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        General >> Problems >> Throttle Position Sensor (TPS) problem
        
(Message started by: davem on Nov 16^th, 2008, 3:53pm)

Title: Throttle Position Sensor (TPS) problem
Post by davem on Nov 16^th, 2008, 3:53pm

Cosworth 1995.
A bit of history. Car running perfectly. Went away on hols for 3 weeks. Didn't drive car for those 3 weeks plus another 2 weeks. So 5 weeks since started.

Starts 1st time (even after 5 weeks) throttle cable seems a bit stuck. An extra bit of pressure and its free. This maybe a red herring.

Problem now, auto gearbox wont change up while accelerating. Only changes up when I take my foot off the throttle, and even then, anything but very gentle acceleration will drop down to 1st gear. (or down to 2nd if "snow" button is selected)

I put the car on an OBD2 scanner (thenks to Paul - Pegasus) and sure enough the only code was TPS.
The exact code is P0123 T/PPSACHI

The OBD2 scan also shows 100% as a value for TPS, which Paul said was far too high.

OK, we then changed the TPS sensor (got one from a breakers) but the fault and code are still there, even after Paul cleared the code, it came back.

Next I did a bit of voltage measuring. Three wires on the connector to the TPS. The TPS is mounted vertically and I measured the following connector voltages (on the end of the wire) with the connector disconnected and igntion in position 2.

Top pin = ground
Middle pin = +12v (really strange)
Bottom pin = +5V

I expected the top pin and bottom pin results, but I didn't expect +12v on the middle pin, especially with the connector disconnected.

The way I understand it works is that +5v is supplied (from ECU) on the bottom pin, top pin is a supplied ground (from ECU) and the middle pin returns the variable voltage (varies depending on throttle position) to the ECU.

All this looks fine, but when you have +12V permanently on the middle pin, it sort of messes up the plan. No surprise the OBD2 scan shows 100% for TPS and a TPS error code.

Where is this +12v coming from? Should it be there? I don't think so.

OK. I checked the circuit diagram and the only items in this circuit were the ECU, Connector C110, the TPS and its connector (Connector C869)

OK, C869 is OK. I can see that and check it OK. I also checked C110 (one of the 2 large bolted connectors in the engine compartment, left hand side). I unbolted the C110 connector, checked it, cleaned it etc and refitted.

While C110 was disconnected, I re-tested the voltages. All pins the same, except the middle pin had now dropped to +5v (with connector connected - 0v when connector disconnected). This is what I expected it to be all the time.

Reconnected C110 and I still have +12v on the middle pin, and the same fault.

Yes, I tend to agree .... +12v on a pin that should read between 0.6v and 4.5v is a bit high. ;)

Any ideas anyone?

Title: Re: Throttle Position Sensor (TPS) problem
Post by gost on Nov 16^th, 2008, 3:58pm

Davem, you lost me on about line 3 ;D

Title: Re: Throttle Position Sensor (TPS) problem
Post by davem on Nov 16^th, 2008, 4:00pm

hahahahaha. Very good

Title: Re: Throttle Position Sensor (TPS) problem
Post by harry.m1byt on Nov 16^th, 2008, 8:47pm

I am a bit confused. Is it wired like this >

TPS -to- C110 -to- C869 ?

C869 being at the ECU?

Assuming the above is correct - If unplugging C110 and /or C869 brings the 12v down to normal voltages, then the 12v must either be coming from the ECU itself or there is a short somewhere in the wiring between the middle TPS pin wire and a 12v sourcing wire.

As a temporary, could you cut/disconnect the entire bit of wire at each end and bypass it with a new wire from TPS back to ECU?

Title: Re: Throttle Position Sensor (TPS) problem
Post by davem on Nov 16^th, 2008, 8:56pm

Hi Harry. Thanks for the input.

No, according to the circiut diagram it's wired ECU to C110 to C869. C869 is the connector on the actual TPS.

I guess it maybe a short or corroded wire somewhere. I would think between C110 and C869, but as you say, could be a fault on the ECU.

Yes, I thought about that bit of re-wiring but I think I need to prove the wiring between the 2 connectors first, before checking back to the ECU.

I will try it next weekend, if I can find the wire colour at the C110 connector end. I can then chop it at both connectors and run a new wire, first making sure that +12v is not present on the source C110 end.

Any more input appreciated :-)

Thanks