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Discussion Starter #1
First off - I am pretty much a complete idiot when it comes to electrical stuff - so PLEASE type slowly.

I have a VDO speedo in my wagon that uses a 3 wire (red, black, white) Hall Effects sender. I am trying to trouble shoot a problem I am having with my GearVendors control box. It doesn't appear that the box is receiving any signal from the sender. Since the speedo is working fine, I am trying to trace the wires from where they connect into the sender cable. I have spliced into the black wire (which is ground) and I know how to test this wire. I have also spliced into the white wire (which I have no clue what it is).

So my question is - what type of electrical test can I do to confirm the signal from the white wire?

Thank you very much for your help.
 

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Now this is just a wild stab..... I'd also suggest you call GV or VDO to find out for sure.

Here's my take. One wire is ground, one is +12v, and the third one is a switched 12V return. This third wire is the "trigger" signal if you catch my drift.....
 

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Discussion Starter #3
Thanks John

I probably need to call VDO. I know that the black wire is ground.

I just wasn't sure what setting to try on my digital multi-meter to confirm that the signal is being sent to the GV box. I think I might just try 2000m Vdc. It has to start low and increase as the mile per hour increases. I have the car up on stands so I can pretty easily start the car - put it in gear and run the speed up 30 - 40 mph. I have tested the jumper cable from the sender cable to the control box for continuity and every thing appears to be OK. So either my splice connector on the sender cable isn't making a proper connection or the box doesn't like what is being sent to it.
 

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Hall Effect Switches

The trouble here is not knowing what the signal coming from the sensor looks like. It MAY be possible to measure it with a volt meter, or it may not. If the sensor has an impulse output (a very brief pulse) every time it senses the tooth in the gear or cog its detecting, a voltmeter likely is not responsive enough to display it. Digital voltmeters average the voltage readings over time. Analog meters arent responsive until the pulse is large enough.

I'd call VDO.

B
 

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Hi,
In my experience the following information should get you out of trouble 99% of the time,
Most hall effect senders require a pull up resistor to make the signal visible wich you will be able to buy from any electronics parts store about 5 cents worth, connect the resistor between the ignition supply and the output of the sender unit (Red and white wires). most hall effect senders have approximately 8 pulses per one revolution of the sender, that is, the output will change state from high (12 volts) to low (approx. 0.8 volts) 8 times for one turn of the sender. place the volt meter across the resistor to measure this. with the ignition turned on and earth connected, turn the sender slowly by hand to watch the change on the volt meter,hope this helps.

Theo
 

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Discussion Starter #6
Theo

As I mentioned earlier I am not real smart on electronic stuff and then we have this Aussy - American English translation thing :p . Can you explain what a "pull up resistor" is? Can I go to a local electronics store and ask for such a thing and the sales guy will know what I mean?

It is sort of a mute point as I was really just trying to prove to myself that the my splice was good. I was able to do that by testing the continuity between the sendor and the control box for the GearVendor. I talked with VDO and they said is was a 16 pulse hall effect unit and it should pulse between 4 volts and 0 - 16 times a revolution. I can see the voltage go up and down with my cheap multimeter but I can get the clean breaks which I assume the resistor will do for me.

Thank you for the info. I guess I am still alive as I continue to learn something new every day about my car.
 

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Hi,
Sorry for the novel, forgot to mention that the resistor you need is a 10K OHM ressistor that any electronic parts store will have, it is very tiny the body is only around 5mm in length,connect one end of the resistor to ignition and the other end to the signal wire,just splice into the wires anywhere before the end of the wires of the sender.
Hope this helps,

Theo
 
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