TPS sensor problem

[bdearnley]

I have a 1999 subaru wrx with an SMC pnp ecu. I have recently tried to install an Innovate thermocouple amp to log each cylinder's exhaust temps (with Innovate software) and apart from the problems with that not working intermittently, I now have TPS problems with the Autronic. It reads 5% when fully closed, and when I open the throttle it goes to 0%. I have tried doing a TPS learn, and it worked for a few seconds but then the same thing occurs. Occasionally it reads around 55% if the throttle is slightly open, say about 10%. So I removed all the wiring from the Innovate Amp install and the TPS is still playing up, even after another learn. What have I done!?

[Wroom]

Could it be something that you connected that fried the trace in the TPS?

[bdearnley]

What do you mean by 'fried the trace', and how do I fix it?

[Ian]

What do you mean by 'fried the trace', and how do I fix it?
If you had the lid off the ECU with the wiring loom plugged into the ECU there is a Battery supply to the ECU and it is very easy to short out on the bottom of the case. The loom plugs must be removed before opening the case.

[bdearnley]

I haven't opened the case, but I was splicing into the ecu 12v power supply wire for power to the Innovate box. The car still runs OK apart from the TPS signal, so I haven't completely screwed the ecu... just don't know how to fix this TPS signal.

[Wroom]

What do you mean by 'fried the trace', and how do I fix it?
If one has happened to put voltage, even shortly, to the wiper connection of the TPS when the trottle is closed and thus the wiper is close to the ground connection, then the current flowing through the resistive trace in the TPS will burn a hole around the wiper contact in the resistive trace, or in worst case it will burn the trace right off.

I'm not saying that this is 100% sure what has happened to you. But i have seen a bunch of defect TPS, and in many cases they behave like you describe.

If it is the TPS then the only solution is to replace it.

[bdearnley]

Wow. OK, I'll get a new TPS and see if that fixes it. I don't quite understand how this could have happened when I was just mucking around with wires in the passenger footwell, where the ecu is mounted, and not the TPS wire anyway. All I was soldering into was the ECU power supply wire. Could it have happened if the 12v wire was accidently grounded briefly before it was connected to the Innovate box?

[Ian]

Go to M0 - Monitor All screen and watch the Throttle volts as you slowly open the throttle. If you can report what happens we may be able to work out what is wrong.

[bdearnley]

OK, I'll try this after work today and report back.

[bdearnley]

It reads 0.00 volts closed, and 1.72 volts fully open. When the throttle then closes again, it goes down to about 0.23 volts then gradually sinks down to 0.00 over about 10 seconds. This doesn't sound right... am I up for a new TPS?

[Wroom]

It reads 0.00 volts closed, and 1.72 volts fully open. When the throttle then closes again, it goes down to about 0.23 volts then gradually sinks down to 0.00 over about 10 seconds. This doesn't sound right... am I up for a new TPS?
It should have been 0.5 +/- 0.3 Volts closed, and 4.3 +/- 0.3 Volts fully open.

Where there any nonlinearities in the voltage when you measured now?

Can you measure the voltage between the supply and ground terminals in the TPS connector with ignition on?
Should read 5 Volts.

The TPS share supply with the pressure sensor. Have you connected something to the pressure sensor?

[bdearnley]

Voltage between supply and ground in the TPS connector is approx 4.7 volts. I have connected nothing to the pressure sensor.

I just did another TPS reset this morning and it looked fine (0.6 v closed, 4.3 at 100% open) but when I started the engine it went back to zero volts.

I haven't tested the linearity yet, but from memory there is 0.00 volts until it is approx 50% open, and then it reads linearly up to 1.72 volts. I should do what Ian said and slowly open the throttle while checking the voltage at M0.

I find it strange how a TPS reset will work for sometime, but then as soon as I start driving the car it becomes faulty again.

[RAC]

Is the coolant temp and air temp sensors still reading the right values?

[bdearnley]

Hey Ray, good to see your name on here :-)

yep, everything else was reading fine. I just got a second hand TPS today and the problem was still there. So I had a bright idea and unplugged/replugged one of the ecu connectors (the blue one on a 99 wrx) and now the tps signal behaves normally. 0% is 0.43 volts and 100% is 3.74 with a nice linear increase as I open the throttle. I would have expected full throttle to be showing over 4 volts, as I am getting 4.7v with a multimeter at the tps connector, but it works anyway. Must have been a dirty pin at the ecu or a fractured wire due to all my fumbling around trying to solder in these Innovate boxes.

I still don't quite understand why I was getting any voltage at all at 100% open when the problem was there...increased resistance from dirty pins??

Ben

[RAC]

All good then, now you can get on with your egt tests.

[bdearnley]

Yes, keep your eyes peeled on the 22B forum Projects section for some screen shots when I finally get it all done.