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MG MGB Technical - Fuel pump shenanigans

I've just spent a fortune getting the pink slip and everything else needed for rego this year, driving to the RTA to pay the actual rego fee and my B dies. Well maybe it's just unconscious, I'm not sure yet.

Called the NRMA expecting the problem is electrical as the charge/alternator light had started to glow a bit dim over the last couple of days. It was and it wasn't. He finds there is no fuel getting to the filter and discovers the pump is only working when I'm cranking over the engine. As soon as I release the starter motor the power drains out of the pump and no electricity flows to it.

To get me going the NRMA mechanic hooked the pump up to my tail lights and followed me right back to my house, warning me that technically he should organise a tow for me as it is not safe to run the pump this way in case of an accident (yeah I can understand that easily enough).

The pump itself it almost new, so I assume this is totally the fault of the electrical system and possibly related to a wire shorting out. Is this is common thing to happen with the electrical system? Is there something particularly simple I should look for to find what has caused this?

Regards,
David
Sydney, Australia
D O'Brien

If worse comes to worse you can always run a power line from the wire that connects to the + side of the coil but before the ballast resistor, if you still have one. Put an inline fuse in.
Mike MaGee

David,

Doesn't the fuel pump receive a direct feed from the ignition switch? If you trace the wires back I think you should find that power should go to the pump whenever the ignition is on. Failing that is your alternator charging enough?

regards,

Dirk
Dirk Richards

David. No, this is not common. I have never seen it before. (And have something over 35 years experience, so you do not have a "common" problem and the fuel pump, and its circuit, are very seldom a problem.) As others have noted, the fuel pump is powered by the ignition switch along with the distributor. What is fairly common on these cars is that the connectors for the wires will get some corrosion build up over the years and the flow of electricity will be somewhat erratic. Get a wiring diagram of your car and check to see where the power should be coming from, then, trace the wires back to find out where the problem is. The worst case is that you have an internal break in the wire going to the fuel pump. This can be determined by using an ohm meter which will show no resistance (zero ohms) with good continuity and infinite resistance with a broken wire. If that is the case, simply run a new wire from the orignal connection back to the pump.

My experience is that these cars, when properly maintained, are wonderfully reliable and a joy to drive. Do your "debugging" to correct the problems that have built up over the years, then enjoy what you have accomplished. Les
Les Bengtson

David, Did the cars shipped to Australia have the rollover switch that was installed in the cars for the U.S. market. On the U.S. cars it was located under the dash on the left side. It has a reset button on the top. I have seen this same problem on one car and it was a matter of reaching up and pressing the reset. Good luck with it.
John
John Davis

Firstly I'll just point out that it's been sitting at the auto electrician shop all morning and he just called me to say it is fine. Apparently once he disconnected the "makeshift" wire the NRMA guy installed, he checked the power going to the fuel pump and it was okay. He then tested everything associated with it, turning the engine over and leaving it running for a while (found out that I need to check my coolant) and went for a drive.
All the while he could not get the pump to fail.
He has left the spare wire installed on the tail light with the connection covered in insulation tape so I can do the same "makeshift" fix on the road if it happens again. He traced all the wiring he could get to and found no problem. He is mystified.

Now just to answer a couple of the points some of you made here...
The car is British made and was brought to Australia by the original owner who I then bought it from.
I was hoping the wiring was simply a direct line from either the ignition or alternator, and it sounds from the descriptions here that is correct. The auto electrician suggested it was possible some sort of relay switch might be hidden in the circuit between the pump and the ignition, but this doesn't sound like the case with my car. Unless relay switch means the rollover switch...
The rollover switch John mentioned sounds like a possibility. I certainly didn't roll the car, but I guess the way I drive you could be excused for thinking otherwise. I'll definitely be looking for that switch when I pick it up this afternoon.

Lastly, a special note to Les. I love my B. It's a love/hate relationship though. When she runs well I can think of nothing I enjoy more. When she breaks down on me that is a different story.

