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MG TD TF 1500 - Positive/Negative earth fuel pump

I was at an autojumble yesterday and found a SU fuel pump identical to the one on my 1953 TD, since I have had some recent difficulty with my own fuel pump I bought the pump (a real bargain!). I know SU makes this pump in both positive and negative earth. How can I tell, and if it is a negative earth type is it easily converted to positive earth.

All information appreciated.

Kind Regards

John Clack
J Clack

John -- Take the cap off the end of the coil. If there are points inside, it will work with either polarity. If there are electronic comnponents that is another matter.
Cheers,
Bob
R. K. (Bob) Jeffers

Thanks Bob

Have taken the end cap off and indeed it had good 'old fashioned' mechanical parts. I fitted the fuel pump and got a good fast 'ticking' noise as I would have expected but no fuel is being pumped!
I put the old one back on and it pumps fuel fine.
Any ideas?

John
J Clack

John, please check if there's a kind of condensor fitted parallel to the mechanical points, it should have it because otherwise the life of the points is reduced drastically. I think they were fitted from the 70's onwards.
How the red wire from this part is fitted decides if the unit is pos. of neg. earth; the red wire should be connected to the live-wire.

BTW if you connected the pump with "wrong" polarity, the condensor if dead, kaputt, finito. But not expensive to buy from several sources including Burlen.
Willem van der Veer

Re. Condenser, The early AUA 25 pumps didn't use a condenser, it depended strictly on the internal swamping resistor for arc suppression. The first pumps that used condensers (small diameter silver cylinder with a wire coming out one end and attached to the attachment point of the upper contact spring of the points set) is a non-polarized, 0.47 microfarad unit. The unit that Willem describes is a diode/resistor unit what was pressed into use much later (sometime after 1985) and is indeed polarity sensitive. If the red wire goes to the power terminal, then the pump is negative ground. If the black wire goes to the power terminal, the pump is positive ground. This diode/resistor unit will withstand a reverse polarity for a short period of time (up to a couple of minutes), but when hooked up in a reverse polarity, it will make the arcing at the points worse even than with no arc suppression circuit.

The fast ticking with no fuel delivery would indicate an air leak on the inlet side, debris in the check valves that is keeping them from closing or just valves that are not closing tight enough to start pulling fuel from the tank. Try squirting some light oil into the outlet port and allow it to seep down into the pump or take the pump off and fill it with fuel through the inlet port until fuel starts to dribble out of the outlet port and then try the pump again. If this doesn't work, then diassembly is required to clean out the interior of the pump and insure that nothing is in the valves to hold the open, This can be done by simply unscrewing the inlet port and lifting the outlet valve cage out to gain access to the inlet valve disk. You should also remove the filter plug and filter to get that whole area cleaned out. Since this operation does not disturbe the diaphragm or its adjustment, the only thing to worry about is that the valves are put backin the proper order and that the outlet port and the filter plug are reinstalled tightly. Also check that the inlet port fitting is screwed in very tight. As a final check, insure that the six screws holding the coil housing to the pump body are all tight. If it is still just sucking air after the above, then you have a diaphragm that has lost it's integrety. Good luck - Dave
David DuBois

Thank you all for your comments,the fuel pump is of an early type and with good advice from you all it turned out to be 'dry' compressed gaskets connecting the pump body to the alloy valve housing allowing air into the system combined with small debris fouling the valve.

Thanks again

John
J Clack

Figured I'll hitch a ride on this post. I'm finishing up re-building my Type L fuel pump too, and noticed that there isn't a Valve Cage. I'm pretty sure I didn't see or lose one when I disassembled the pump. According to the diagrams I have, the 2 discs should go on either side of this Valve Cage. The order of assembly I have (working from the pump outwards) is:

suction disc
fibre washer on valve cage
valve cage
delivery disc
spring clip
thick orange washer
outlet union

In the Moss TD catalog, the parts for the SU Fuel Pumps aren't broken down very much, as opposed to say the Austin pumps. Am I looking for a problem where none exists?
Richard

Question related to fuel pumps... If you change to neg ground, do you have to change pumps, or hook the wire up differently?
gblawson - TD#27667

Richard - You have the assembly order correct and there should be a valve cage in the pump. Moss doesn't sell the valve cage, but you can order one from Victoria British http://www.victoriabritish.com/ Since Victoria British doesn't carry T series parts, you will need to order the valve cage for a MGA or early MGB high pressure pump (not the mga twin cam or late MGB pump). The valve cage is the same on the low pressure and high pressure L type pump.

Gordon - If you have the standard, low pressure, L type pump installed, it is not polarity sensitive. Check under the end cover. If there are points only or points and a capacitor (silver cylinder with a wire protruding from one or both ends), then it is not polarity sensitive. If it has a black cylinder with a red and black wire protruding from one end, then the wires will have to be reversed if you change polarity of the car. If there is a printed circuit card with a bunch of components on it, it is an all electronic pump and will only work with one polarity and there is no simple way to reverse polarity. Cheers - Dave
David DuBois

Richard,

Contact me off list (lpalmer@roundaboutmanor.com), and I'll send you a valve cage. I have plenty as I routinely replace these when I rebuild pumps.

Cheers,
Lew Palmer
Lew Palmer

This thread was discussed between 21/08/2006 and 04/09/2006

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