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  #1  
Old 09-01-2010, 02:35 AM
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How to test w123 Speedometer Cruise Sender Switch

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

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Old 09-01-2010, 01:01 PM
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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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Old 09-01-2010, 01:13 PM
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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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Old 09-01-2010, 02:02 PM
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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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Old 09-01-2010, 02:13 PM
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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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  #6  
Old 09-01-2010, 02:43 PM
Craig
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I believe the GDL site had detailed test procedures, but I think you need a scope to do it correctly.
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Old 09-01-2010, 02:58 PM
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Quote:
Originally Posted by compu_85 View Post
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
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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Old 09-01-2010, 03:10 PM
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http://gdl-online.com/test024.html
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Old 09-01-2010, 04:08 PM
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"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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Old 09-01-2010, 04:20 PM
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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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