Thanks once again everyone. I only posted this thread 4 hours ago and I'm already just about back on the road with a whole heap of new knowledge about my car.
D O'Brien

David. Thanks for the "special note". Yes, this is an interesting group of people. We are all dilettantes in this field. (From the Latin dilettare--to delight.) Thus, I share your delight in having a rare UK spec car and being the second owner of same. As you are probably aware, England, after giving almost everything it owned in WW II, began to derive hard currency from the US (who had minimal damage from that war) by shipping sports cars there. The MG, which had been the main, leading Brit Sports Car in the US prior to WW II, was immediately a success in the US market. Not only had it been the sports car of choice amoung the "in crowd" prior to WW II, but it had been seen and driven by the soldiers and airmen stationed in England. These two factors, the "charisma" of the marque and the familiarity with it by returning servicemen, lead to a curious situation. A fine British Sports Car that was, mostly, oriented towards sales in the United States. Something like 90% off all the MGs sold, after WW II, were sold in the US.

Thus, you have a very special car. A UK specification, imported to Oz, and sold to you. I can fully understand your "love/hate" relationship with the car. I have a similar relationship with my 79 LE, a far more common car than yours. All I can suggest is that, in my case, the love has outlasted the hate. I can only hope that your car will reward you as fully as mine have me. For me, they have returned the "love" as fully as a faithful wife, child, or dog has. They have become something beyond the mechanical and have entered a different relm, that of faithful companion. We discussed this interesting aspect of ownership once on the midget board. I believe the thread was something like "Does a Midget have a soul". Many of us, particularly the ones who had years of ownership and some feeling for machines in general, felt that yes, the cars do have a "soul", something that was put into them by the dedication of the people who built them. This is a very strange concept, but anyone who has ever had to work with machines over an extended period of time realizes that some machines are superior to others. The MG is a superior machine, in all examples, and will reward your efforts to make it better. Les
Les Bengtson

Cranking shouldn't bypass the roll-over switch, if it does the wiring has been tampered with, and could be causing any number of strange effects. If connecting the pump to the tail lights (and presumably having them switched on? Or was it a ground connection?) caused the pump to run satisfactorily then it does sound like a loose connection that until found and fixed is likely to crop up again at any time. You don't say the year of the car, generally the pump had its own connections where the white (or white/brown) in the main loom joins the rear loom by the pedal box and a bad connection here can cause the pump to fail and nothing else. It was only North American rubber bumpers that had the electrical pump cut-out, which incidentally also powered the overdrive. The above connection is a good and easy place to insert an in-line fuse as mentioned, whilst not common it has been known for that wire to short out, which can take the main and rear looms with it. If it was shorting out now you would know about it, and the makeshift wire would have burnt out too. Whilst on about fusing the overdrive feed has also been known to short out and is unfused taking the gearbox and main looms, another in-line fuse between the yellow in the main loom and the yellow/red in the gearbox loom protects that.
Paul Hunt

British made model, so apparently I don't have a rollover switch. Checked for the other one which is occasionally under the bonnet lefthand side but this isn't in my model either.
The auto electrician never could get it to stop working for him, so without these switches it would be assumed my problem was a simple wiring issue. There must be a wire broken inside the insulation somewhere between the pump and the ignition. I decided to investigate further as follows...

Built myself a little 12volt test lamp and connected that to the power terminal on the fuel pump and ran the light on the dash board (I pulled out the LED for the alarm indicator and used the same hole... temporarily). This was just so I can keep an eye on the problem for a little while and see if it happens again. For all I know this power failure could have been going on for a long time but just never for long enough to run my fuel out of the carbies and stop the car.
I've just taken the car for the first run with the test light all hooked up and I might be onto something. In all my tests in the garage before I drove the car I found the fuel pump always had power whenever the ignition was switched on. When I "crank" the engine over it has power and when I release the key to allow the engine to run it still has power. I fiddled around in the boot with the terminal to make sure it was firm and making good connection. I left the engine idling to warm up to normal running temperature. Finally I took the car out for a drive. The whole time the light showed full power to the pump and didn't flicker at all. The light changes colour to red if the voltage drops below threshold (tuned to 11volts for my purpose) and yet even this never happened. Green light all the way.
I drove a reasonable distance (more than 10km) and stopped at a friend's house for coffee. About half an hour later I decided to head back home. Checked for green light when I turned the ignition on. Good. Green light still on when I turn the engine over. Green light all the way home... until just as I'm turning the corner to my street (right hand corner, if it matters) and the light goes out completely. All other electrical stuff is working fine. Radio still has power and headlights are fine. Indicators are flashing (not steady, which is my first sign of trouble normally). The fuel pump has simple stopped receiving power.
I made it into my driveway and turned off the engine. Checked everything I could possibly check. Tried starting the car again but still no green light (car started no problems though). Opened the bonnet and checked the fuel filter and it seemed to be okay, but the pump is definitely not getting any power still. Hooked a multi-tester up to make sure it wasn't a case of my own test light failing, there was no power going to the pump at all.
Either there's still enough fuel in the lines and carbies for the engine to run, or I'm seeing a pump that doesn't "need" to work all the time. Is this all my "problem" has been all along? Does the pump only switch on when required or am I going to find myself "out of fuel" shortly after I start the car tomorrow if the pump hasn't started working again? My father suggested to me that "older cars" didn't actually pump fuel all the time, and that it should only me powered when I'm starting the engine. During normal running he guesses the electric pump isn't needed and a manual pump from the engine is what delivers the fuel. I'm thinking from my original problem the other day that my pump is supposed to be working all the time or I wouldn't have broken down 5 minutes after filling up at a service station.
BTW - There is definitely a full tank.

