MGB 1974 MGB SU fuel pump voltage issue

[wkilleffer]

Hello everyone,

It has been quite awhile since my last post. I've been rennovating the brakes on my 1974 MGB. This has been an interesting journey, and it took longer than I ever would have thought. Well, now the brakes are working again and it's time to get the car back on the road. There's just one problem...

The car's been sitting close to a year. I checked the battery voltage and the multimeter said it's at around 12.30v. I switched on the ignition, and didn't hear any clicking from the fuel pump. That said, the battery voltage was enough to crank the starter even though the car woudn't start due to no fuel in the carbs.

I then checked the voltage at the white wire on the fuel pump, and it says around 11.35v. That's with the meter set to VDC and grounded at the negative battery terminal. Is that enough power to get it going? I hooked up a jump start device to the battery to see if that might give it a little more juice, but no difference. The pump was almost new at the time I took the car off the road to do the brakes, and it was running then.

I was thinking about driving my old Jeep back to the garage and hooking up an old school pair of jumper cables since it tends to put out around 14v with the engine running.

Any more ideas? I'm trying to stay optimistic because it's at least getting some voltage, but I sure would have loved to have taken a drive today. It's a lovely 80 something degree low humidity day.

Thank you,
-Bill

 

[DrEntropy]

The pump was almost new at the time I took the car off the road to do the brakes, and it was running then.

What pump is this replacement? SU points type or transistor switched? An aftermarket pump?

 

[wkilleffer]

What pump is this replacement? SU points type or transistor switched? An aftermarket pump?

It's an SU with points, IIRC. Pretty sure it's not the transistorized one.

Since I posted, I went out and put a new connector on the white wire bullets near the firewall after giving the bullets a scrape with some sandpaper. This got the voltage to 11.65. I'm afraid it could be the ground point on the pump itself since it's in a pretty vulnerable spot with plenty of chances for dirt and moisture to dirty up the connection. The frustration is that it was running before I did all this...

 

[BobHaskell]

Burlen states in their technical section that an AUF200/AZX1200 or AUF300 /AZX1300 series pump should run with a minimum voltage of 9.5 vdc. The pumps I've tested normally start lower than that - below 7 vdc.

 

[DrEntropy]

Well, Bill, this may sound like lunacy...but try whacking the pump (with ignition on) on the steel body of the pump. Not a hammer blow, but a few good taps with something solid. A small hammer like a two ounce ball-peen or even a piece of rock. The impact will jar the points enough they may make/break contact.

 

[NutmegCT]

If it sat for a year w/o being used, sure seems a light tap might get the pump running. But ... in the very rare likelihood that the carb bowl(s) are full, the SU wouldn't need to pump yet.

Just sayin'

 

[DrEntropy]

If it sat for a year w/o being used, sure seems a light tap might get the pump running. But ... in the very rare likelihood that the carb bowl(s) are full, the SU wouldn't need to pump yet.

Just sayin'

Dormant for a year, the bowls are not full. Ethanol alone would cause enough evaporation to lower the floats.

 

[wkilleffer]

Well, Bill, this may sound like lunacy...but try whacking the pump (with ignition on) on the steel body of the pump. Not a hammer blow, but a few good taps with something solid. A small hammer like a two ounce ball-peen or even a piece of rock. The impact will jar the points enough they may make/break contact.

Ok, so what I'm taking away from this is that the voltage should be enough to turn the pump, but it may have become stuck from sitting so long.

When you say to tap it on the steel body, are you talking about that area on the back of the pump where the fuel lines are? That would be an easier place to tap, for sure. I have a small ball peen hammer that would be right for the job.

To the best of my memory, the fuel tank is nearly full and was ehanol-free.

 

[DrEntropy]

The black steel body of the pump. The casting with the fuel fittings is aluminum, not good for tapping. And the "cap" where the wiring attaches is plastic... you get the idea, I'm sure. Try to get to the middle area. Even use something like a metal drift between hammer and pump body.

 

[YakkoWarner]

I always used the small wooden chunk of 4x4 that I kept in the trunk as the emergency parking brake. A little percussive maintainence and off you go....

 

[wkilleffer]

The black steel body of the pump. The casting with the fuel fittings is aluminum, not good for tapping. And the "cap" where the wiring attaches is plastic... you get the idea, I'm sure. Try to get to the middle area. Even use something like a metal drift between hammer and pump body.

A couple of light taps to the correct area of the pump with a small ball peen hammer, and we're back in business.

Thank you to everyone who helped out with this thread. This sort of issue may have happened to me once before in the dim and distant past, but I couldn't remember it. It may seem a little silly, but I'm kind of proud of myself for not going into some kind of panic. I remembered to check a wiring diagram, at first thinking that I might have been dealing with an inertia swich issue. But my car wasn't fitted with one, so it was a matter of verifying the circuit and checking voltage. I was glad to see voltage at the pump, so at least it wasn't a broken wire somewhere.

 

[DrEntropy]

Good!

Now that the issue is found, may be a good idea to dress those points. A piece of #600 wet-or-dry run through 'em and it should not be a problem again for a while. Bit of a PITA to get to them, but better than being out someplace and realize your little hammer is home in the garage.

I've carried a wedge-shaped two pound piece of granite behind the passenger seat as the parking brake/wheel chock for decades (and in three MGB's). On a trip to Miami years ago in the '64 B, the pump began to fail half-way across Alligator Alley... pump on the older chassis is next to the bulkhead behind the right seat, next to the battery. Used that rock to thump the bulkhead and made the Miami side, pulled into an auto parts store and got a low pressure Facet pump, MacGyver'd it in place and it lasted 'til we could replace the SU pump in Tampa after the trip.