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04' C230 Kompressor sedan W203
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Discussion Starter #1
Folks, need help with my inop tach. 83' 300D. The tach has not worked since i owned had the car. Finally getting around to T/S it. Here's what i've done so far:

1983 300D Turbo. Tach does not move at all.

1. pulled cluster verified it is connected in the back.
2. purchased new replacement Tach Amp & installed.
3. inspected sensor and it looks as though it is actually touching the metal pin on the balancer. So it is surely close enough.

With Amp removed.
Pin 2 to Pin 6 reading 12 VDC
Pin 2 to pin 3 reading 10 VDC
* Reading 77ohms from sensor ground (pin 7) to *Sensor + (pin 9).
* Sensor shield (pin 8) to pin 9 is reading open.
* Pin 7 to pin 8 are reading open.

With Amp installed.
* Glow Position Pin 8 to Pin 9 reading 2.4 VDC
* Glow Position Pin 7 to Pin 9 reading 2.4 VDC

Assuming here that Pin 8 is a shielded ground (shorted to pin 7).

I believe all reading are as they should be for a working Tach. What have i failed to check other than the Tach itself failed internally?

Thank you for your time.
 

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77 240D
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i believe the trick with debugging a system like this is to take measurements using an oscilloscope instead of a multimeter. what you are looking for is a waveform of a particular period, and only when the voltage is visualized over time will you see whether the pattern is present. that said, not having done this, i'm not totally sure what the waveform would look like, but i'm going to guess that there is a peak for every revolution of the magnet past the hall effect sensor, which is quite low-voltage before the tach amp, and somewhat higher voltage between the amp and the cluster.
 

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04' C230 Kompressor sedan W203
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Discussion Starter #4
Thanks brickton. Don't have an O scope. Read the link you provided. So this means that i should have variable VDC at the cluster connector from 2-12 VDC depending on RPM? Would this be lower voltage at lower RPM or Vice Versa? Come to think of it ...i don't remember checking ground at the tach cluster connector. I will go back and check today and report back.

BTW.... was out in SF last week. Really liked Saucelito and Muir Woods. Talk about a contrast in weather. Bay area night temps in 40s no humity to GA highs over 100 and 80% humidity.:surrender::surrender:
 

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1981 W123 300D non turbo, 1992 190E 1.8 <=> 2.0
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I'm not sure how you should go about checking the operational effectiveness of the sensor but I found this

Hall Effect Measurements Resistivity

perhaps you can make sense of it?

I don't think you're going to have much joy with a standard digital multimeter - a 'scope would probably be better as said before - but as we can't magic one out of thin air for you...

The bit where you say "actually touching the pin on the balancer" makes me a bit worried though. Have you pulled the sensor off of the car and checked it for damage?
 

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04' C230 Kompressor sedan W203
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Discussion Starter #6 (Edited)
I have not pulled the sensor yet to check for damage yet. I did get as close as possible beneath the car to see it and turned the crank to see the pin rest at the sensor. You couldn slide a cat hair in between the two. Supposed to be that close? What is the sensor tip made of?


Army, that link made my head hurt....
 

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77 240D
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hey cool, marin county is real nice, i was there yesterday. what brought you out there?

in light of mach4's post i think you are right about the VDC at the cluster; that, you should be able to test with a plain voltmeter. do let me know what you find as i also need to get mine working (240D did not come with a tach; i have the tooth, amp, and cluster, but haven't linked them up yet.)
 

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04' C230 Kompressor sedan W203
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Discussion Starter #8
I come out to Fairfield 2/3 times a year for my job. Caught the Vallejo Ferry to SF. Passed a salvage yard on that main road but they were closed that day. I will post my findings. I do have a 3 prong cluster where mine is 2 prong. I think i will try and slave it in and see if i get Tach movement on that one.
 

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77 240D
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i would like to try that ferry, never have. i also have a 3-prong cluster. i'm not sure i have the wiring which connects the tach amp to the cluster at all. it's going to take some poking around - let us know what you find.
 

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04' C230 Kompressor sedan W203
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Discussion Starter #10
Checked pins 2 and 3 with engine running: idle 44 mVAC. Voltage actually dropped with throttle advance. High RPM gave me about 24 mVAC.
Reinstalled Tach Amp and checked voltage cluster two pin connector. Here read 13 .4 VDC from fuse box. Also signal wire from Amp read 275 mVAC and increased with throttle advance as it should.

Although not positive i am almost convinced it must be the Tach itself. Just don't like swapping components without being more certain.


Update: scrounged up a cluster schematic for Tach. BK/RD wire is hot from fuse box and the other wire GN/BK is signal wire from Tach Amp. The signal wire is a straight run from Tach Amp pin 3.

Funola, i misinterpreted your above post and read pin 3 voltage at amp connector. I went back today and read at the cluster as i should have the first time. If i understand the pickup correctly then having this mVAC at cluster signal wire along with increase with throttle tells me the pickup and circuit through amp is acting correctly.

Looks like the tach body serves as ground to the cluster body then through the large multi pin cluster connector. So i either have a ground issue or dead Tach. Does anyone know what the actual Tach resistance should be reading at the back of the cluster(two pins)? Mine is reading .76 meg ohm. I would think a functioning tach would read higher than this.
 

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77 240D
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i am not clear that it's correct to read volts AC at the cluster. i would have guessed that the signal would be DC. whichever one is true, you could also try connecting the tach directly to a voltage source to see if the needle moves that way.
 

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04' C230 Kompressor sedan W203
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Discussion Starter #12
I was going off one of the posts i read to measure for presence of signal from Tach Amp. Use the mVAC scale to pick up signal. Not real sure where the ground comes from for the Tach now??
 
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