I assume this is the Blue.White fuel vacuum switch:


How do I test that it works as intended?

 

My understanding of the system is as follows (always subject to correction):


The switch should pass current when NO vacuum is applied. Disconnect the two connectors from the loom. If you use a test lamp on the connectors with the car not running and vac line removed, the light should light. This is the equivalent of WOT and the switch earthing the circuit thus making the ECU go to a rich mixture. This is to avoid any leanness at WOT and consequent piston problems.

If you keep the test light circuit going and re-attach the vac tube and start the car and leave it at tickover, the light should go out as the vac opens the switch and allows the fuelling circuit to the ECU to function normally.

If you want to test at exactly what pressure the switch operates, then you need to test with a vac apparatus of some sort with a manometer attached.

 

I vacuum pump with gauge would do the trick, I'd think

Mityvac

Besides testing for operational/non-operational it would be helpful if we knew at what point the switch is intended to close. Does anyone know? I don't recall seeing a specification anywhere.

I'll throw out a guess and say 5 in/HG

Cheers
DD

 

Sorry, didn't quite get the first part. (Probably just misinterpreted.)

If I disconnect the two connectors, where do apply the test lamp? To the connectors on the side of the fuel vac switch? Or to the connectors from which the switch was unplugged? If the former, how would lamp could turn on, because once disconnected the switch should not have any source of electric current, right?

I am sure I am missing something

 

I assumed Greg was referring to a self-powered test lamp; 'continuity checker'.

An ohm meter would work as well


Cheers
DD

 

I see. I don't have a self-powered lamp tester at the moment.

Will try with DVM/Ohnmeter

Thanks all!

 

As Doug, explained, my idea was that you apply an external 12 v source to the two disconnected terminals coming from the switch.

 

The switch get 5V. 12 volt will be a problem for the ECU

 

The switch simply sends a ground signal to the ECU.


Cheers
DD

 

The switch will be disconnected from the loom when you test it, as the connectors are disconnected. So the ECU will not see any of the test voltage

 

Disconnected from the loom, 100%.

A standard medical sytringe (about $2 at the local pharmacy minus needle),medium size, works just fine.

NO/lo vac = continuity.

Vac applied = Open circuit.

Doug's 5" is pretty close from memory.

I drove mine with vac gauge attached, and threaded thru the door window, and noted the vac readings at certain driving conditions, then adjusted that switch to open/close when I wanted it to.

A "T" in the syringe line, vac gauge attached, and it was a simple procedure to get it where I wanted it.

 

Grant
How did you adjust the switch, please?
Greg

 

Aussie secret maaaaaaate. Lucas special tools required.

I will do a detailed write up tomorrow, too bloody cold in the shed now, and email to you, and post on here obviously.

 

There was a discussion on JL where the original switching point was determined to be 4 in hg. With age this drops lower with 2 in hg switching not uncommon. This was considered a "dangerously" low switching point. Failure of the throttle WOT microswitch in addition to a 2 in hg or less switch point on the vac WOT could result in no WOT enrichment which apparently leads to bang/smoke/cursing.

An available replacement part was located.

--Wolesley

 

As I recall from other discussions not all V12s used the redundant enrichment switches, Jaguar apparently deciding only certain variants/markets would benefit from having them.

Am I correct?

If so, I'd be happy with just the mechanical switch.

Cheers
DD

 

Doug,

Some markets had this Blue/White switch, some had the older PreHE style switch, some had the mechanical only, as you have noted. I think it was dependant on the emissions junk required.

It was/is a fairly safe switch, coz if the vac fails, hose falls off, diaphragm splits, the valve "sees" NO vac, so the contacts close, the ECU goes into Open Loop, and richens the fuel by about 10-15%ish.

The biggest issue with them is some need the vac to drop waaaaay too low to close the contacts, so the ECU stays in Closed Loop when a richer mixture really is needed. At some point of the loud pedal depression, the mechanical switch takes over, and all is OK.

Adjusting that switch to suit your engine, and your driving style, is possible, and as I said, I will write this up tomorrow, with pretty pictures. I have a spare switch in the stash.

 

You are Doug. USA cars had the vac switch and a microswitch on the rear of the capstan. They were wired so both or one would do the job. UK cars just had the vac switch. I believe the redundancy was because of the low octane number of US unleaded fuel at the time of manufacture.

 

As promised, here ya go.

V12 Full Fuel Switch adjustment.pdf