Here's the scenario. It's raining, it's dark, and your 20v/vt won't start... and there's a red light on the dash.

Curses, you cry, the only fault that prevents starting *and* shows a red light is a cam or crank sensor fault - I wonder which it is? If only I had some sort of test box I could plug in...

Coincidentally, I've just demo'd the first prototype of such a device - it plugs in between the crank sensor and the loom connection and flashes an LED as every pulse is received. It doesn't affect the ECU and it can't tell you anything about the crank sensor (except that if the cam is working, and you need to sort the crank out!) - it's purely to confirm that the cam sensor is putting out pulses.

I'm thinking of a small metal box - about 1" by 1.5" by 3" - and a couple of cables to the loom, long enough that you can position it to be visible from the driver's seat. I expect the price to be under forty quid.

Is there any interest? Is it worth me knocking a few up?

(As an aside: I have heard - but do not know for certain - that in the case of a cam sensor failure, the engine can *sometimes* be fooled by injecting a fake pulse on the line. I suspect I can include something along that line as a possible 'get you home' emergency start...)

I would be.Why not.

Cool. Would have given me something for that 20 vis I called you about a while ago. In fact it still would, as it's still sitting there.

sounds good Neil .

Ah, the infamous "flashing LED" trick to fool the cam sensor.

But I'm not sure if you mean the crank/rpm or camshaft/phase sensor? Which one does it test? Isn't the cam sensor signal easier to check than the crank/rpm?

The blue cam sensor connector IIRC is easier to get to (behind manifold approx in line with oil filler cap) than the crank sensor connector.

The thing with the cam sensor is it's only there to help the ECU send the sparks at the right time, in phase with the right stroke of the cylinder, to make sure it's not 1 engine revolution out of sync. Much like the wasted spark system on the 16v/vt, I've always thought that the ECU could use just the crank/rpm sensor to send out sparks, guessing if eg: when cylinder 1 is on compression stroke, send a spark, if engine doesn't start within say 2-3 more revs, then switch to the other phase since cylinder 1 must have been on exhaust stroke, then after 2-3 more revs if it still won't start, switch on the injector light and stop (or just keep going and if it starts, switch injector light off).

Not a perfect system, but not bad as an emergency stand by, is that what you had in mind?

It tests the cam phase sensor; that's a hall effect sensor which can be monitored without affecting the input to the ECU. The crank/rpm sensor is a variable reluctance sensor which outputs a sine (ish) wave and is a lot harder to give an unambiguous yes/no on.

What if there is an earth or power problem and the cam sensor is actually OK.

Hmm. I'll think about a good way to indicate that. It's still probably not going to be able to check that the earth connectivity is correct, though; some things you need an auto electrician to test!

Sounds good to me. 8)

The only issue with tricking it:

If it's doing it's job because your timing is out and you start it up and destroy your engine as very almost happened to me a few weeks back.

I think you'd be safer having a "is it the crank sensor" and if it's not then find a specialist.

I had considered that... haven't decided which way to go yet.

I'd be interested in this purely as a diagnostic tool. Would rather forego the 'get me home' option for the reason voiced by Big_Muzzie above.