I changed my "Starting" procedure I was still getting the cam sensor code. :icon_scra

[Don in VT]

Hi All,

Cliff asked me to elaborate on my calibration of the TPS sensor on my wife's 90 ZR1. I have a lot of experience adjusting the TPS on my 87 L98 and I was happy to see that the voltage setting and procedure is the same.

First, I use a plug in interface harness that I purchased from Mid America Motorworks that make setting the TPS voltage a snap.

Procedure.

Install the MA interface. It will only install one way so it is mistake proof.
You should have a DVM to set the voltage correctly. Any good DVM is OK. I use a Radio Shack Auto ranging Digital Multimeter.

1. With ignition "OFF" disconnect the connector from the TPS.
2. Plug in the TPS jumper tool. Use caution as not to bend the connector pins.
3 Turn ignition "ON" engine stopped, use the DVM to measure voltage between terminal "A"(black wire) and "B"(red wire).
4. The voltage reading should be between 0.465 and 0.615 at closed throttle. Note: Your specific voltage may vary but I use 0.54v as my set point on our 90 ZR.
5. If reading outside of spec, loosen the TPS mounting screws (should be Torx #25) and rotate the sensor to achieve your particular specifications.
6. Tighten screws and re-check voltage setting.
7. Turn off engine and remove the jumper tool. Reconnect the TPS connector.

It is certainly possible to just pin probe the Red and Black wires with the DVM and it should work just as well. The jumper tool just seems easier.

I also have a poor mans field test for the TPS that I will pass on as a FWIW procedure if you are stuck somewhere and you suspect the TPS as being the culprit you can try the following.

If the engine will start and somewhat run (even with rough idle) Locate the TPS actuating lever and manually move it to a position about 25% open. If the engine speed increases with no dead spots (even though the throttle plates are closed) then the TPS is "probably" OK. If you hit dead spots or the engine does not increase in speed, then you need to consider replacing the TPS.

This procedure has worked for me 100% of the time on my L98 and it should be worth a try on the LT5 as well. I realize that the LT5 and the L98 intake systems are very different it should give an indication of the health of the TPS. Again, this is a FWIW procedure.

Regards,

Don in VT












Tom, thanks for the reply. Yes, it just did not reappear inthe next starting sequence. I am not sure if these things are prone to phantom codes, but it was certainly stored in memory for me to extract.