Tachometer Repaired Successfully with new resistor and capacitors - PeachParts Mercedes-Benz Forum

David Wilson · #1 03-03-2017, 02:35 PM

Been working to fix the tach on my 87 300d. Has not worked for many years. Checked the signal and power and both were good.
Finally removed the tach, noticed a burnt resistor.
Replaced the resistor, and the two capacitors on the board.
It now works correctly! RPM's jive with the idle speed adjuster setting.
The resistor and Cap specs are; Resistor 51ohm 1w
Caps 100uf & 1uf

I posted this for all who have searched for tach repair info and could not find it in the archives.

tjts1 · #2 03-03-2017, 03:35 PM

Nice! Check RPM at freeway speed. Mine looked correct at idle but freeway rpm was off by about 300rpm after replacing the caps so I had to adjust it.

David Wilson · #3 03-03-2017, 04:50 PM

How did you determine it was off at freeway speed? How did you adjust?

tjts1 · #4 03-03-2017, 05:08 PM

I know in 5th geat @ 70mph the tach should be @ 2000rpm but it was reading 1700. Also I used a gear ratio calculator.
I adjusted it with the small potentiometer on the PCB. I had to drive the car around with the tach out of the instrument cluster, drive down the freeway, stop, adjust repeat.

Here's the same gear ratio calculator for the automatic but this assumes no torque converter loss. The capacitor is a multiplier so the higher the RPM gets the bigger the error.

I was surprised to find a change in the tach even tho I installed exactly the same value capacitors.

David Wilson · #5 03-03-2017, 06:02 PM

Good info! I'll have to check it out at speed.

David Wilson · #6 03-03-2017, 08:51 PM

2800 @ 70mph GPS.
Speedo is off 10%

E30M42 · #7 03-07-2017, 08:17 AM

VT220D · #8 03-07-2017, 04:36 PM

You can calibrate a tach after rebuilding using an audio generator program on your computer. If your sound card doesn't produce a large enough signal you may need a preamp.

When I rebuilt the tach for my 85 300D (3 pin type) I powered the tach on the bench and used a program to generate a tone corresponding to the RPM I wanted to display. I used a preamplifier to adjust the signal level to a value safely above the point at which the tach drops out.

To get the frequency in Hz (cycles per SECOND) that corresponds to the RPM (revolutions per MINUTE) you wish to display you have to know which type of tach you have. On the W123 the tach used before 1985 uses the front mounted crank sensor. That sensor generates one pulse per revolution. At 600RPM the sensor will generate 10 pulses per second (600/60). When your generator is set to 10Hz your tach should read 600RPM. To display 6000RPM the generator should be set to 100Hz.

The 85 tach is different but the principle is the same. This tach uses a sensor on the engine adapter that counts flywheel teeth (140). For this tach a 600RPM reading is given at 1,400Hz (600/60*140). The 6,000RPM number is 14,000Hz (6,000/60*140). Any number in between can be calculated. For example 4,500RPM is 10,500Hz (4500/60*140).

In fact, 14,000Hz is stamped on the back of the 85 tach. This number is the full scale (6K) frequency. I calibrate my tachs to 4500RPM and I find the error at any other rpm is minimal. The adjustment pot is normally used to calibrate full scale but my OM617 is never going to see 6K.

I used a program on a computer connected to a preamp on my electronics bench for this. I haven't tried this without a preamp since the computer was connected directly to it. I suspect the later tach read directly from a computer sound card. However, the early tach uses an amplifier on the fender well and may need a higher amplitude signal than the computer alone will provide.

The system I used is packed away for a move and I don't have the name of the program handy. However it was free and had several nice features like the ability to choose a waveform type and sweep at a variable rate between two selectable frequencies. The sweep lets you run the tach from 600 through 6K to check for a sticking movement etc. Even a program without sweep can give you information. Start at 600RPM then jump to 3K and note the needle position. Then start at 6K drop to 3K and note the needle position. Any combination should give accurate repeatable results.

I found that the tach reading was unaffected by waveform type and worked over a wide range of signal amplitudes. It ether read the signal or it didn't.

While you're in there change all the caps in the tach and clock.

I don't know what the frequency of the W124 tach is but if it uses the flywheel method it will be possible to derive the right frequency from the tooth count. Or it may be stamped in the back like the 85 W123.

tjts1 · #9 03-07-2017, 04:41 PM

The W124 diesel tach uses the 144 tooth starter ring gear for the tach signal. It might be easier to just buy a photo tachometer and read off the crank pulley.
https://www.amazon.com/Signstek-Professional-Automatical-Switchover-Photoelectric/dp/B00EUY6FNI/ref=pd_lpo_263_lp_t_4?_encoding=UTF8&psc=1&refRID=X4SXKEHC4V7BS3F0TQ2Y

BodhiBenz1987 · #10 11-09-2017, 04:21 PM

I seem to have a similar problem in my 87 300D tach which I have apart to repair. The top resistor is broken from the board and has burn marks around the solder points. The paint has flaked off so I can't tell the specs of the resistor, though if I read the bottom one correct that one is 51 ohm. Are the two resistors the same? Is the top one also 51ohm? It reads 27 across it with a multimeter but since it's obviously failing that doesn't tell me much. I just want to make sure I pick out the correct resistor to replace it. Photo attached.