Citroen XM Forum

Anyone got one please? The Hella part number is 5GA 004 397-17. I know a Xantia S1 one is the same as well but not the Xantia S2. How bizarre...

Full story of why I need one in my ES9 Topic in the members cars...

TIA

Will have a look Jim. I have a Hella ECU from a 2.1 TD.

Thanks Robert, that should be perfect

I also have one from a 2.1! in fact the whole system except the pump

Gentlemen, happily I no longer require one. I've fixed mine by the replacement of the three duff transistors and one duff zener I found... Now working a treat

Nice fix Jim, now can you fix my radio?

Hi Jim, good fix. Have you considered the cause of the failure ? Old age or an external component, ie pump ?
robertm

robertmnorton wrote:Have you considered the cause of the failure ? Old age or an external component, ie pump ?
robertm

I have Robert and came to the conclusion that it may have been used on a Xantia. It's an oddity that the Xantia S2 cruise ECU is slightly differently wired in that the vacuum pump motor control and the vacuum control electrovalve are wired opposite; the pump is on pin 7 on the XM and Pin 6 in the Xantia whilst the vacuum electrovalve is on pin 6 on the XM and 7 on the Xantia.

The pump takes a highish current and is driven by a BD535 power transistor. The vacuum electrovalve is a low current device and is therefore driven by a pair of little BC337 transistors. these were short-circuit, as was the 24v zener across the pump supply. the BD535 was tested OK but I swapped it as a precaution.

So, I think the vacuum electrovalve circuit tried to drive the pump and that took out the BC337s and the pump circuit trying to drive the vacuum electrovalve caused a big enough spike of back-emf from it to pop the zener...

So the moral of this tale is if you think an S2 Xantia has a duff cruise ECU don't try substituting an XM one. A quirk is that an S1 Xantia uses the same cruise ECU as the XM I can't explain that one either!

Hi Jim, many thanks for info, I suspected the high current for the pump was key but thought mechanical issues with the pump the likely cause. Would never have considered a foreign unit (xantia) anyway.
robertm

It's bizarre Robert but every model that uses the Hella cruise have much the same ECU but the connector pin-outs are different in every case. I've got hold of some hand-drawn circuits of the ECU used an a VW Passat. It's very, very similar to the XM one but all the pins are different.

I guess the microprocessor program is slightly different for every model of car and this is a way of ensuring they can't be swapped between cars with potentially dangerous results..