P.S. Not being sure if my overdrive is related or not from what Paul says above, so I checked with a multimeter for power to the solenoid under the car when the switch on the gear stick is on and I'm in 3rd/4th gear. I remember this test when I had to fix the overdrive. Power is working no problems down there.

P.P.S. The light on my tachometer started playing up on this test drive. Tell me I'm crazy but I'm convinced the ghost of Lucas is haunting my car now. Calling an exorcist tomorrow.
D O'Brien

Oops.. forgot. 78 model. Almost-new replacement SU fuel pump (new style). I'm still waiting for you to move to Sydney and work on it fulltime for me Paul. :)
D O'Brien

With the SU pump, it will indeed stop pumping when the fuel level in the carbs gets high enough to raise the floats high enough to close the needle into its seat. However, the pump will still have 12 volts supplied to it the whole time the ignition is switched on. Once some fuel is used and the level drops a bit, the pump will start again. But the pump should have 12 volts all the time.

Still sounds like an broken wire/poor connection to me. Disconect the wires to your coil (so it won't overheat), switch on the ignition and play "wiggle the wire". Wiggle, shake, tug and push the wiring loom from the engine bay to the rear, listening for the characteristic tick of the SU fuel pump to start. If your new modern SU pump is one of those that don't "tick", the grab an assistant to watch your test light while you wiggle the wires.

My B had obviously had a similar problem at some point and the fuel pump came with a new piece of wire running from the fuse box, under the car and thus to the pump. It's still there, 5 years and 70,000 miles later.

Good luck in your search. If you were a bit closer, I'd come and give you a hand.

TTFN
Derek Nicholson

Definitely broken connection. The way the SUs work (and most carbs for that matter) means that a warm engine will continue to run for several minutes *after* the pump has stopped working, partly on pressure in the pipe from the pump after its last solenoid stroke, but mainly from the reservoir in the float chambers. Move your test-lamp to the connector where the white from the rear loom joins the white/brown from the main loom near the fusebox, preferably as a bit of wire in the same socket as the wire to the rear loom. If that shows green the next time the pump stops then it does look like the conductor broken inside the insulation as there are no other connectors. But if the lamp again goes out with the ignition on then the problem is either iside that connector (check all bullets are clean and tight) or towards the fusebox. However this is unlikely as that same feed supplies the ignition warning light, electric cooling fan and OD - unless the ignition warning light *also* fails to come on when turning on the ignition after the problem has occured.

BTW, I'm surprised your lamp didn't change to red during cranking as that usually causes even a good battery to drop to about 10v.

Even further BTW, re waiting on the move to Sydney, you and me both :o) But having seen all that rain during the rugby (what a heart-stopper that was) now I'm not so sure ...
Paul Hunt

David, start checking all your ground wires, especially for the rear harness and the engine. An intermittent bad earth will give you these problems. Also try flexing the wiring harness in your hands with the car running. Follow from the fuel pump up toward the front of the car - especially where they pass through the crossmember. You may find, or cause an intermittent short. Also, make certain your fusebox is in good condition. I know you don't have any problems with other circuits - but there's a reason your dash light comes on and the signals flash slowly as well. The only things they have in common (I believe) is the ground and the steering column.

By the way, did they import gremlins with these cars, or do they just enjoy the occasional ride?

Jeff Schlemmer

This thread was discussed between 19/11/2003 and 23/11/2003

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