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Modify an m104 tachometer to work with an om603?
tjts1 Offline
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#1
Modify an m104 tachometer to work with an om603?
The tachometer on my 87 300d died and I wasn't able to fix it. I have a spare 94 e320 cluster which I was planning to use parts from for the improved lighting. For shits and giggles I plugged the e320 tach into my 300d and it pegged at 7k as the engine was idling. The 300d reads rpm off the starter gear and the e320 reads off the 3 timing windows built into the flywheel.  Has anyone successfully modified a gas tach to work in a diesel? The car already has an M103/M104 flywheel from the 5spd swap so I guess one option would be to move the crank sensor or add a second crank sensor so it can read the 3 timing Windows. 

I appreciate any experience or ideals with the problem.

EDIT: Looks like one guy was able to do it on a 190d but I can't find any details. 
https://www.youtube.com/watch?v=iSLWW2ePaDg
[Image: 208104.png]
(This post was last modified: 01-09-2017, 10:27 PM by tjts1.)
01-09-2017, 10:18 PM
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Tobulus Offline
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#2
RE: Modify an m104 tachometer to work with an om603?
I did that once, wait till today evening when I get home
01-10-2017, 04:21 AM
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maxypriest Offline
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#3
RE: Modify an m104 tachometer to work with an om603?
I used the W wire from the alt into a Dakota Digital devise and then into the tach.
Works for me.
W124 om606 HX40 finished and it's a beauty 450bhp/456ft-lbs
01-10-2017, 06:37 AM
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Tobulus Offline
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#4
RE: Modify an m104 tachometer to work with an om603?
You dont need any additional devices to connect a gasoline tach to either the generator or the engine speed sensor
01-10-2017, 07:09 AM
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barrote Offline
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#5
RE: Modify an m104 tachometer to work with an om603?
talking in tacho´s .... odo´s shouldn´t be that hard too, anyone know how to hook a 616 gear box speed sensor to a w124 cluster? or the ABS speed sensor .....
thanks
FD,
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01-10-2017, 07:48 AM
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Tobulus Offline
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#6
RE: Modify an m104 tachometer to work with an om603?
Look at my project-thread for speedometer-informations

Here is information about tachometer-adjustment:

All the w124 tachometers I have seen shared the same board layout. Newer versions have a modern style, green board, the older ones have a brown board. But the layout is basically the same. They are all built for different rev signals, but basically the only difference is how many pulses the tachometer gets per crankshaft rotation. Also they are very tolerant to what sort of input signal you feed to it.

There are several resistors and capacitors on the board, you are looking for a small non-electrolytic-style capacitor. In the picture, it is shown in the lower right corner, the green one. This capacitor defines, how much the needle responses to each signal impulse. The higher the capacity, the greater the response. On this board, it is a 4.5nF capacitor for signal input from starter gear sensor (144 pulses per crankshaft rotation). As you can see, I soldered a bigger capacitor parallel to it to increase capacity by 47nF. I did this to modify this tachometer for signal input from generator terminal W.

   

The dimension of the components on these boards are different from version to version, I recommend to look for the smallest capacitor on the board.

There is also a variable resistor on that board to fine-tune the instrument. With this resistor, you can put an offset to the instument. This is for fine-tuning only.

Grüße
01-11-2017, 03:17 PM
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tjts1 Offline
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#7
RE: Modify an m104 tachometer to work with an om603?
Grüße
Thank you very much, this is exactly the information I was looking for. Did you try multiple capacitor values before arriving at this solution? Do you think matching the capacitor value of the original 6k diesel tach would be a good starting point?

I think I found a burned resistor on the diesel tach so I'm going to try replacing that one, then give the gas tach another try if that fails. 
Cheers
[Image: 208104.png]
(This post was last modified: 01-12-2017, 02:44 AM by tjts1.)
01-12-2017, 02:43 AM
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tjts1 Offline
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#8
RE: Modify an m104 tachometer to work with an om603?
I was able to fix the 6k diesel tach by replacing the 2 electrolytic capacitors and 1 burned up 51ohm resistor. The Caps are 50v 1uF and 100v 10uf. The 10uF cap was short circuited internally. There was no resistance across the 2 pins. The new caps are on the right and the blue resistor on the left. I used a 2w vs 1w originally. EDIT: I also replaced the little black diode in the upper left hand corner because I had one on hand. The new blue resistor still gets warm to the touch which is probably not good but the old 1w resistor survived 30 years so the 2w resistor I installed should last at least as long. 
[Image: NVIB48q.jpg?1]

After the fix I found that the tach was reading about 200rpm too low so I adjusted it with the potentiometer built into the circuit board.
[Image: 4jl9ONV.jpg?1]

While I had the 300D and E320 clusters apart side by side, I swapped the E320 outside temperature gauge and plug (mine was broken), light tubes for improved lighting at night and orange needles. Mine were faded to yellow.

