Tps will not stay calibrated!!!

[r1rilley]

Ms2 v3.57
Im currently going insane trying to figure out why I cannot seem to maintain a stable tps signal. I've had the problem for a while and chose to ignore it but I just can't anymore. At first it was real jumpy but I've fixed the ground and ran it to the sensor ground on pin 7 and it cleared up a little bit but now every time I calibrate it the adc count continues to drop ever so slightly just sitting there. So if i calibrate it it will start at 0% and slowly decrease into the negatives and my full throttle percent will also drop respectively (0-100, -2,98). I currently have the throttle body and sensor with pigtail off the car and sitting in the car with wires going from the pig tail straight to the ms wire harness. If I hold the throttle body half way open I get a solid number that doesn't move or barely moves if anything. I haven't ruled out a bad sensor yet. Also ADC counts are high. Closed is 560 and full throttle is 747. Any help is appreciated, thanks

[trakkies]

I'd check with a stim. If it does the same, you have a problem with MS. If not, the TPS or wiring. You can check the TPS to some extent with a DVM set to resistance. Open the pot very slowly, and the resistance should change smoothly. A pot with a worn or dirty track will cause the reading to jump suddenly from the correct resistance to a much greater one, then come back down as you move it. It is most likely to be worn at low throttle openings.
Also with your DVM, check Vref at the pot to ground, That should be approx 5v.

[r1rilley]

I don't currently have a stim readily available to check with. I have tried every wire combination just in case (only 6 ways of variation) but I know the wiring is correct but still have a decreasing adc count. Could a battery that is losing power when the car is sitting in the on position cause this? I have checked vref and it has a steady 5v I also checked the signal wire as I opened it and it also had steady readings as I move the throttle in VOLTS but i have read that may not mean anything. My main issue is how come the ADC count drops and will not remain constant.

[trakkies]

Checking the volts vary smoothly as you open the throttle is as good a way to test as measuring resistance. Did you do this by back probing the DB37?

If the battery volts were going low enough to effect the regulated 5v, you'd likely have lots of other symptoms.

A Stim should at least isolate the fault to either the MS or TPS and wiring.

[r1rilley]

I checked with a DVM by probing at the signal and ground at the tps and moving the throttle. It seemed smooth but I'm not sure what readings I should be getting or even if that matters as long as it is smooth. the 5v always reads fine I just know something is whacky since the ad counts are no where near where they should be. I will try to get ahold of a sim and might borrow another tps just to see if I get the same result. Im leaning towards a problem in the wiring, not sure what I've moved grounds and had no changes, or faulty tps. I don't see how the ms unit could be at fault i have never changed anything inside besides pots for ignition.

[trakkies]

A TPS never goes fully to the end of its travel. Always some 'spare' to allow for wear etc in the mechanics of the throttle. So it won't go from 0 to 5 volts.
Maybe 0.5 - 4.5v or whatever. MS doesn't care where it starts and finishes as the calibration takes that into account, but it should change smoothly as you open the throttle.

[r1rilley]

Interesting...when i probed them last time i was receiving around 0.3v to 1.5v or something along those lines when moving the throttle from open to close. I will have to check again to make sure

[r1rilley]

I take that back. I am getting ~2.41v at closed throttle and 3.37v open. The weird thing is I could see the voltage decreasing as it sat there closed. It started off at 2.5v and slowly decreased and ended up settling at 2.41v until I moved the throttle some then it went down to 2.37v. AGH sounds like a bad or worn TPS imo.

[trakkies]

This is using a DVM? I'm racking my brain to think what could cause a negative reading. is this a normal resistive pot? Or some clever electronic version?

[r1rilley]

Yes using a dvm and it is an e36 variable tps which has been used on e30 setups countless times so I know they are able to work. I am going to try another tps and see what I get I think the part is worn or is loose inside. Voltage increases and decreases very smoothly still though.

[trakkies]

Sorry - read your post as a negative (-) rather than (~)

I'd expect a worn pot to jump from the correct reading, rather than change slowly. It's where a Stim would help isolate if the fault is your TPS/wiring, and not the MS itself.

[r1rilley]

Got it New tps and she calibrated perfectly. Thanks for you help! Off to tune

[trakkies]

Got it New tps and she calibrated perfectly. Thanks for you help! Off to tune

Result.

It can be difficult to check a TPS for wear with simple equipment like a DVM.

A lateral thinking way is to rig it up as a volume control between an audio source and amp with speaker. A worn or dirty one will make the audio break up or crackle when moved.

[r1rilley]

Ill have to remember that! good idea to test with, thanks again!