MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Ignition / electrical problem

I started having a problem with my 77 MGB the other day and I'm not sure where to start to fix it. I've had a Crane ignition installed for a couple of years now and it's been fine. The other day the car started up fine but after driving it for a while it started missing. If I keep the RPM's up it's fine but if they go down below 3000 it begins missing and will often stall. I thought maybe it was getting too hot and that the plugs could use replacing so I did replace them. The same thing still happened. It was only then did I make the connection that the engine would misfire when the cooling fans were on. As soon as it cooled down enough to shut the fans off the car ran fine. Any idea where I should start to look for the problem? Any suggestions would be appreciated.

Michael
Michael Powers

Michael, when the misfire starts unplug the fan,test drive the car, if it runs smooth with the fan unpluged, plug it in ,if the motor misfires with the fan running the fan is causing the problem,if you can't control the misfire by unpluging the fan the problem might be in the fan circut but It is more then likely heat related.If the misfire is controled by the pluging and unpluging of the fan look at the fan!- RIC
R E L Lloyd

Michael, I have a problem I thought was caused by a Crane system, see the Thread: Possible Crane XR700 ingnition failure. Your mention of the problem happening only when the fans are on makes me think your alternator may not be charging enough to keep up with the current demands. It may be a good idea to have your alternator checked. Good luck, Clifton
Clifton Gordon

Hi.

The two options that occur to me are :

1)The brushes on one of the fans are badly worn, and the fan is producing lots of spikes on the power feed, which are upsetting the ignition electronics.

2) The battery voltage is dropping significantly when the fans cut in (as suggested by Clifton).
While the alternator may be the culprit, I would expect a healthy battery to have enough reserve power to cope with the fans... does the engine run well when you have other large electrical loads (eg headlights) drawing current ?.
Some 'dodgy' electrical connections are a real possibility.. checks with a voltmeter would soon narrow the possibilities.

Don
Don

A 77 powers the fans and the ignition from the ignition relay. This could be developing high-resistance contacts so dropping the voltage (and getting hot), much like a low-output alt would. The battery would only be a factor if the alt was not up to the job, either because of a fault in the alt (ho ho) or circuits drawing more current then the alt was designed to supply. Measure the voltage on the white/brown at the fusebox with fans on and off, then the brown at the alt.
Paul Hunt

This thread was discussed between 16/08/2003 and 18/08/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.