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Crank position Sensor and mis-fire

Black Beauty

Black Beauty

Member
I have an on-going issue with a mis-fire/back fire at the mo and I think I have found that it is due to the crank position sensor, but I am not sure how to interpret the resistance readings

When I test the resistance as per the Clarks guide, when the engine is cold the resistance is pretty much to guidelines as is the speed sensor.

However, after going out for a run, the resistance of the crank position sensor is a third of what it should be - what I want to know is, is this normal or should I replace it based upon this evidence.

The speed sensor resistance when hot or cold is relatively the same and well within spec.

Thanks
Spence

PS I don't know where I would be if it wasn't for the Clarks guide
 
The two sensors are identical so I would expect them to behave similarily. I think the one that changes resistance when hot is probably fubared.

Also check the dizzy cap isn't tracking (this can be very hard to spot).
 
There is no reason why they should change at all. The are only magnetic pick ups. You should get a new sensor mate.
 
If you're referring to the reference sensor on the top of the bell housing (assuming the Lux's have a similar set up to the turbo) that sits alongside the speed sensor then that is only used on startup and once the engine is running it serves no function so cannot be the cause of your misfire. When I was having reference sensor issues the car would sometimes not start, but once it started it would run fine. It turned out the sensor was fine, it had just loosened itself and so only needed a nip up with a socket.

If you suspect reference sensor problems you can always swap the speed and reference sensors over as they are the same part.

 
The DME may be able to cope with an intermittently faulty ref sensor due to slight misalignment. If the sensor has moved away from the teeth then the voltage spikes it produces will lessen. It may be the case that at low cranking speeds the voltage was sometimes too low to trigger the circuits in the DME. Once the engine speeds up the voltage output of the sensor increases as it is related to the speed of rotation. It probably had enough volts to reliably trigger the DME circuits once the engine was running. If the sensor is really damaged then who knows what signals it sends to the DME. I wouldn't rule it out.
 
have you checked to see if the misfire is associated with a particular cyclinder ?
i would do a simple test with pulling the spark plug caps off one by one and see if there is any significant drop of rpm with any particular cyclinder. Check the colour and deposits on each of the plugs also .
If in doubt do a compression test to eliminate broken valve springs/poor valve seats etc.

Its probably just a bad earth or HT tracking somewhere, Might be worth checking and cleaning your main earthing points also .Won't hurt anyway
 

ORIGINAL: sawood12

When I was having reference sensor issues the car would sometimes not start, but once it started it would run fine. It turned out the sensor was fine, it had just loosened itself and so only needed a nip up with a socket.

If you suspect reference sensor problems you can always swap the speed and reference sensors over as they are the same part.
Click to expand...

I have intermittent hot start issues, so this could be the cause of it.

I think I will replace it based upon your advice

Thanks
S
 
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