Why TPs don't work when wired backwards ??

[SotonS2]

Purely out of interest , I took an original Ford Throttle Pot and ran some tests on it. The graphs show the transition from throttle off to throttle max to throttle off etc. The higher voltages are throttle on.

The TPs have a three pin input. For orientation, with the TP spindle hole facing you, Pin 1 is left, Pin 2 is centre and Pin 3 is right.

The outputs depending on which way round the TP is wired are way different.

Wired 'backwards', its no wonder the ECU cannot work out what the heck is going on. Hence lumpy and erratic running.

Wired what I believe to be 'correctly', the output is smooth and consistent.

I can run the same tests on a Cerb TP (once I get it out of the S !) and publish the results if anyone is interested.

Correctly Wired


Wired Backwards

[GreenV8S]

Those voltage traces look very lumpy especially round about 0.2 Volts (right way round), is it working OK?

The throttle pot itself should work just as well either way round (its just a variable resistance) but obviously does need to be wired up the right way in order for the ECU to interpret the signal correctly.

[SotonS2]

GreenV8S said:

Those voltage traces look very lumpy especially round about 0.2 Volts (right way round), is it working OK?

I think so ! I don't think the signals are bad enough for the ECU to switch into LOS mode which is where mine seems to be all the time. Running very rich, cutting out at max throttle position, a bit jerky at constant throttle position etc. These symptoms remain irrespective of which throttle pot is in place - and I've tried three ! The original, another silver tipped Ford unit and a Cerb unit. The only one I haven't tried yet is the Ford gold plated one.

I am going to invest in a new Coolant Temp sensor as they are only about £12. I'm also going to check / replace all of the vacuum pipes. The AFMs and under plenum are all destined for a clean up at the weekend as well.

I'm hoping the running problems are not the result of a fried ECU. Apparently, at full throttle position, the ECU uses a different data set for the mixture. If this data set got bolloxed, then this could explain the abrupt cut in power when the pedal is pressed to the floor.

If I can't get it sorted this weekend, then I think a trip to Austec is looming.

Any feedback welcomed here guys.

[wonderland]

Hmmm.... i'm confused (it's not hard).

My S2 started to run like a bag of undesirable and reading various threads it sounded like a TP problem. Ordered a new Cerb 4.2, removed old Ford, replaced and car runs OK (with a touch of cleaner and a lot of WD40)

Unfortunately, about 5 months later now and it's going the same way again, so once again I've reordered another Cerb one. Last time needed no wiring mods at all and thank god because while I'm pretty competent with small jobs like this, you've all completely lost me with what needs rewiring and what doesn't etc etc etc. Can anyone post pics of what it looks like if it does need rewiring, what it looks like if it doesn't and the process if it is needed?

Sorry for moaning, but will someone clear this up once and for all - and particularly for the simple of mind...? Cheers!!

[wonderland]

Just as an additional thought, is it worth a trip to a rolling road tuning place after a new TP just to check all is OK? I'm having a new exhaust on Friday and should get the new TP fitted tomorrow or Thursday so was planning to book in a session for a tune-up anyway.

Wondered if this had ironed out any remaining problems for anyone??

[SotonS2]

wonderland said:

Hmmm.... i'm confused (it's not hard). My S2 started to run like a bag of undesirable and reading various threads it sounded like a TP problem. Ordered a new Cerb 4.2, removed old Ford, replaced and car runs OK (with a touch of cleaner and a lot of WD40) Unfortunately, about 5 months later now and it's going the same way again, so once again I've reordered another Cerb one. Last time needed no wiring mods at all and thank god because while I'm pretty competent with small jobs like this, you've all completely lost me with what needs rewiring and what doesn't etc etc etc. Can anyone post pics of what it looks like if it does need rewiring, what it looks like if it doesn't and the process if it is needed?

I think most of us are confused !!

The wiring mod consists of swapping the two outer wires on the throttle pot connector. Most, if not all, Ss will have had this mode done by now I would have thought. There is a replacement loom set from Ford for not much more than a tenner IIRC which makes it easier and gives you connector peace of mind at the same time. It may be worth investing in one of these.

As regards rolling road, Austec's £250 for a service and RR session seems good value.



edit - just noticed that you are nowhere near Austec ! Perhaps someone else can recommend a decent S tuning centre in Wales / the NW ?



>> Edited by SotonS2 on Tuesday 22 February 14:59

[chris_chim]

Not sure you are measuring the correct thing from the title on the plots. Pins 1 and 3 should have the constant voltage applied (i.e. 0v and 5v) and pin 2 (i.e. the middle pin) is where you take the reading.
When pins 1 and 3 are wired 'backwards' the volts will go from 12 to 0 rather than 0 to 12.
Any basic electronics guide should cover the difference between a variable resistor and a potentiometer (or voltage divider).

[SotonS2]

Yup Chris , I think I was being a muppet and relying on some incorrect info supplied (not from PH !) defining which pins were which - no excuses, I should have checked

Pin 2 IS signal. Repeated the tests on the Cerb TP this morning.

Got the following (longer sample and more throttle on/off transitions). Much more what I would have expected.



Hey, at least I know this TP is OK and I now have some sort of test procedure !!

[chris_chim]

This test doesn't necessarily mean the TP is ok.
All my dud Ford TPs perform fine in this sort of test but not on the car. I think the problem is heat but this has been covered at great length elsewhere so I will say no more...