[Image: 7n0dvCz.jpg?1]

Ignore the 7k tach lol
[Image: ctrwzjz.jpg?1]

Light tubes. These carry light from the back of the cluster to the front. They can be swapped out by drilling out the plastic pins holding them in place. The 87 one is on the left, 94 on the right.
[Image: FoMyylO.jpg?1]
[Image: 208104.png]
(This post was last modified: 01-16-2017, 08:22 PM by tjts1.)
01-16-2017, 08:17 PM
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hents99 Offline
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#9
RE: Modify an m104 tachometer to work with an om603?
Very good thread. 

Mercedes Benz (VDO) Outside Temperature Display Repair:


https://www.youtube.com/watch?v=CHpzMeb1L0c
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W201, C126, W463 STD, W140
01-17-2017, 02:56 AM
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mrfajita Offline
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#10
RE: Modify an m104 tachometer to work with an om603?
This may be the answer to my tach problems, also Hents99 I may have to try to repair my temperature display as well
'77 300D, 617.912, HY35 15psi, 3" side pipe, maxed out stock MW pump (Sold, now converted into a ute)
'87 300TD, 603.960, questionable mods from the PO including a ghetto 3" straight pipe
01-19-2017, 01:19 AM
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barrote Offline
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#11
RE: Modify an m104 tachometer to work with an om603?
(01-11-2017, 03:17 PM)Tobulus Wrote: Look at my project-thread for speedometer-informations

Looked and it looked chinese to me.

Small aside. There is the mech way too for the tacho. If car uses the alternator as source, just open it and get the wire. If it uses the phonic wheel above the block can be drilled and installed the pick up. Place to drill is perfectly visible. If the pick up is in the pan the pans have the place to drill to clearly marked. 
If its gas conversion then uses one of the 3 options above. 

There must be a easy way of using the odo with electric imput signal. Like the tacho
Could u have a look and start a discussion on this matter. 
Thanks
FD,
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01-20-2017, 08:20 AM
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Tobulus Offline
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#12
RE: Modify an m104 tachometer to work with an om603?
There is an easy way for the speedometer, too. I am working on it right now at another project car. The w124/126 speedometer uses an inductive sensor in the gearbox (4 signals per propshaft revolution). U can connect awheel speed sensor to it and mount it i.e. at the propshaft flange. The speedometer can be easily adjusted by replacing a resistor on its board with a variable resistor.
01-21-2017, 07:30 AM
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barrote Offline
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#13
RE: Modify an m104 tachometer to work with an om603?
Sorry, but we shouldn't be talking about the same thing....
W 124 i know use the warm cable which power a magnet inside and gives the odo motion by dragging.
The counter is mechanic.
What i can't understand is how i can add a resistor to a mech thing... unless i'm missing something .
Apologise but i never dissasemble the thing to its bones at least the w124 . Just assumed its like the 123's.
Inductance signals we can gather in many places . Including building one at some point in the drive line .
Danke
FD,
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01-21-2017, 08:13 AM
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Tobulus Offline
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#14
RE: Modify an m104 tachometer to work with an om603?
Oh, you are talking about cable driven speedos... I was talking about the electronic V8 speedometer
01-21-2017, 09:00 AM
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barrote Offline
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#15
RE: Modify an m104 tachometer to work with an om603?
Yes cable driven!!! Unless u fimd me one electic driven.
FD,
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01-21-2017, 01:00 PM
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hents99 Offline
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#16
RE: Modify an m104 tachometer to work with an om603?
W124 v8 models and w463 has electronic speedometer.
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W201, C126, W463 STD, W140
01-22-2017, 02:56 AM
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barrote Offline
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#17
RE: Modify an m104 tachometer to work with an om603?
Apreciate that info , doea anyone has one for sale at a decente price ?
FD,
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01-22-2017, 06:42 AM
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JoeB Offline
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#18
RE: Modify an m104 tachometer to work with an om603?
Any w126 speedo, just requires a little modifcation of the plastic on the rear to fit onto the 124 cluster. See the speedohealer thread.
02-12-2017, 07:10 PM
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EvoPeter Offline
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#19
RE: Modify an m104 tachometer to work with an om603?
My w126 speedo in w201 and Dakota Digital box from rear ABS sensor.

https://www.youtube.com/watch?v=Yb0n4aZT0_k
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02-17-2017, 04:52 PM
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R-3350 Offline
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#20
RE: Modify an m104 tachometer to work with an om603?
anyone worked with a w116 tach before? it looks to be based off the same frequency to voltage conversion set up but with a slightly more complicated front end. im also having trouble getting a pin out for the main chip its a SN29707P. just trying to figure out which of the small MKP caps is the main biasing cap for the chip. the standard w123,126,124,ect,ect tachs use a LM2917 which is easy to find pin outs for but im not having any luck with the 29707.
(This post was last modified: 04-19-2017, 10:10 PM by R-3350.)
04-19-2017, 10:08 PM
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Petar Offline
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#21
RE: Modify an m104 tachometer to work with an om603?
You could design your own amplifier/converter. First a VR sensor amplifier then an ATtiny85 arduino chip. Total cost would be less than 10 dollars if you DIY and you can get the attiny to output any signal you want. Variable voltage, variable frequency etc.
04-20-2017, 09:25 AM
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