5 replies to this topic

[Stevie W]

Evening All,

 

My Mini has recently been taken out of hibernation and had a new fuel tank fitted. The car started "on the button" warm or cold last year.

Whilst it now starts immediately (cold or warm) initially I had no spark at the plugs. During an inspection of the electrical ignition system, I found nothing wrong to account for the lack of spark.

 

Taking one plug out then asking my lady to crank the engine over suddenly resulted in a spark on the removed plug and the engine bursting into life unexpectedly! Whilst I'm happy that it now starts, I can't explain why there was no spark and then there was.

 

That aside, the engine runs perfectly but the rev counter has stopped working. It's an after market Smiths/Caerbont electronic unit which was working perfectly. I'm assuming the rev counter works by sensing the 12v pulse from the - terminal of the coil, turning this varying speed pulse into a DC voltage which moves the needle.

 

I'm getting switched ignition power to the rev counter and the earth is good and using my digital multimeter I seem to be getting a random DC voltage at the pulse input terminal on the rev counter.  

 

What should I be seeing? I'm aware that my multimeter won't respond fast enough to see the 12v/0v/12v pulse but even taking this into consideration, what I'm seeing doesn't seem right.    

 

Tomorrow I'm going to set-up at rough 12v pulse at the rev counter input under the bonnet (12v dc supply) by touching then removing the 12v from the input wire, surely if the rev counter is working, I should at least see the needle flick.

 

Any other ideas or suggestions welcome :)

 

Cheers, Steve.

[dklawson]

The test you propose may work.  However, if there is a pulse on the coil (-) terminal, you would also see a very low amplitude flicker on the tach needle while cranking on the starter.  The needle won't jump much off of zero but there should be a slight pulse or flicker.

 

While this should not happen on a modern tach like your CAI unit, tachs can go bad where their sense wire shorts to earth.  When that happens, it's like you have a kill switch on the ignition system which prevents making sparks.   Therefore, I suggest disconnecting the tach from the coil for a few days to see if all your starting/running problems go away.  If they do, I would be talking to CAI about how to test the gauge and/or what is required for them to troubleshoot and repair it.

[Stevie W]

Doug,

 

Many thanks for the info!

 

The needle used to jump slightly when the ignition was switched on and flick during cranking, this no longer happens.

Your theory about the sense wire shorting to earth does make sense and would explain the lack of spark initially.

 

I took great care when making up the original auxiliary loom for the tacho so hope that's not the problem. Since I'm getting some sort of signal at the "-" coil terminal with the engine running and the same at the tacho sense input, I'm confident the loom is ok. 

 

Might be time to look at a new tacho, although this one has only done about 5000 miles, so I'd be a little miffed if it has given up already!

 

Thanks again for your usual methodical help.

 

Cheers, Steve.

[Stevie W]

Just a quick update... 

 

I thought I was getting ignition switched power to the rev counter but the other night I was tired and it was getting dark. I think the power feed was intermittent as I couldn't see it all today.

Remembering I had placed the 3A fuse holder under the dash, I pulled it out to find the plastic fuse holder had cracked and the fuse was hanging out!

A new fuse holder later, and my rev counter now works!

 

Cheers, Steve.

[dklawson]

Great !  Thanks for posting your findings !

[Stevie W]

No problem Doug...and thanks for your help. Cheaper fix than a new rev counter!!!!

 

Steve.