RHZ not pulling in Fuel Pump Relay

[addo]

There's a catch, I can't communicate with the BSI.

There's another catch - I don't want to spend forever on the car.

On the tachymetric relay, one coil pulls in just fine - but the other, which is grounded by internal switching of the ECU - isn't getting its required ground.

I'm thinking of using the functional side of the tachy relay, to operate a third relay that pulls in a ground contact to operate the fuel pump, and leaving the original ECU wire open circuit. Can anyone see a problem with this?

Thanks, Adam.

[wheeler]

I assume this is a C5 seeing as you mentioed the BSI, the tachymetric relay is integral to the under bonnet fusebox, how are you checking the primary side of the relay in this case ?

[addo]

It's actually a late 406 (ssh!) with the hybrid relay panel underbonnet but the BSI was only a fringe issue - I mentioned it because of inability to access fault logs.

For reasons I cannot fathom, the car ran fine with a substitute tachymetric relay today. The owner took it for the weekend, I'm expecting it'll come back next week again. The impression it gave me with some thinking about the multimeter screen, was a dying transistor (that switched the ground) in the ECU. It would conduct the ground for a moment with maybe 0.4V drop, then let it go altogether.

[wheeler]

Ahh so its a 406 without com2000 then ? that means it has a separate 'double' relay ?
First things I would do here is run an additional earth from the chassis/engine block straight onto the casing of the ECU + remove & clean the main earth points where they eventually terminate from the ECU.

I'm thinking of using the functional side of the tachy relay, to operate a third relay that pulls in a ground contact to operate the fuel pump, and leaving the original ECU wire open circuit. Can anyone see a problem with this?

Now I don't have a wiring diagram in front of me but if I remember right the first side of the relay operates all the time with the ignition on right ? the second side of the relay should only ever operate if the engine is turning (with the exception of the initial couple of second priming when you first switch the ignition on). This is a safety feature & should never be bypassed, it sounds like if you did what you were intending the fuel pump would run all the time with the ignition on. If the car were in an accident where a fuel line was severed the fuel pump would continue to keep pumping fuel out until the key was turned off.

[addo]

Thank you very much. You're right on how the relay works, the only caveat being I would have used the inertia switch as a breaker for the added-in relay ground. The extra earth I can see a lot of merit in, I've worried at times about capacitative loops but that's probably irrelevant on a fairly rugged piece of kit.

Much of the reading I did, focused on the rail pressure switch (allegedly) screwing things up. I note they have changed the part number in Service Citroën and this one appears original (243000km). Cussed thing runs like a tractor down low, and is a real laggard with the boost, think it has more troubles than the electrical issues dumped on me.

[wheeler]

I've worried at times about capacitative loops but that's probably irrelevant on a fairly rugged piece of kit.

I don't even know what that means but I have ran direct earths to injection ecu's with no problems, it can save ECUs when jump leads have been connected the wrong way around or when a live has been shorted on the ECU casing, usually xsara picasso as the positive jump point is right next to the ECU, I've seen people moving it out the way to do clutch changes etc without disconnecting the battery.