speedometer sender unit - how does it work?

[jimsunz]

I have a 740 automatic estate(wagon ?)in the UK. The Haynes manual
says that the sender unit is mounted on the back axle. This sender
unit sends pulses to the electronic speedometer.

How does this sender unit work?
Are there magnets in the axle inducing pulses in a coil on the sender
unit?
Or is it an optical unit reading rotations of gear teeth?
Or some other method?

Secondly, Where does the cruise control get its pulses from?
The Haynes manual says from the back of the speedometer. Does anyone
know the correct connection?

I have a kit cruise control that has been on the last 5 cars, various
makes, all manual gear change. The pulses were taken from the ignition
coil.
I had it on my last 740 which had manual gearstick.
The ignition coil pulse cannot be used on an automatic. Instead the
kit has magnets which are strapped around the prop shaft, together
with a pickup coil.
I was wondering whether I could use the speedo connection mentioned
above, instead of crawling under the car etc.

Many thanks guys for your help.

 

[James Sweet]

I have a 740 automatic estate(wagon ?)in the UK. The Haynes manual
says that the sender unit is mounted on the back axle. This sender
unit sends pulses to the electronic speedometer.

How does this sender unit work?
Are there magnets in the axle inducing pulses in a coil on the sender
unit?
Or is it an optical unit reading rotations of gear teeth?
Or some other method?

IIRC it uses a reluctance pickup, the sender is a coil stuck in the rear
end, and there's metal teeth on the differential housing that pass by the
end of the coil. You'd have to use an oscilloscope to see exactly what sort
of signal this puts out but it will be pulses of some sort.

 

[Robert Dietz]

IIRC it uses a reluctance pickup, the sender is a coil stuck in the rear
end, and there's metal teeth on the differential housing that pass by the
end of the coil. You'd have to use an oscilloscope to see exactly what sort
of signal this puts out but it will be pulses of some sort.

As you said, the pickup is a coil wrapped around a permanent magnet. As
the tone wheel castellations pass the end of the magnet the flux changes
and induces an A/C current on the wires. As the speed increasesboth the
frequency and amplitude fo the signal increase. If I remember correctly,
the diff generates 48 pulses per revolution. Since the ABS uses the same
pulse set for control it has to be modified to matched the pulses
genrated by the front wheel sensors. If I remember correctly they use a
64 or 80 tooth wheel. The rear signal is split, one leg remains unchanged
and goes to the speedo. The other leg is fed through an AD converter and
divided to represent a pulse rate indentical to the fronts and sent to
the ABS controller. The raw signal from the rear sensor drives the speedo
directly. There is an AD converter in the speedo that outputs a digital
square wave that is sufficient for the factory cruise control unit.
However if the amplitude is falling due to broken strands in the sender
wires at the diff (when the signal is clipped if the amplitude is too low
then the square wave is discontinuous at both ends)one can splice (jump)
the two wire sets together in circuit to increase the cruise output
signal strength.

As for the Dana type cruise control, it's looking for two pulses per
revolution and I'm afraid the OP will have to crawl under the car to
install the two magnets and the pick up coil.

Bob

 

[James Sweet]

As for the Dana type cruise control, it's looking for two pulses per
revolution and I'm afraid the OP will have to crawl under the car to
install the two magnets and the pick up coil.

Would it matter if the pulses are not exactly the same number per turn? The
cruise control needs only a relative value, not an absolute speed
measurement. Of course it needs to be in the same ballpark otherwise it'll
be out of range at some speeds. Could also use a counter chip to divide down
the signal to something appropriate for the cruise control.