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MG MGB Technical - Fuel Pump - Low Voltage

'77B: I drained fuel and removed the tank, to gain access to something else, replaced tank and fuel, switched on ignition, and pump didn't activate. Voltage at the pump varies from about 8-11VDC, and doesn't power the pump. I put an external 12VDC to the pump terminals, and it worked. Problem first occurred months ago, worked awhile, and has now returned.

I noticed in another thread (Question involving smoke/fuse) that Paul Hunt said the following:

"The output side of the pump should have a single white coming off it going to a 4-way bullet connector, with two more wires in it..."

Paul, by "output side of the pump," do you mean this 4-way bullet connector is somewhere near the pump, or is it under the dash? I was going to tackle this problem this weekend, but your comment got me curious. I expect a resistive connection is my problem, so this bullet connector is suspect.

If the problems you've helped me with in the past are an indicator, I'll bet you know exactly where my problem is.
Fred Doyen

Fred,

Paul will have to tell you for sure, but I think it is under the hood (bonnet) on the right side of the engine bay.

The thing I can contribute is that if you go to a gun shop and get a wire brush for cleaning barrels of a pellet gun, it works to clean the female side of the bullet connectors.

I think the size you want is 0.177 caliber. It makes quick work of cleaning them up. Buy several, they wear out pretty quickly. It will probably take two or three to do the whole car.

Charley
C R Huff

Fred - The 4 way connector is in the engine compartment close the firewall on the passenger side of the car. One of the wires going to it is from the fuse block, on the input side (the unfused side of the block). You may find, after cleaning up the bullet connectors, that the voltage is still low, which would indicate that one of the bullet crimps on the wire has corrosion in it and will require a new bullet be put on the end of the wire. Cheers - Dave
David DuBois

I think I must have meant 'output side of the inertia switch' as 'output side of the pump' doesn't make sense. This will be in the engine compartment as Charley and David say. For 76 there seems to have been a single white coming off the inertia switch, through a 2-way bullet connector between the main and rear harnesses to the fuel pump. For 78 it seems to have been a 4-way off the inertia switch with three wires, the extra wire going to the overdrive. The difference seems to depend on whether you have an ignition relay or not, the 76 doesn't but the 78 does. In either case the feed is from the ignition switch, branching to the inertia switch in one direction and the fusebox in the other. The feed doesn't come from the fusebpx to the interitia switch. The problem could be in the inertia switch as well as in that connector, whether it is the 2- or 4-way, you need to measure the voltage both sides of both. However you will probably only see the lower voltage when the fault is present and the pump isn't operating.
Paul Hunt

For a good, colorized, easy to read wiring diagram for your car, go to Advance Auto Wire http://www.advanceautowire.com/, click on Stock Schematics and then scroll down to the diagram for your car and print it out - or better, download the whole file to a flash drive and take it to Kinko's and have them make a full sized print (14 X 17 or some such size) and then laminate it. Cheers - Dave
David DuBois

Thanks Dave, that's what I use, and it's definitely an improvement over my other resources.

Paul, my 77B has the ignition relay, but the previous owner (rest his soul) has run two yellow wires from the inertia switch, so I'm not sure what else he's done in the circuit. I'm on vacation this week, so I'll look into it starting this afternoon.

Charley, good suggestion, as I might have tried my 22 caliber brush - but I'll stop by Walmart today (buying a bike for the granddaugter) and get the proper caliber.

Gents in general, in the 5 years I've owned this car, I've fixed probably 15 electrical faults, and there's two things I can count on: 1)You gents have never failed to provide valuable suggestions, and 2)based on those suggestions, the fault WILL be repaired the first try (so, thanks in advance, you've made it easy).

Fred
Fred Doyen

Fred - One of the things that I have found, is that the bullet connector sleeves are often brittle, which is why I always keep a stock of them handy (available from British Wiring). I have quit trying to clean them and just replace any that are questionable. Cheers - Dave
David DuBois

I just glazed over the conversation while eating some lunch. I had a problem getting my fuel pump going after verifying with an external power source that it infact worked (New out of the box). I didnt have the negative terminal plugged in which is on the "outside of the pump" as compared to the inside of the pump in terms of it being in the trunk or not. I also had fuel venting problems. Once I took the gas cap off it pumped fine after it was earthed correctly.

-James
James

This thread was discussed between 21/05/2009 and 27/05/2009

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