During the past couple of years that I've had my MX-5 I've noticed a slight variation in speed at low (or zero) throttle settings, and I decided that the problem must lie with the adjustment of the Throttle Position Sensor. I followed the clear instructions in the Haynes manual (the TPS is the later type post-1994) and, sure enough, there was permanent contact between the top two contacts of the plug with the 16 thou feeler inserted against the throttle stop.

Trying to adjust this, I rotated the sensor housing to the full amount in each direction, but still the contact remained unbroken. Simple, says Mr Haynes - just replace the sensor. What he doesn't tell you is that this simple moulding costs 3 pence short of 500.

It's no consolation to hear that these things rarely fail - "Never heard of it before" was the usual comment.

I haven't yet taken it out for examination, but can anyone suggest exactly what might have failed - is there a spring-loaded internal contact which might have siezed, or might the control spindle have jammed or sheared?

If so, would that be part of the sensor, or of the spring-loaded throttle quadrant which operates it? Would a good dosing of WD40 do any good? Mileage is under 18,000 so it shouldn't be badly worn. Any suggestions would be appreciated as to what could be done (other than spend 500). It would surely even be cheaper to have a replacement fabricated from a cannibalised unit than pay that sort of money! I've tried looking for a used part (trying all advertisers in the E&M classifieds) but nobody has one. (1/01)

As the result of a helpful reply from Malcolm Gray-Stephens, it has been found that the Haynes manual is incorrect, and those of you who have one of these books may care to make a correction to page 4-12.

In the Haynes manual they name the four terminals A, B, C, D from top to bottom. They say that the continuity test must be carried out between the top two, A and B. Only on 1993 models and before, they say, do you test between C and D.

However, I see that the moulded numbers on the plug are in fact 1 to 4, starting at the BOTTOM! And those with red and black/blue cable are the lower two, which are the wires to be tested.

So - I've repeated the adjustment process using the LOWER two terminals and it's possible to find the correct position. Result - no need to spend 499.97. Phew! I haven't yet road tested the car - if it still hesitates, I'll try the LED analysis with the gadget I've borrowed from a friend, to see if there's a fault code. (1/01)