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MegaSquirt and TPS problem

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
I finally got the car running again and having some issues with MegaSquirt. I've had this problem before but not finding a fix.

I calibrate the TPS, and when the car is ON (motor off), it shows 0 at closed and 100% at WOT. But when the motor is running, it's showing ~10% at idle and causing the rpm to sit about 1300rpm. When I rev, it revs fine, but coming back down takes a while due to MS still thinking the throttle is open

The TPS is still entirely hooked up to the stock ECU. I have the 5vref wire hooked to the middle pin on the TPS. Should I fully disconnect the TPS from the stock ECU and wire up a ground from MS directly to it? The flapper adjustment screw is backed all the way out, and plate is fully closed, there is no tension on the throttle cable at idle.

Any ideas? Thanks
 

VelocitàPaola

Moderator
5,069
153
Jun 14, 2005
Ft. Lauderdale, Florida
Try replacing the TPS's power wire with the "TPS Vref" wire from your MegaSquirt harness. The fact that the sensor's reading is changing after the car starts makes me think the sensor doesn't have a stable voltage reference.
 

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
So completely cut off the stock wire? Right now the 5v is T-ed into the existing wire
 

VelocitàPaola

Moderator
5,069
153
Jun 14, 2005
Ft. Lauderdale, Florida
So completely cut off the stock wire? Right now the 5v is T-ed into the existing wire

You have the TPS Vref wire from your MegaSquirt harness tapped into the existing +5V Vref wire? If so, yes! Cut either the stock wire or the new one; having both connected is sure to cause problems.
 

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
I seen this after I worked on it some :(

I had the TPS wire from MS hooked up right at the stock ECU. I cut that, and re hooked it up right at the TPS sensor itself. Now, at idle the tps is reading 2-3% which is MUCH better than before. It's not revving on its own anymore. I will cut the existing 5v wire and just use MS and see what happens then.

I'm having the same problem with the AEM UEGO as well. It's off by 1-2afr at times, sometimes more. Seeing how the wiring is making all the difference, I may run a ground from MS to the WB instead of where ever the hell I originally hooked it up to.
 

#40Fan

Proven Member
392
2
Sep 18, 2009
Pueblo, Colorado
You only need one wire powering the TPS. Having both of them tied together will confuse things. You could just leave the PCM powering the TPS and hook the MS up to the return wire while the return is still hooked up to the PCM. This will allow the PCM to control the idle and cruise if you don't already have the MS controlling the IAC.
 

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
You only need one wire powering the TPS. Having both of them tied together will confuse things. You could just leave the PCM powering the TPS and hook the MS up to the return wire while the return is still hooked up to the PCM. This will allow the PCM to control the idle and cruise if you don't already have the MS controlling the IAC.

Actually the reason I hooked up the 5vref was because I was having really bad noise in the TPS when I first installed MS. The 5v cleaned it up some, but then I had to tear the car down and it sat for a long time and just haven't been the same.
 

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
Finally got the car running this weekend and got around to cutting the 5v wire from the ECU. Oddly, the car had problems starting afterward, and sometime wouldn't hold idle, which I found very weird. I reconnected, and it was fine.

I regrounded the WB to the same ground as MS, which is the (-) battery terminal at the moment, and got an exact on reading FINALLY between the 2. So I'm going to do the same to the TPS, coolant and IAT. All sensors are kind of shaky, so since the WB cleaned up, I'm guessing it'll help the others as well.
 

VelocitàPaola

Moderator
5,069
153
Jun 14, 2005
Ft. Lauderdale, Florida
I regrounded the WB to the same ground as MS, which is the (-) battery terminal at the moment, and got an exact on reading FINALLY between the 2. So I'm going to do the same to the TPS, coolant and IAT. All sensors are kind of shaky, so since the WB cleaned up, I'm guessing it'll help the others as well.

Absolutely, ensuring all sensors are using the same voltage as ground (as we all know, what's supposed to be 0V isn't always 0V in a car) is paramount to accurate sensor readings. Take a look here:

megasquirt:grounding [SymTech Laboratories]
 

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
I was always told MS needs to be grounded directly to the battery which I found weird. So ideally, I can ground MS to the chasis right where it's mounted right now, and just wire all the sensor grounds to that same spot, and it should work?
 

VelocitàPaola

Moderator
5,069
153
Jun 14, 2005
Ft. Lauderdale, Florida
I was always told MS needs to be grounded directly to the battery which I found weird. So ideally, I can ground MS to the chasis right where it's mounted right now, and just wire all the sensor grounds to that same spot, and it should work?

Yep, that would be ideal actually.
 

david-b

Proven Member
562
2
Aug 15, 2005
Chicago, Illinois
Guess I'll do that when I get a chance. Would everything being on the negative terminal cause a drain? MS has been there since I got it and never had a problem.
 
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