More on throttle position sensor - or best £4.60 invested

My GTA turbo has had very intermittent variations in idle speed and jerky progress off / on throttle. After finding a vacuum leak, cured sticking idle speed control valve and high resistance in certain connectors to main engine management sensors, it was mostly better, but not quite.
Like Paulrob who I spoke to at RATS open day, I looked at the TPS. The microswitches functioned, but there was high resistance when they closed - between 50 and 150 Ohms on a meter measured at the connector (disconnected). Also the time to respond from open to closed varied when manually operating the throttle. They operate as a mechanically operated logic gate to the ECU - Idle low resistance, part load infinite resistance, full throttle no resistance. If the poor thing doesn't recognise if it's meant to read high or low, it can't decide the right fuel/ignition.
I went off to Maplin to get 2 replacements. They really are identical, off the shelf, £2.30 each, and easy to fit. The sealed plastic cover on the TPS prises off cleanly through the plastic weld. It was sealed afterwards with silicone RTV adhesive/sealant. The most skillful part is to gently prise off the metal clips holding the microswitches into the location pillars. Go slow and carefully.

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Transformation in driving experience was huge and immediate, like driving a brand new car. That's why it is the best £4.60 investment.
I would recommend anyone with a standard GTA turbo to check their condition do the same if switch resistance is greater than zero (say 0.8 Ohm max). It's a miracle that the microswitches last as long as they do.

It made a huge difference to the way my car drove. Like you say, its probably the best value for money thing you can do.

[The Saint]

I have a band new one of these, somewhere in my garage....
Since I now only own an A610, and I think these are only fitted to the GTA's. I don't need ot anymore...
Must dig it out and sell it..

I notice the TPS has slots either side if the unit. Presumably to set its position correctly.
Hence is there a set up procedure ?

Also, is there any connections you can tap onto to check resistance without removal of the whole unit?

....... is there any connections you can tap onto to check resistance without removal of the whole unit?

If you follow the three wire cable form back from the TPS unit, quite close you will find a connector ..... disconnect it, and identify the three wires. one is Brown, one is Black, and the other is Yellow.
The Black is common to to both switches, so one meter lead on the Black, the other meter lead on either of the other two, manually rotate the throttle to check resistance across the contacts ...... then move the meter lead to the other lead, rotate again to read the resistance of the second switch.
I hope that makes sense.

Well this is interesting.
I disconnected the connector and put a multimeter across the black & yellow and black & brown.

With throttle resting I get a reading of 0.4 ohms on black & yellow and nothing on black & brown. If I then move the throttle slowly i get o/c across both blk & yllw and blk & brn.
I can only hear one switch being operated and not the other.

I get no reading across blk & brn under any circumstances.

Are we certain both switches are used as I know sometimes its easier for car manufacturers to share components 'as is' rather then modify them ?
For instance, looking at the photo above, I notice one switch has a lever but I cant see how the other switch is mechanically switched on and off.

Both switches are used according to the injection & ignition manual. There are two separate cams operating the microswitches. The one without the metal lever is operated directly by a plastic cam. It tells the computer that full throttle is being used, so enriches the air- fuel ratio. Unless you actually push the accelerator pedal close to the floor, it won't be activated. You moght not notice.
The idle to part throttle switchover is very noticible every journey.
Hope that is correct, as I am on holiday without reference manuals at hand.

OK, thanks. I'll get the wife to stand on the throttle inside the car and see if that makes any difference

Yes. Need to depress the accelerator fully inside the car to activate the second micro switch. Mine appear to be ok, but it's something to keep an eye on.

I gave your advice a try this weekend. The odd time I feel the low end pick up seems poor.

I found 0.8 on the idle micro and 12+ Ohms on the full throttle micro.

A trip to marlin with£4.58 in hand and job done.

I think it helped a smoother power delivery and easier start off!

The picture is before I stared, looks like the idle micro has already been done before.