I've printed out the FAQ on speedo troubleshooting for the 900s, but must have missed something.

Is there a way to test the speedometer sensor in the differential, either in the diff or removed from it? I have a "test harness" made up that I could plug into the sensor, but not sure if that's the way to go.

My speedo & cruise are both dead, odometer too. All other gauges work fine.

I checked continuity between sensor connector and the connector from the back of the instrument cluster - it's OK

I also tried the "sensor bypass" trick - running jumpers from the sensor connector (unplugged from the sensor) and rapidly making/breaking the connection between the two, to simulate the action of the sensor.

The sensor output is an AC voltage of around 1 volt increasing to about 3 volts as speed increases. Of course the frequency increases as well. It is easy to measure at the cluster connection and drive the car around the block. You could also measure by jacking up a wheel and spinning it, I believe the drive shaft must turn to get an output.

Never heard of the sensor bypass. I doubt you could make/break the connection fast enough to see a result at the speedo.
--
David Hunter

David -

So disconnect the harness wires at the sensor and use jumpers to the sensor for your AC voltage test.

Does it matter which side of the sensor I put my voltmeter + lead on? (My 940 has 1 brown and 1 green/white wire connected to the sensor)

Thanks

the sensor bypass test -
http://www.brickboard.com/FAQ/700-900/ElectricalSpeedometerRepair.htm

You could do as you described. It is an AC signal so meter polarity does not matter. I would check the signal at the instrument panel so that you know the wiring is good too. By the way you can drive your car with the instruments completely removed from the car. This makes it easy to get at the connection from the sender. Watch you don't speed now.

Interesting about the test in the FAQ, never read about that. Would be interested to know how that works if youi decide to try it.
--
David Hunter

I also tried the "sensor bypass" trick - running jumpers from the sensor connector (unplugged from the sensor) and rapidly making/breaking the connection between the two, to simulate the action of the sensor.

Where did this one come from?
--
Art Benstein near Baltimore

A plateau is a high form of flattery.

Art -
the sensor bypass test -
http://www.brickboard.com/FAQ/700-900/ElectricalSpeedometerRepair.htm

Thanks,

I think I read that article twice without seeing this:

REAR SENDER TEST:

Faults are usually either in the gage itself or the rear sender.

A very effective and easy way to diagnose the rear sender is to disconnect the rear connector on the differential case and connect two long pieces of wires to the plug coming from the vehicle wiring, bring the “extension” wires inside the car, start the car, and start shorting the wires at a pace of 4-5 shorts per second. If the gage is OK you should start to read a speed over 15MPH. The faster you short, the higher the speed.

As I told another poster, I know diddly do about 7/9 speedometers, so I can't say it isn't so. It could be the sender's amplifier and limiter (inside the speedometer circuit) are sensitive enough to interpret this action if the surrounding electrical noise being picked up by the long extension wires is shorted out repeatedly. But as a conclusive test, there are too many environmental variables to rely on it.

Despite my diddly-do 7/9 knowledge, I would stick with David Hunter's qualification of the sender. First, determine the wiring and sender's coil show the continuity and expected DC resistance by using your ohmmeter up front. Then, up at the dash end, measure the AC voltage generated when you drive. Polarity is not an issue using your meter in AC mode. Use clip leads, or some other method of having secure connections into your meter, as poor jiggled probe tip connections will send your AC meter bouncing erratically. The usual problem is a break in the flex harness going into the differential, so it will work at times, when the wires are touching, but get flaky with movement.


--
Art Benstein near Baltimore

From the AMAZING HOME REMEDIES category: If you are choking on an ice cube, don't panic. Simply pour a cup of boiling water down your throat and presto. The blockage will be almost instantly removed.

Art -
Let me impose on your time some more ...

What resistance would be typical for the sensor coil?

And I assume you mean to dothe driving test with the 4-pin speedo connector removed from the instrument cluster, and the multi-meter test probes applied to the 2 pins on the connector.
In other words, the differential sensor will generate an AC signal by itself ...

Thanks

Again, without experience on the 900 series, I can only make a guess its sender is similar to the 93 240 sender, which Bentley says should range from 600 to 1600 ohms. The 240 sender is a reluctor, meaning a coil with a magnetized core, that will generate current in proportion to the disturbance of its magnetic field, such as the openings in the differential's tone ring will create as they pass it.

The idea of the measurement is to be sure the wiring is there and it is not shorted, not to figure out whether the sensor has the right number of turns wound on its core. So why not make your measurement, and then ask us if it sounds right, in case it doesn't seem right to you. Someone here, in the 900 forum, will know. Maybe you already have, and I haven't followed the thread.

Same goes for driving without the wires connected. David proposed the idea and it sounds good to me. Whether the 940 needs some special treatment, again, is something for the 900 group to set us straight on. I just wanted to clarify using an AC voltmeter in a sometimes electrically noisy automotive environment.
--
Art Benstein near Baltimore

When cheese gets its picture taken, what does it say?

Art -
Much obliged. It appears that the 240 & 940 use the same signal generator, if I can trust the online parts seaches.

The Haynes UK 940 service book groups the speed sensor in the ABS section.
If the sensor has failed, should it throw an ABS trouble code?

Thanks

Good thinking!

I tried to see if the sensors were the same, but apparently didn't look hard enough. On the ABS error code, the lack of one would seem to vote in favor of a good sensor and cable.
--
Art Benstein near Baltimore

Why isn't the number 11 pronounced onety one?

Many thanks to Art & David for the speedo testing ideas.

I ran the AC signal test at the speedo connector & saw an increasing voltage as speed increased. So it's the speedo head that's defective.

How I did it single-handed - (in case it will help anybody else)
I took 2 male crimp-on spade terminals, trimmed them on each side of the flat spade to narrow them to about half the original width, then crimped them lightly to the tips of the probe wires on my multi-meter. This gave me a flat tip to insert in the connectors.
Pulled the 4-pin connector from the back of the instrument cluster, located the 2 speedo sensor wires (#1 & #2, Green/White & Brown), then inserted the probes/adapters into the BACK of the 4-pin connector, alongside the wires, NOT into the front of the connector where it plugs onto the cluster.

Great forum you have here, guys.

I can't wait to try this on my 1993 940! Thanks.

I also wondered about the ABS. My ABS light does not come on with the other warning lights before startup. Does your light come on then?

Best,
Steve