Citroen XM Forum

What is that bulletin number and content?

Just out of interest, I put a multimeter on a brand new 6160 70 (3-pin) sensor here. (The latest one, fitted to many PSA vehicles.)
Expected to find two of the pins were the ends of a Hall effect sensing coil, and would show as a moderate d.c. resistance.

Surprised to find resistance between pins to be
(pin view, black potting at the top)
Outer pins: infinity, irrespective of meter polarity
Left-centre: 25M with -ve probe to left; infinity +ve probe to left
Centre-right: infinity -ve probe to left; 8M5 +ve probe to left

Was a bit surprised at that.
Suggests additional components inside, not just a magnetic wheel and coil.
Not about to break this brand new one open to find out...

xmexclusive wrote: Sun Nov 03, 2019 3:27 pm What is that bulletin number and content?

Bulletin No. 66, dated 24/11/1995. For MY-1995 3.0, 12v & 24v.

Engine misfire at low speed, speed sensor code 27 recorded by the engine ECU only, no record of speed sensor code 24 in the Hydractive ECU.


This would be a H2 equipped, Series 2 car, so doesn't answer whether the 3 pin speed sensor can be used with H1 systems.

White Exec wrote: Sun Nov 03, 2019 9:55 pm Just out of interest, I put a multimeter on a brand new 6160 70 (3-pin) sensor here. (The latest one, fitted to many PSA vehicles.)
Expected to find two of the pins were the ends of a Hall effect sensing coil, and would show as a moderate d.c. resistance.

Surprised to find resistance between pins to be
(pin view, black potting at the top)
Outer pins: infinity, irrespective of meter polarity
Left-centre: 25M with -ve probe to left; infinity +ve probe to left
Centre-right: infinity -ve probe to left; 8M5 +ve probe to left

Was a bit surprised at that.
Suggests additional components inside, not just a magnetic wheel and coil.
Not about to break this brand new one open to find out...

I'm not sure whether 3 pin sensors have built in amplification, or use a transistor to shunt the signal line to Gnd. I suspect they are amplified, hence my caution in connectiing one to an H-1 hydractive ECU.

The new one I have just taken to bits has 6 two contact components in addition to the Hall device.
No transistors or IC's so probably a passive circuit.
The biggest performance changes since the original XM speed sensors were in field strength and understanding how to design with Hall devices.
Modern magnetic materials offer up to a five fold increase in field strength for a given volume.

H-1 ECU's accept both amplified and non amplified signals.

Well done, John, in opening up a new one. I had only bought a new spare a month ago, so didn't want to butcher it.
(The car is on its third, which has been there since its second year on the road (2001). The first two sensors both gave up, and were replaced under warranty. At the time, it was remarked "These fail..." )

Good news about an H1 ECU accepting either arrangement.

Hi Will

Thanks for that link.
I found it in XM 1 which is the engine section of the bulletin manual.
I had been looking for it in XM 9 which is the suspension section.

There is a clear statement that it is a mod to correct a V6 engine ECU malfunction.
It also confirms that the fault did not affect the suspension ECU.
Note that models with other engine types were not modified and continued to use 3 wire speed sensors.
As you point out the ECU in this case is a H-2 system (Hydractive 2) while your question is in respect of the pre 1993 original Hydractive ECU's.
The other oddity is the bulletin drawings showing mechanical speedo drive on a mod for a series 2 XM.
I assume that a late series 1 XM diagram was to hand and such V6 cars would also need the mod.

I assume that all these speed sensor designs predate HALL effect ones.
I can therefore understand your concern about the knowing that the speed signal levels and types are correct for your older ECU's.
When I get the chance I will have a search for older items that may help us to know more.
Just now medically limited in what I can do until my blood test results get back to normal.