Hi,

we have a V12 1988 convertible which has been stood for a few weeks and now wont start.

I have noticed i can no longer hear the fuel pump when the ignition is on, is there a fuse for this please ?

thanks

 

Hi Rowland

Have you checked the Fuel Pump cut out switch by the door pillar?




Fuel Pump Cut Out Switch by Door Pillar 'push button in top to reset'

It could also be a problem with one of the Relays, so do a Test and see what you can find, this is on Page:46 of my 'Cherry Blossom' restoration thread 'Click on the Link below'

How to Test the Fuel Pump Relay on a Pre-Facelift XJS V12

 

I just went through this. First, check the main relay (for the ECU). OB can point you to where it resides (mine was inside the rear fender - 1992).

The ECU controls the fuel pump relay. If the ECU relay is not switching on, the FP will not be turned on by the ECU.

You can also check the FP by disconnecting the pump from the harness, and jumping it directly from the battery.

Apparently, I haven't learned from experiences with all my other 1990's toys: always carry a spare main ECU relay and a spare FP relay. Locate where they are, write a note, and shove it in the glove box.

 

Hi Rowland

The Fuel Pump Relay and The Main Relay are in the Boot/Trunk in a Black Box by the gas strut hinge of the Wing/Fender

The Relay in the Red Holder with the Red Stripe is the Main Relay and the Relay in the Black Holder is the Fuel Pump Relay and they both need to be working for the Fuel Pump to work

These Relays are Not interchangeable, as the one with the Stripe has a Diode

As has been said, you could also try putting 12v directly onto the Pump to see if the Pump works on its own




Main Relay is in the Red Holder and the Fuel Pump Relay is in the Black Holder (Not interchangeable) as the Relay with the Red Stripe has a Diode

 

Hi,

thanks for the replies, as soon as the rain stops I'll be out there.

I have already looked at the door switch, this is in the down position so will be going for the relays next.

many thanks

Rowland

 

Hi Rowland

If it turns out that the Pump has died they're as cheap as chips on ebay, so get back and don't spend $200 on a New one

 

Ok, so my testing so far

There is no power getting to the fuel pump.

There is power getting to the fuel pump relay, just looking for the post which tells me what pin does what so i can test if there is power from the ignition

 

Pins 85 and 86 are the pins that switch the relay off and on. One is powered by the main relay, the other by the ECU.

Then are Pin 30 provides the battery power. It's a dual output relay, so one of the pins 87 are the output power to the fuel pump. The other is used to provide heating power to the 02 sensors.

To check and see if the relay works, remove it. Place +/- 12v across pins 85 ad 86. Check for continuity between pin 30 and both of the pins 87.

 

awesome explanation thank you.

the fuel pump relay im sad to sad is working ok.

does the ECU relay ( red stripe ) get tested the same way please ??

cheers

 

this is at the side of those 2 relays, what does this do please ?

 

The main relay gets tested in exactly the same fashion.

They are both dual 87 relays. The difference is that the main relay has a diode across the switching terminals to minimize the transients cause by switching fed to the ECU control lines.

 

ok, the red stripe relay is not working then the same, presumably this is my problem why the car wont start.

where is the best pace to get one in the uk please ? ebay ?

 

The ECU relay is more commonly called the "main relay"

The fuel pump relay is triggered by the main relay so, if the fuel pump relay is operating correctly, the main relay is OK

But, to answer your question, yes, it is checked the same way. Apply 12v "+" to pin 86 and apply ground/"-" voltage to pin 85. The result should be continuity between 30 and 87

Cheers
DD

 

Being stateside, I have no idea where in the UK. If you can't find the original, you can use a double output (87) relay. But you'll have to wire your own suppression diode.

 

That's the fuel interface unit. Simply put, it provides info to your trip computer for the "miles per gallon" calculations.

Cheers
DD

 

Working Ok when installed on the car? Or working OK when bench tested?

Cheers
DD

 

fuel pump relay ok when bench tested, ecu relay not working on bench test, it hash a bosch ref number on it but struggling to get the number in my old age

many thanks for your help, most useful.

 

Have ordered myself a ecu relay ( should be here tomorrow )

many thanks for your help

 

Just back from the shop. Interesting find. Both the main relay and the fuel pump relay have diodes wired into the wiring harness. I unwrapped the electrical tape coming from the relay sockets to splice in my own diode. There they were.

 

Diodes which might be present in earlier cars. There were several sub-variations of these circuits depending on year and market.

Very roughly speaking I'd say that 1981-1986 HEs were pretty standard across the board but changes started coming along in 1987-88 with the addition of 'idle enhancement' relays and such

Cheers
DD