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 - Press the brake pedal - engine stalls.

This was a strange one, so I thought I would post it for the benefit of others.

I started the B the other week after not having used it for a couple of months and all seemed well. Well that was until I put my foot on the brake pedal, whereupon the engine stopped dead. So I restarted the engine and it was fine. I repeated the process several times and the same thing happened. Every time I pressed the brake pedal the engine stopped dead. There was no run-on or spluttering or poor running it stopped dead.

Initial thoughts were servo problems. However I noticed that the rev counter was returning to zero in a flash. This made me think it was an electrical problem. I disconnected the brake light circuit at the pressure switch, restarted and pressed the pedal; the engine continued to run.

So I took out the multi-meter and buzzed out the break circuit – dead short to earth. I eventually traced the short to the LHS brake bulb fitting. I re-seated the bulb and all is now ok. It seams weird that a short circuit could develop after standing for two months. None of the fuses blew.

Any one else had this problem?

Cheers

Richard
Richard Thompson

This is not an answer to your question, but a funny story. Once my sister rented a car and every time she would apply the brakes, the engine died. When she went back up to the rental car desk and said, "Every time I put on the brakes, the car stops", the man in line behind her said, "What do you want it to do lady?"
Robert Browning

hehe !

RT
Richard Thompson

Richard If you have a vacuum booster on the brakes it will be a hole in the diaphragm. It opens up when the pedal is pressed causing a massive vac leak. Its not that uncommon. Denis
DENIS H

Richard - I suspect you have another problem which is still lurking ready to catch you out at some future point.

Normally with a short on the brake light circuit you would blow the green circuit fuse (2nd one up) and the engine would continue to run. I suspect you have a 'weak link' further back towards the ignition switch, or possibly down by the solenoid, which is giving way before the fuse can blow then repairing itself, I've had a very similar thing at the solenoid on my 73 (totally stumped Halfords when it happened during the MOT). If the coil and ignition switch wires share the same spade connector i.e. the engine still runs when the whites are removed from the fusebox, then check the connections to the ignition switch or the switch itself. If the interior lights (for example) also extinguish when the engine cuts out then the problem is almost certainly down by the solenoid. If removing the whites from the fusebox also cause the engine to cut then it could be the spades and connectors there.

In my view it is worth shorting out the brake lights again to reproduce the problem (or just conenct a ground to the green side of the 2nd fuse up), so you can find and fix this lurker.
Paul Hunt 2

Hi Paul,

Thanks for your comments. I have just looked at the wiring diagram to understand your suggestions. I will re-investigate in the way you suggest.

When tracing the short I isolated the LHS lamp unit form the car and measured the short circuit there, re-seated the bulb and then measured some ohms, indicating the bulb. So all ok there. This doesn't explain the non-blowing of the fuse, however what you suggest does explain it. A connection going high resistance/open circuit under high current flow, produced by the short, would explain it all.

May be the connections on the solenoid just need some attention.

I'll investigate and report back

Many thanks

RT
Richard Thompson

Had a similar problem with an aftermarket " Third eye "
type brake light that was installed by the PO.....it made a satisfying clunk sound when it hit the bottom of the dustbin. ( Still ended up pulling all new harnesses ).
Jim
Jim K.

A lady friend of ours complained that when ever she braked the cars lights came on. She had an old Ford Fiesta.
I knowingly went to check, as If!!, but sure enough all the cars lights came on when braking.
I am sure you have all guessed, but it was new to me, The two filaments in one of the brake/side light bulbs had fused. When 1 was energised the other circuit came on too.

Rich, thanks for the info, tales like yours can often came in handy.
Regards
Dave
D M Tetlow

Richard - Check the wire going into the socket for the offending brake light. I suspect that you will find that it has been pushed over where it exits the socket and is laying hard against the metal edge. I had a similiar experience with our 66 MGB when on a long trip, I had stuffed my camera case into the fender well in the boot. My symptoms were a bit different, in that when I stepped on the brakes the tach would drop and the headlight warning alarm would chirp even though the lights weren't on. When I started investigating, I found that the wire had been pushed over so hard against the edge of the entrance hole in the back of the socket that it cut through the insulation and caused a short. I did a temproary fix to the wire and when I got home, I made up some shields to go over the back of hte sockets to keep the wires from being pushed on by whatever I stash in the boot (a heck of a lot when we make a three week treck to southern California each year to visit family). If you find tha tyou have the same problem and want to make up some protective shields, e-mailme and I'll send you pictures and directions. Cheers - Dave
David DuBois

Dave D,
Thanks for the tip. This probably is the root cause I guess. I haven’t investigated the wiring inside the lamp unit but I will.

I did some tests to check for Paul’s suggested root cause, by simulating short circuits, however the fuses just blew and the engine stopped. The engine stopped because the power to my Luminition ignition control box is powered from the fused green line. (I don’t like having electronics on un-fused lines as some failure conditions could cause a short circuit; we all know how serious this can be in a car)

I temporally swapped the Luninition to the white un-fused side, shorted out the green and the fuse blew and the engine kept running. So I wonder if the original short in the lamp unit was sufficient to drop the voltage to the electronic ignition, and therefore cause it to stop working hence the stall, but not sufficient to blow the fuse.

I now need to restock on 17A/35A fuses !

Cheers

Richard
Richard Thompson

This thread was discussed between 27/09/2006 and 07/10/2006

MG MGB Technical index

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