how do you bench test the small relay/switch/sender thingy on the back of the speedometer using a multimeter?

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i don't know for sure but i would figure ohms, to test the functionality though i would hook it up to measure ohms so you can get a stable reading and wave a magnet close to the coil. you could also use a simple diode or "beep" test to check continuity through the coil.

give me 5 minutes and i'll go measure the resistance of the good sender i have here and post that up for you.

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OK here's what I got. MB P/N: 004 542 6517 VDO P/N: 88 374 007

Im getting 941 Ohms across the contacts of the coil. Like I said in my last post it should respond to a nearby magnet so if you hook it up to your DMM and wave a magnet near it the resistance should change as the magnet gets closer and further.

I hope this helps you out man.

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nick, you clearly know what you're talking about. here is the multimeter i own:



can you tell me what settings i should put the multimeter to and if it makes a difference which pin i put the black or red lead on? i'm going to use a telescoping tool pickup magnet to test it. does the size of the magnet affect how much the ohms increase or decrease, or is it mainly the distance?

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That's a hall effect sensor, just a coil of wire around a metal piece. Test if it has an open circuit or not.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
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2014 Cadillac ELR
2013 Fiat 500E.

I believe the GDL site had detailed test procedures, but I think you need a scope to do it correctly.

wait, ok. so, how do i test it using the multi-meter shown above?

craig: the gdl website didn't s how any procedures for testing this relay. the part number posted by nick is correct

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http://gdl-online.com/test024.html

"B) Speed Sensor Test (meter common to ground)

This test must be performed with the rear wheels turning. Please have two qualified people present at all times. Pin 11 Speed Sensor loaded (with the amplifier plugged in) must be at least 2.0 Vrms at 25 mph. The amplifier actually measures the frequency of this signal. At 60 mph the frequency should be 131 Hz."

so this says the speed sensor has to be attached to the speedometer, with the car jacked up or driving. given that the relay is behind the speedo, and that the speedo cable can only be stretched so much before it requires replacement, and how hard it would be to get a multimeter (with two hands) behind the speedo while driving or the car on jack stand and someone else revving the car until it simulates 25mph, i figure it would just be easier/cheaper/less time consuming to just buy a new one.

or is there some other way to bench test it which doesn't require an octopus

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Yes, Check the resistance of the coil on the back of the speedo.

If its around 1K then it OK.

Cruise faults are generally the controller or 'amplifier' --as people round here call it....

--This device suffers from a few maladies, mostly dry-soldered joints on the board, sometimes the tiny reed-relay develops a high resistance leak as well as the set-speed cap and the FET. The FET's however are no-longer available, they are a two-gated P channel device....

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....