[Mgs] Intermittent miss

[PaulHunt73]

The CEI system is very good, so I'd be looking for an intermittent 
connection before replacing it without doing a proper diagnosis.  You could 
replace it and still have the problem, which would annoy me (having just 
bought a new external disc drive which had the same problem as the old one, 
then paid #50 to 'fix' a USB problem, only to discover for myself that it's 
probably a faulty hub, that the external drives weren't even plugged in to).

The question is, does the tach drop because the 12v supply to it has been 
lost, or because it is no longer receiving pulses from the ignition?  If the 
former then it can only be the relay, ignition switch, or connections around 
them.  If the latter then it could be the ballast resistance, coil, 
amplifier, trigger or connections around them.  If other ignition powered 
circuits are affected as well then you know it is the former, if they 
continue to work normally then it is the latter.  The temp and fuel gauges 
are thermal and very slow moving so the power would have to cut for a second 
or more before you would see anything on those, so you need some other more 
instantaneous indicator.  In the past when I've had an intermittent problem 
I've hooked up a meter or test light in the cabin connected to various 
points in a circuit to track down a problem.

PaulH.

----- Original Message ----- 
> My 1980 MGB has developed and intermittent miss.