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MG MGB Technical - 78 fuel pump

I recently purchased a 1978 MGB in non running condition. One of the problems is that the wire (white)which is to furnish power to the fuel pump is dead in the boot. This car has a fuel pump mounted through the front of the trunk wall on the right hand side. The body is plastic with the wires inside the boot and hoses outside the boot.

Can someone tell me where the power feed wire connects and are there any inline fuses involved.

All 4 fuses in the fuse block are good.

I'm sure I will have other questions as I get deeper into this project. I have had other B's('65,'67.'71GT)add currently have a '76 midget and '61 Mini, but have very little exposure to later model B's.

Thanks

Roger
Roger Nagy

Roger. The first thing to do is get a copy of the workshop manual and have the wiring diagram expanded to about 300%, then laminated. Your fuel pump is part of the white wire circuit and power to it goes from the ignition switch, throught the inertia switch located under the dash on the left hand side (US), then to the fuel pump. Les
Les Bengtson

Hi Les,
I removed what appeared to be an inertia switch located on the shelf between the Brake master cylinder and heater. It was not currently in the fuel circuit. I think this device was mechanical as I don't recall disconecting any wires. I'll check tomorrow.

This device had what could be a knob on top an inlet and outlet port and was mounted on three legs.

Roger
Roger Nagy

Roger. This device, designed to cut the power to the fuel pump in case of a roll over accident, is located under the dash. If memory serves me, it is about 3/4" in diameter and has a button on top to reset it if it is tripped. There should be a powered wire going into it and a powered wire going from it to the fuel pump. Most of the cars I have seen have had this feature bypassed. But, if yours is still hooked up, this would be the first place to start. The white wire circuit, as a whole, must be working if you are getting power to the ignition relay and the third fuse from the top on your fuse pannel. Les
Les Bengtson

Roger - If the entire body (not just the end cap that sticks into the boot) is plastic, the pump is not a SU pump it is the New Zeland Autopuls pump. It has boteh the power and ground connection going through the end cap, whereas the SU pumps have a ground lug on the pump body that is under the car. Be sure that the ground is going to the right place and that the point where the ground wire goes to the body is well cleaned of all paint, dirt and corrosion. See my article on ground point preparation at: http://www.omgtr.ca/technical/General%20Technical/grounding.htm
Good luck- Dave
David DuBois

Roger, what you describe is the fuel shut off valve.
The inertia switch is under the dash as Les describes.
It was designed that if the car overturned there would be no power to the fuel pump.
On my 77 The inertia switch is bypassed. Everything works fine on that circuit.
Dan H.
Dan Hanson

Thanks, for the input.

The pump is definately not Lucas, I believe it says ECCO on the end which protrudes into the boot.
The pumpis well groundes and runs fine when a jumper wire is connected between the the + wire on the pump and the output side of the 3rd fuse. The white wire which is not "live" exits the loom about 2 inches up in the right rear corner of the boot. Does this wire go directly to the inertia switch or is there some intermediate connecter between it and the boot. If so where is it?

When connected with the jumper, fuel is being delivered to the front of thr car.

Thanks.
Roger
Roger Nagy

Roger. "As designed" the white wire in back is supposed to come directly from the inertia cut off switch. However, after something more than 20 years, what you started with and what you actually have might be quite different. Check the inertia switch and see if you have power in and power out of it. If not, that would be a good place to start looking. If the switch is still hooked up, you can verify that the wire going out of the switch and back to the fuel pump is good by using an ohm meter. Les
Les Bengtson

I found the switch behind the dash and depressed the button on top. There was a definite click. I have not yet had a chance to check for power in the boot. Probably check tomorrow. Part of what confused me is that the metal device mounted next to the Master Cylindes has the words "inertia switch" cast into it. Can anyone tell me how that dwvice works?

Thanks

Roger
Roger Nagy

Roger. All of the RB cars had the "fuel pump inertia switch". This description from the driver's handbook contained in the Bentley copy of the factory workshop manual, "The electrical supply to the fuel pump is switched off by an inertia switch if the cr is subjected to a moderate impact. The switch, shown in the off position, is located under the facia on the left hand side. To reset the switch unscrew the three screws to release the bottom panel put the plunger into the switch body and refit the bottom panel."

What you were looking at is the "fuel cut off valve". "The supply of fuel to the carburetter is mecahnically cut off by an inertia valve if the car is subjected to moderate impact. The valve shown in the off position (fuel flowing) is located in the engine compartment. To reset the valve push the plunger into the valve body." This is noted as being used on 77 and later cars.

Thus, all of the cars had the fuel pump inertia switch which cuts power to the fuel pump (your problem), while 77 on had an additional switch which cut off the fuel flow near the carbs, but had no effect on the power supply to the fuel pump. I assume this is something required by US safety standards. Les
Les Bengtson

The inertia switch also cuts off power to the OD, FWIW. There is a 4-way connector after the inertia switch where the whites branch off to OD and fuel pump, that could be pulled out or making a bad connection too.
Paul Hunt

Thanks, What is FWIW???

