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MG MGB Technical - Fuel Pump/Electrical Silliness

OK,


1979 B Roadster, Tatty looking but a good runner with its share of minor but easily fixed problems. Recently pulled the engine to repalce a faulty throw out bearing. No problems since then.

Take off for a quick trip last night and it quits running. Look for the usual suspects fuel and spark. Good spark but no click-click from the pump. But while walking around it at night I also notice the two rear side marker lights (no tail lights)are illuminated with the ignition on and the lights off ???. Its dark and cold so I give up and go home. Next morning I get on with the trouble shooting after a look at the wiring diagram. I suspect a chafed fuel pump and marker light wire but this doesn't make a lot of sense since the right front/left aft and right aft/left front are on two different circuits. So after a lot of different combinations of checking for shorts and checking where I do have and don’t have voltage I find no short between the pump hot lead and the rear marker lights I find that with the hot lead to the pump connected I get two rear marker light and when I disconnect the lead from the pump the marker lights go out and there is no direct short through the pump to ground. One final clue, with the pump connected and the lights and ignition on, both rear marker lights and the left front are dimmed but there is no discernable effect on the parking or tail lights.

Anyone ever see this before?

Thanks,

Dan
Dan deCamp

Dan,

I suspect you have an earth problem somewhere and maybe the pump is somehow earthing via the lights? Check the fuel pump earth.

John
J Tampkins

Dan,

Like John said, when things seem weird, it is usually a ground.

I don't know about 79, but the fuel pump became intermittent on my 68 when the license plate bracket was loose. That's a ground point for several things.

Charley
C R Huff

Dudes!

Dead on. I cleaned and tightened the grounding lug and away she went! Makes sense since that is where the pump and the lights have a common point while the tail lights ground through their frames (and both my license plate lights are out).

Thanks Again,

Dan
Dan deCamp

Yeah, it's nice when things work out that way.

Charley
C R Huff

Dan: had the same issue a few years back. It turned out to be electrical failure due to time afflicted corrosion in the fuse box. You need to pull all the fuses, super clean all the contact ends both internal and external of the fuse box and replace the fuses. I went one step further and replaced all the connector ends on the wires to the fuse box. Problems went away. I am sure the process will start all over again. But this is how I solved it.

cheers

Gary
79 MGB
gnhansen

Re. Garry's suggestion - see th article, Electrical Connection Preparation in the Other Tech Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/
Cheers,
David DuBois

I don't know about Gary's car, but no MGB came out of the factory with the power supply to the pump going through the fusebox, or any fuse. Fusing the pump (along with the overdrive) is a very good idea however, both mine came to me with harness damage from shorts at or near the pump, see http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'Pump Fusing'.

Go to 'Gearbox' and 'OD fusing' for how to fuse that as well, especially important on cars with the OD switch in the gear knob.
Paul Hunt

This thread was discussed between 16/12/2008 and 29/12/2008

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