What function does the clutch switch serve to provide?

  1. My model of MX-5 is: NC __
  2. I’m based near: Leeds__
  3. I’m looking for technical help or recommendations on: Clutch switch function.

If I remove this switch what would the effect(s) be? It particularly annoys me that every time the clutch is pressed the revs automatically rise.

Surge dampening apparently.

I think the revs should rise slightly when you press the clutch due to reduced drag from the gearbox. Same as they reduce when the aircon kicks in


The switch is nothing to do with the rise in revs. It’s there as part of the traction control, ABS and cruise control.

So what triggers the rise in revs?

On this particular car the wiring is quite extensively modified and the abs completely removed.

Good question, perhaps a lack of drag from stirring the gearbox oil?

Back in my bad old banger days with the inevitable dodgy battery the engines were always easier to start if the clutch pedal was pressed to the floor.

Now it is standard practice with a lot of cars not allowing one to start them unless the clutch is depressed.


Something to do with ignition timing and engine load, it works in conjunction with the neutral switch. Clutch engaged and in gear needs different ignition timing to clutch engaged and not in gear.

It probably serves more purpose than just that but it’s 23:55 and I’m not in a position to look up all its functions right now.

Well all bets are off as to what it is if the wirings been messed with and parts removed.
You never asked that in your first post. Unplug it and see what happens is the answer there.
Also there are loads of workshop manuals available on line which have walk through procedures for faults, what does what and why, wiring diagrams etc.

I found when I had a 1958 Morris Minor that depressing the clutch slightly dropped the revs because the carbon faced release bearing caused quite some drag, unlike the modern ball race type. Therefore I always started it with the clutch engaged. Mind you, a mildly tuned (ahem!) A series with a large SU carburettor never failed to start whatever the weather.

I take your point, although I think there is still a valid discussion to be had.

As far as I’m concerned the increase in revs is 100% linked to pressing the clutch pedal. And the amount of the revs increase is far in excess of what might be attributed to any reduced load because the gearbox input shaft ceases to be driven.

So what I thought was a reasonable deduction is that the whatever is actually going on with the revs is triggered by the movement of the clutch switch plunger (I assume is has a plunger, rather like the brake pedal switch?) making or breaking a circuit, and that this change of state then got communicated back via the canbus to the ecu which then, for whatever reason, gave an instruction to the throttle body to open up a bit.

To be totally honest I can’t recall if this rev rise was present from Day 1 of ownership or only from the point at which the wiring was loom was modified. (This may seem a bit strange, but I’m actually the car’s co-owner and driving it isn’t what I do with it - so I’m not even sure I ever drove it prior to the wiring changes.) The wiring changes were undertaken by a motorsport professional and with total awareness that canbus systems have lots of knock-on interactions that may be far from obvious.

If indeed the the function of the clutch switch is only in relation to traction control and cruise control, neither of which systems are any longer functional on this car (the wheel speed sensors are disconnected), is there any sensible reason why this rev increase occurs? If cruise control was fitted and active at the time I assume it would disengage it? (I’ve never has cruise control on a manual car). But how would sensing a clutch pedal action contribute in terms of traction control?

What I think I will try is removing the switch from it’s current position and either discarding it completely or tying is off somewhere behind the dash. If anyone has any better ideas I’d be very glad to hear them.

From the Engine Control System Relation Chart, clutch pedal position switch is involved in…

Drive-by-wire control
Fuel injection control
ESA control (Electronic Spark Advance I think)
EGR control
A/C cut-off control


Thanks for all the responses so far.

Upon further investigation I now need to swallow a large piece of humble pie and admit that the rev increase appears not to be related to the depression of the clutch pedal (and therefore anything to do with the clutch switch), but is triggered upon selection of a gear (any gear, not just first gear).

So what is happening to cause this? Is there an electrical sensor that detects whenever a gear is selected/engaged? And if so, does it serve any useful purpose other seemingly to increase the revs? And is it deletable?

Sorry again for setting hares running by attributing the rev rise to the clutch switch.

Sounds very strange. The ECU doesn’t know what gear you’re in (unless it calculates it roughly by road speed) and there are only two electrical connections on a 'box. One for speedo, the other reverse light switch.
It’s not a wire being caught by the gearlever or something like that is it?

There is a neutral switch, it works in conjunction with the clutch pedal position switch to ascertain an in-gear state.

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Ok, that’s interesting. The (quite extensive) changes to the wiring system have resulted in the speedo ceasing to work, but my understanding is/was that was to be expected if the wheel speed sensors aren’t connected (abs delete and all that). And I can’t honestly recall if we still have a working reverse light!

Maybe I’ll simply have to put the rev rise phenomenon down to a canbus quirk?

Sorry, Robbie is right. The two on the gearbox are neutral switch and reverse light. Speedo comes from the rear N/S ABS sensor, this is why they’re so expensive.

I feel like I’ve been rumbled…lol

If you ‘blip’ the throttle whilst in gear and stationary then try ‘blipping it’ out of gear you will hear a sharper throttle response.
Going back to when I first raced a mk3 by bypassing the neutral switch on the box it allowed me to keep the sharper throttle when sat on the start line…hmmmmm #win

Nothing in the regs said I couldn’t lol


Great story!!

So would you see it as an advantage more generally (ie not just on the start line) to bypass the neutral switch?

Without incriminating yourself too much (more!!), did you effect the change by physically removing/isolating the switch or in the mapping? In a series like Supercup don’t they stipulate use of a control map?

I believe the neutral wire also connects to the hill assist if fitted.

Yes we did run a controlled map, but with the above mod it wasn’t found or identified as cheat

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