Fuel pump conundrum

[ChrisDixon]

I've replaced the tanks and cleaned up the fuel system. To get the car running I put a few litres in the RH tank until I could hear bubbling when I blew up the pipe on the inlet side and reconnected the RH pump . Nothing, looks like the pump electrics failed (points burned and springs not closing them). I swapped it for the LH pump and still nothing. Examined the LH pump and it looked ok, no crud inside punp head, valves clean. It would click once then do nothing. Although I had checked the pump was a dual polarity AZX1308, just for fun I reversed the connections (live to pump body and earth to the terminal) and the pump ran (the right way). To rule out that pump I tested a spare that came with the car and the same thing happened. Of course I can't install it like that, but having done some research I am none the wiser. Am I missing the bleeding obvious?

[piman]

Hello Chris,
while the nimber may be correct it seems that it is not dual polarity.

From memory S.U. used a diode in those pumps, so I think you should be able to reverse it's connections within the pump?
I just looked at my S.U. manual and it says there are two types of diode depending on polarity, CZX 104 or CZX 105 (+).

Also a AZX 1308 is not dual polarity but negative earth, according to the manual.

Alec

[ChrisDixon]

Hi Alec according to Burlen the AZX1308 is dual polarity. The CZX2004 they inform me is a dual polarity diode whose function is to limit arcing between the points, and is connected between the points arm and the earth side of the points. Burlen suggested I disconnected the diode to see if the pump runs normally, and guess what, it does. Must be coincidence that 2 pumps are showing the same symptom, but I will order 2 new CZX2004.

[piman]

Hello Chris,

While my manual is quite old, I can't see that they would change the specifications and retain the same number? It is in black and white that it is single polarity?

The diode is not fitted across the points in a single polarity pump but in series with the solenoid coil, so reversing it changes the polarity. While they have two different diodes I expect it's because the connections are fitted making it harder to reverse.

However if you are going for new pumps that should sort things out?

Alec

[ChrisDixon]

From the Burlen website:

Part Number: AZX 1308
Description: HP Electric Fuel Pump - Dual Polarity

I'm told that the only polarity specific pumps they now make are the electronic ones which are AZX 1308EP and EN respectively.

[piman]

Hello Chris,

obviously they decided to retain that part number rather than give a new one. If you happen to come acraoss an old stock pump and try and identify it it could be incorrect for the application it is wanted for.

Also from their web site when I searched for the equivelant but opposite polarity pump, AZX 1319 (Positive earth) it comes up with this:-

AZX 1308. Single ended points type Pump. Now Dual Polarity. Supercedes AZX 1319. High pressure 12 Volt with minimum flow of 15 gallons per hour.

Slight confusion.

Alec