Hi everyone, I have a MF373 in very good condition but there’s a problem with the electronic tach:The tachometer sometimes doesn’t show any RPM for years and it seems the problem got worse. Now it rarely works which means the needle keeps its zero position, it does not move at all. I do know the circuit diagram very well and did some checks. The alternator seems to be fine, it definitely charges the battery and I can measure some voltage and frequency from the RPM-pick-up labeled "W".
The tachometer (Made in Spain, 1989) has a label on its back: “360 Hz = 1500 GIRI” (giri must be RPM). I know that alternators usually have 6 pole pairs.
I figure, this should be a working formula for calculating RPM signal frequency:
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f = n / 60 * i * p
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f … AC frequency [Hz]
n … engine RPM [1/min]
i … (pulley diameter) ratio, value of about 2.4 makes sense
p … alternator pole pairs, probably 6 At engine idle (750 RPM) according to the formula above the RPM signal should have 180 Hz and this matches the tach label text exactly.
I own a multimeter which has a frequency mode but no oscilloscope. One time when I had unplugged the dashboard completely, I measured about 175 Hz at idle RPM but very low voltage of about 0.3V AC from the RPM-wire coming from the alternator. Sadly, all other measurements ever since – even directly at the alternator with RPM-wire detached – reported much higher frequencies of about 750 Hz (which I cannot explain) and voltage like 8V AC. I disassembled the alternator partially, took of the plastic cover at its back. There are 3 diodes, RPM-wire well attached on one winding before the diode. My measurements assume that these diodes are ok. Everything is in very good condition and looking fine. Problem is, so far I don’t even know where the problem is. Alternator or tach? Or is my multimeter confused by something and reports wrong values?
Does anyone know what signal type (plain AC I guess), frequency & voltage the signal should have?
I know I can detach the whole dashboard easily (just 4 round connectors) and feed it 12V. I already checked some warning lights – everything exactly like the circuit diagram. But I don’t have a signal generator to test the tach. 1) Is there any way I could improvise like using a sinus signal generated by a computer from the loudspeaker plug? I know how to generate the sound but would this work using a different source for powering with 12V at all?
2) Any suggestions how to measure correctly without buying an oscilloscope?
3) The manual says the alternator is brushless but without disassembling it completely, it still looks like it has something like slip rings in there near the bearing to power its stator. Does brushless only mean, it doesn’t use coals like in older days? Thank you for any suggestions!
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