Citroen Saxo 1.5d rev counter and running issue.

[JoshDieselSaxo]

Good evening all
This is my first post on here so if I've done anything wrong please do tell me.
I've inherited my grandfathers 2000 Citroen Saxo 1.5d which he bought new. I've got it up and running again having been sat in a field for 5 years, given it a service and sorted a few things out. At some point in the past the supply to the glow plug relay has been a bit iffy so someone has added a separate switch. This in itself isn't a problem. When you turn the ignition on you flick the switch and the glow plugs come on, start the car and everything works as it should, turn it off and the rev counter stops working, the glow plugs stop working and the engine becomes noticeable more noisy. That all makes sense.
This arrangement was all fine until about a week ago when I changed the alternator. Now with the switch on the glow plug warning light comes on and goes off as you'd expect on its own timer and the car starts up no problem at all, so the glow plugs and definitely still working. However, the rev counter doesn't work and the noisy running is now constant regardless of where you put the switch. I've scoured all of the wiring diagrams and bought a replacement TDC sensor to compare resistances. Both the new and existing sensor match and fitting the new sensor didn't fix the problem. I'm no electronic engineer and am rather out of my depth, despite being 22, with anything less than 50 years old.
Any help would be enormously appreciated as I would very much like to keep the car alive.
Cheers
Joshua

[mickthemaverick]

I'd be tempted to replace the iffy glow plug relay and put the wiring back to standard and see how it goes. No idea how the rev counter is involved but I'm sure one of our more Saxo familiar members will be able to offer some advice. Good luck.

[CitroJim]

There's a connection here between the rough running and the rev. counter not working.

It occurs to me the injection pump ECU is not in control of the pump dynamic timing and thus the pump is running on its fail-safe/fallback timing curve; if so then running will be a bit rough. The usual default/fail-safe curve retards the injection timing quite a way from optimum.

Can you help us by describing the nature of the rough running please?

Also, do check the wiring to the pump ECU - both its supplies, connection to the pump and also check the signal from the TDC and needle-lift sensors are present and correct. That a modification to the wiring was made to incorporate a glowplug switch suggests there was and likely still is, a significant electrical problem.

I'm not near my Saxo documentation currently but if need be I may be able unearth it and help investigate.

[JoshDieselSaxo]

Hi there.
Thank you for your response. The rough running isn't necessarily rough like a misfire, but more rough like a tractor. It sounds like a proper old tractor diesel engine, very knocky. I think you're absolutely right with the pump being in failsafe, I just need to figure out why. I can't remember whether I made it clear initially, but it is behaving EXACTLY how it used to when I switched off the glow plugs, the only difference currently being that the glow plugs still work. I think you're probably right with it being control related. I have checked the wiring into the pump and there is nothing obviously out of wack or damaged, although I suppose it's possible I disturbed something while removing the old alternator as this all started once I removed the alternator. I fitted a good second hand unit yesterday so it's now charging fine but no change in the running unfortunately. If you have any Saxo literature and you might be able to find a connection that would explain the issues I would very much appreciate it.
Many thanks
Joshua

[CitroJim]

I'll see what I can do Joshua... Sounding like a tractor and being knocky is a sure sign the timing is not optimum...

Watch this space

[CitroJim]

Here's a useful wiring diagram Joshua....

I was incorrect in saying the diesel injection pump is under ECU control. It's fully mechanical in fact. However, do check that the throttle position switch (Item 15a in the attached PDF diagram) is working and the wiring to the cold advance solenoid on the side of the pump is correct and that the solenoid is actuated when the engine is cold and idling...

Check the post-heat thermal switch (item 17) as this also controls the glowplug relay to ensure the glowplugs continue to operate after the initial cold start and until the engine is up to temperature.

Any problems in these areas can account for rough running but only when cold. If it runs roughly when hot then there is either a mechanical problem with the engine or pump; it's unlikely to be electrical.

The tacho. is operated directly from the TDC sensor (Item 24) and has no connection with the injection system.

I trust this is helpful...

2026-08-05-0003.pdf

(945.79 KiB) Downloaded 26 times

[JoshDieselSaxo]

Here's a useful wiring diagram Joshua....

I was incorrect in saying the diesel injection pump is under ECU control. It's fully mechanical in fact. However, do check that the throttle position switch (Item 15a in the attached PDF diagram) is working and the wiring to the cold advance solenoid on the side of the pump is correct and that the solenoid is actuated when the engine is cold and idling...

Check the post-heat thermal switch (item 17) as this also controls the glowplug relay to ensure the glowplugs continue to operate after the initial cold start and until the engine is up to temperature.

Any problems in these areas can account for rough running but only when cold. If it runs roughly when hot then there is either a mechanical problem with the engine or pump; it's unlikely to be electrical.

The tacho. is operated directly from the TDC sensor (Item 24) and has no connection with the injection system.

I trust this is helpful...

2026-08-05-0003.pdf

Hi there sorry for disappearing had a busy few weeks.
Thank you for the wiring diagram, annoyingly it's the same one I managed to find online which shows a 7 pin glow plug relay. My car has a 5 pin glow glow plug relay (although only 4 pins are used). The glow plugs themselves and glow plug light are still working absolutely perfectly and the engine management light works then goes out when you start the car as you'd expect. I'm still having the same problem though of noisy running and no rev counter, exactly the same as I was having when I disconnected the glow plug relay (except the glow plugs are working of course). I can't seem to find a wiring diagram which shows a 5 or 4 pin glow plug relay either.
Any thoughts?

[JoshDieselSaxo]

Additionally on my car with the 5 pin glow plug relay, the rev counter and glow plug relay/injection system must be connected somehow, as the rev counter used to stop working if/when I disconnected the glow plug really. On a 7 pin relay car that doesn't make sense because as you said the tacho is fed straight from the crank sensor, but for some reason this car is different. It's a Y reg 2000 I don't know if that helps. Sorry to go on, but this car is beginning to drive me round then bend

[CitroJim]

Josh, I'm making enquiries on your behalf to see if the correct wiring diagram can be obtained for you... Bear with us for a bit...

[mickthemaverick]

I have dug out this diagram with what appears to be a 5 wire pre heat control unit (1150) which I'm assuming to be the glow plug relay. you may find the guide to PSA diagrams helpful in interpreting this diagram: viewtopic.php?p=782862#p782862

imprimer.pdf

Saxo glow plud circuit

(1.97 MiB) Downloaded 7 times

[JoshDieselSaxo]

That's brilliant thank you very much indeed, I shall have a study of the diagrams and see if I can figure out what's going on.

[JoshDieselSaxo]

It's just occured to me while doing some research and looking at photos, my car has an ECU on top of the battery and quite a few others don't. Do you think the ECU itself could be the culprit? Also I have tried a brand new 5 pin glow plug relay (turns out it's the same as a ZX diesel) but it made no difference at all annoyingly.