5 replies to this topic

[tj80]

It was all going too well!

 

Spent a morning sorting out the little jobs on my wife's "new to us" Mini ('91 Mayfair, 998cc).  I thought I'd fixed pretty much all of them, including the rev counter which wasn't working - but turns out I was wrong!    Next time I went to start it up it wouldn't!

 

The rev counter is part of the factory 3 instrument set, not after market.  The car had Accuspark electronic ignition and coil fitted by the previous owner - as part of this, it looks like he took a new +12v feed for the coil, bypassing the ballast resistor.  I found the original pink/white wire for the ballast resistor, and the white/black wire for the rev counter taped up in the wiring loom.  Connected the white/black to the coil negative terminal and - lo and behold - rev counter worked perfectly!

 

Here's where it gets odd.  I went to start the car again a few hours later and it wouldn't fire - rev counter not even twitching and no hint of starting.  Reversing what I'd done, I disconnected the white/black wire from the coil and the car started right up!  Reconnected it while the engine was running, rev counter started working and engine ran perfectly.  Stopped engine, restarted a couple of times with no problem with the rev counter white/black wire connected!!  Thinking it may have been a bad connection I left it connected.  Tried to start it again today and the same thing happened - no hint of firing until I disconnected the rev counter, then it started straight up!  This time I stopped the engine again straight away, reconnected the rev counter - and it started again perfectly!

 

I'm a total loss as to why this would be happening.  Is it something odd to do with the Accuspark?  What else can I check except for the quality of connections?  Looking at the wiring diagram the white/black wire only feeds the rev counter, it doesn't go anywhere else!  I wondered if it might have been to do with the ballast resistor, but then surely it wouldn't start with the rev counter connected at all.

 

Many thanks,
Tim

Edited by tj80, 12 November 2019 - 01:07 PM.

[sledgehammer]

at a guess - sounds like the rev counter signal wire shorting to earth

 

or the rev counter shorting internally 

 

this would probably stop the electronic ignition from sensing when to fire - hence no start

 

I would get engine running , and start wiggling wires about , you may find the problem that way

 

if not then I suspect something up with the rev counter / or maybe it has got damp

IIRC the circuit strip across the back of rev counter is for the dash light and earth -

 

the wires are +ve & ign signal on the back one bullet & one spade ?

 

as I said all the above is a guess

[Ethel]

The distributor initiates a spark  every time the contact breakers open.  I there's an earth connection for the -ve side of the coil elsewhere there'll be no break and no spark.

[tj80]

Thanks, both.  I did wiggle the connections at the coil with the engine running and rev counter connected, no problem.  I haven't taken the instrument binnacle apart yet, but I guess that might be the next step - if nothing else, running a temporary wire from the coil to the back of the gauge bypassing the wiring loom should show whether it's a fault in the wiring.  The odd thing is that with the rev counter disconnected everything is fine - so the basic wiring of the coil, electronic ignition unit, etc must be OK!

[sledgehammer]

Thanks, both.  I did wiggle the connections at the coil with the engine running and rev counter connected, no problem.  I haven't taken the instrument binnacle apart yet, but I guess that might be the next step - if nothing else, running a temporary wire from the coil to the back of the gauge bypassing the wiring loom should show whether it's a fault in the wiring.  The odd thing is that with the rev counter disconnected everything is fine - so the basic wiring of the coil, electronic ignition unit, etc must be OK!

 

That's what I would do

 

has the rev counter been stored somewhere that could get damp ?

 

at least you can use the car with the rev counter removed if necessary

 

as you may have gathered , the thin PCB on the back is a weak spot in the dash -

 

but shouldn't affect the rev counters function only the back light IIRC

Edited by sledgehammer, 12 November 2019 - 07:05 PM.

[tj80]

In case anyone else comes up against this, you guys were onto something when you said the rev counter may be shorting to earth!  It wasn't actually shorting to earth, but the resistance to earth was only about 500 ohms which I thought sounded a bit low and certainly could be affecting the electronic ignition module.  I added a random moderate value resistor from my electronics junk box (330 ohm half watt) in series between the coil negative and the white/black rev counter signal wire and it seems to have sorted it - the resistance is now high enough to allow the electronic ignition module to do its thing while still being low enough for the rev counter to see the ignition pulses.  No effect on the accuracy of the rev counter because it's obviously just measuring the frequency of pulses not the voltage itself.

 

Hope that helps someone in the future!  :)

 

Cheers,

Tim