Help please, jaguar xjs 2+2 1994 4.0 engine. As part of the fuel pump relay connector in the right rear fender well. The relay is five blade. Extending from the relay connector base is a 1 wire module with 4 diodes (encased and sealed in transparent plastic) the wire in the diode goes between pin 85 which comes from the ecu high power side and pin 86 wich comes from the ecu low power side. I have replaced the fuel pump and when I jumper wire 12v to the fuel pump power wire (the blue red wire), the pump turns on and the relay is new. I removed the module and pump does not turn on. There is 12v at pin 85 coming from the ec high power side. Searching the net I find only that this module might be a voltage control module. there is part no number and nothing in jaguar electic specs. This module connection is not an add on and is integral to the relay connector base. So it seems that without this module the fuel pump will not operate. My knowlege on this limited so all suggestions are greatly appreciated.
John

 

The relay may be 5 blade, but per diagram, the extra (probably load pin - an 87a) is unused. The 94 diagram doesn't illustrate the diode package you describe, instead showing the resister. Diode package across 85/86 as being internal to the relay (typical) which bleeds off feedback inductive spikes in the relay coil itself. It sounds like for some reason an *external* package was setup on your car as if the diode/resistor wasn't internal to the relay itself. This is the control side of the module, and IDK why the relay wouldn't operate w/o the flyback suppressor though it's less safe over the long term for the electronics. Similarly there is an inline filter on the Fuel Pump called the "RFI protection module"

Control side power is supplied via the Main EMS relay to pin 86 of the FPRelay, and it's *turn on* side is the ground side on pin 85 controlled by ECU Pin LI92-5.

The Load side power to the FPRelay is direct battery power on pin 30 which comes via the Right Rear Fuse Panel on 20A fuse #12 (input 52). And load power is, as you know, on the blue (U) red (R) wire via pin 87.

Hopefully that helps a bit in troubleshooting.

 

Thanks Ferrariguy, That confirms what I was troubleshooting. The diode pack on the relay connector is a voltage control diode which only lets voltage flow one way and not the reverse. It is similar to the diode pack on the ecu main relay. So I think the diode is to protect the low voltage feed pin 86. So With the ignition on and the relay removed I probed the pin connections. Pin 30 - 12.40 v, pin 86 - 1.45v,, pin 85 12.38v, but pin 87 was zero and when I jumper wire pin 30 to pin 87 nothing happens. If I jumper wire from the battery to the red blue wire that sends power to the pump the pump turns on. However even then the engine will not start. I am thinking there may be a problem with the ecu. My guess is the ecu is faulting and not sending the correct signals to the relay connector. Your thoughts please.

John

 

Also in the diagram of the relay pump connector and between pin 86 and 87 there is the diode symbol
John

 

My Bad, the diode is between 85 and 86

 

More troubleshooting, the inertia switch was not tripped, but I bypassed it just the same. Something is keeping the circuit open between the 30 and 87 pins. I do not hear the 2 to 3 second burst of the pump priming upon turning the key to the on position. It appears from the relay diagram that this circuit and the priming burst should be closed by power from the low power side of the ecu pin 86, which is probably also why the diode is needed at that location. I have ordered a replacement ecu and am hoping that will solve the issue.
John

 

progress update: The replacement ecu is giving the same voltage readings as the old ecu, so the ecu is not the problem. Which brings me back to the diode pack on the fuel pump relay connector base. It is dbc5471 but only shows as a diode on the elec specs from pin 85 to pin 86 on the fuel pump relay base. it is a voltage control diode pack. It bench tested ok, but when I reinstall it, it does not control the voltage, pin 86 should be low voltage, but it is reading 12.56 volts. pin 86 comes from the low power side of the ecu and controls the voltage to the maf, the egr solenoid vacuum valve, and the air injection relay. It is similar to the diode pack on the ems relay connector base, and on that relay pin 86 is showing .04 volts, so that diode pack is operating correctly. On both diode packs it appears to be 2 diodes and 3 resistors in a series.
John

 

John,

Silly question, but have you tried just replacing the fuel pump relay, which are known to fail sometimes?

Good luck

Paul

 

Hi Paul, Yes of course that is the first thing I did
John

 

It seems to me that the diode pack is intended to prevent the high voltage pin 85 from crossing over to the low voltage pin 86 and is so doing the voltage from pin 85 then goes to the solenoid in the relay to close the power connection to pin 30 and 87 and activate the fuel pump
John

 

Just thinking out loud. On some older jaguar models a relay with the diode built into the relay was used. Pektron and hella each made them. So did jaguar create their own diode pack to be integral to the fuel pump relay connector. thus allowing a standard 5 pin relay to be used? It appears that these relays would serve the same function.If anyone knows please confirm.
John

 

So today I jumper wired directly from the battery to the fuel pump, with the key in the off position. The new fuel pump worked fine and the pressure reading was good and it held pressure for over an hour. So that eliminates the fuel pump and fuel delivery as a problem. I will try the hella relay with internal diode this week, then next week I will receive the replacement jaguar diode pack (new) and can use it if necessary.
John