Fuel changeover valve

(Neville, S1 XJ12, Megasquirt ECU) #1

There has been quite a bit of discussion recently about fuel change over valves. I own a series 1 XJ-12, which was originally fitted with two large and expensive fuel pumps. Some years ago I overhauled one of the pumps and fitted a change over valve from a series 3 car. Ever since the valve has given trouble, sticking closed whenever the car sits for any period. I recently decided to fit one of the motorised ball valves so kindly recommended by Aristides. A bit of research on Ali Express turned up a host of options for what is essentially the same valve. I selected this one: https://tinyurl.com/y4e6mpfb
This valve has a half inch bore which matches the fuel supply plumbing. The factory changeover valve has incredibly restricted passages. I’m very surprised that it flows enough fuel.
After fitting the new valve it burnt out on first useage. While waiting for a replacement I pulled it apart to see how it worked. They have half an “H Bridge” circuit, the input side being handled mechanically with two DPST limit switches. I got as far as working out the circuit diagram when the second valve arrived so I fitted that. Same instant burn out. I guess there is so much back EMF from the fuel pump relay, return solenoid valves and the fuel pump that the poor old electronics can’t cope. Being a series 1 car, none of this stuff is protected with diodes.
No problem. It was a simple matter of replacing the electronics with two miniature DPST relays to reverse the motor polarity and retain the two limit switches to shut the power off when the valve gets to the end of it’s travel.
This works just fine. The gear box, while lightly constructed, appears well made and has been well lubricated on assembly. Changeover is not as instant as a solenoid valve, but only takes 3 seconds, and fuel could be flowing from the other tank in 2, which is quite acceptable.

(Frank Andersen) #2

It’s a smallish drawback, Neville - the instant change over of the standard valve is useful, with instant recovery if the engine falters due to a feed problem…

Being motor driven it won’t stand the constant power designed into the ‘normal’ system, of course.

xj6 85 Sov Europe (UK/NZ)