Roger
Roger Nagy

Roger. FWIW= For What It's Worth. Paul is indicating another area to check if you have power through the fuel pump cut off, but no power at the pump. Les
Les Bengtson

I have power in and out of the inertia switch, but sill no power at the fuel pump. In looking at a wiring diagram for a late North American MGB it shows a junction block between the pump the inertia switch. At this blocl a wire branches off to supply devices numbered 48, 98, & 78. What are these and can antone tell me where this junction is located. Can someone also tell me what device 51 is(appears to be a relay??) and where it is located(would my 78 have this?).

Thanks

Roger
Roger Nagy

Roger.

48 is Overdrive/TCSA switch
98 is not listed as a valid number
78 is the overdrive solenoid switch

This information is taken from the Bentley reprint of the Factory Workshop Manual, 78 and later. (This diagram most accurately relects my daughter's 77 B wiring and accurately reflects the wiring on my 79. The 76 wiring diagram, with a supplement for the 77 year, does not accurately reflect my 77. None of the sources show an item numbered 98.)

Power into the pump comes from a three way connector having a white wire in, a white wire out (fuel pump) and white/brown wire going to the overdrive switch. This connector would be located in the back of the engine compartment.

Power to the three way connector comes from the inertia switch, item number 36. If you have power there, the problem must be in the white wire going from the inertia switch to the connector, in the connector or in the wire from the connector going back to the pump.

Get a piece of electrical wire about 8" long and attach an alligator clip to each end. Disconnect the wires going to the inertia switch, then turn on the ignition switch. One wire, power in, should light a test lamp or show 12V+ input on a volt meter. Connect the power wire to the inertia switch and look for a light/voltage reading at the other terminal of the inertia switch. If you have power, the switch is good. Then, turn off the ignition switch and hook one end of your jumper wire to the power output lead from the inertia switch. Hook the other end to an ohm meter and look for a three wire connector at the right rear of the engine compartment having a single white wire in and a single white wire and a single white/brown stipe wire coming out. Use the ohm meter to measure the resistance between the input wire and the end of that same wire under the dash. You should read zero ohms resistance if the wire is good. If you read infinite resistance, the wire is bad and you have found your problem.

If the wire checks good, reconnect it to the inertia switch, then turn on the ignition switch. Use your test light/volt meter to check for light/voltage at the end of the wire. You should have it if the wire tested zero resistance. Then, connect the wire to the connector and remove the white wire output and check the connector for light/voltage. If it does not show, you have a bad connector. If it does show power, turn off the igntion switch and begin the next test.

Connect the jumper lead from the white wire, output to the fuel pump, to one lead of your ohm meter. Go to the back of the car and check the resistance of the power feed wire. If it is zero, you have a good wire. If not, you have found your problem.

While you are playing with the connector, clean the ends of the round connections on the ends of the wire and, as much as possible, clean out the inside contact areas of the connector. A small, round needle file will work for this. By then, you should have found the problem and be making a plan to replace what is bad. Les
Les Bengtson

Thanks Les,

I that conector on the carb side or the Dist side of the back of the engine compartment?
I actuslly ment device 68 not 98.

Do you know the location of devive 51 which appears to be a relay which connects a brown wire from terminat 7 on the fuse box to terminal and the power feed to the ignition?

thanks

roger
Roger Nagy

Roger. Item 68 is the manual overdrive switch. Item 51, the ignition relay, is located on the right (passenger in the US) side of the inner fender, just ahead of the fuse box. This, however, should not affect your getting power to the fuel pump is you have power into the inertia switch. The white wire circuit starts at the ignition switch, item 45 on the "78 on" wiring diagram, then goes to a three way connector. One white wire goes forwards to the ignition relay and the other wire goes to the inertia switch. If you have, as you mentioned, power at the inertia switch, the ignition relay is not a concern.

The three way, white wire input from the inertia switch is on the right (distributor) side of the rear engine compartment. Les
Les Bengtson

Thanks Les,
I had incorectly assumed that the relay next to the fuse box was the starter relay. Where is the starter relay located(71)?

Thanks
Roger
Roger Nagy

Roger. If you mean a 78, rather than a 71, the starter relay is located on the right hand inner fender about 8" behind the fuse box and slightly above it. Not sure where it is located on a 71, nor if a 71 has a starter relay. Les
Les Bengtson

The bullet connector between the inertia switch and the pump will be where the rear and gearbox looms join, somewhere near the fusebox. The gearbox loom comes up from the right-hand side of the tunnel, and the rear loom comes up in the angle between the right-hand inner wing and the firewall/toeboard.

FWIW 71s did have a starter relay (actually arrived in 70) and should be in much the same place i.e. near the fusebox.
Paul Hunt

Thanks Les, for holding my hand through this issue. I now have power at the fuel pump! One of my main problems is that this car had it's harness butchered over the years, first by the dealer? when airconditioning was installed and later when a replacement electronic ignition was installed. I'm sure it will be a real adventure sorting it all out.

Thanks again

Roger
Roger Nagy

This thread was discussed between 24/02/2004 and 02/03/2004

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