Saxon 500 faulty tachometer

[TheFlyingPolak]

A few months ago I had an electrical problem on my 1997 MZ 500 Saxon Tour. I left it out in heavy rain for a short time and when I started it after that, the main ground wire, which was already in bad shape, shorted to a positive wire. This knocked out the tachometer and several lights. Since then I replaced faulty wires and blown light bulbs but I don't know how to tackle the tachometer. What would be good first steps in diagnosing the faulty tachometer?

[nice2day]

Hi there.

Quickest way is by substituting a known good working one. If this one does not work you are probably lucky in that it is likely to be a connection/wiring fault. Trace all connections as per wiring diagram

If the substituted one does work it would seem highly likely that the old Tacho is now faulty (burnt out) and likely to be unrepairable, certainly not an easy job.

I would think that a used tacho would be cheap enough to buy.

Les

[DAVID THOMPSON]

mount a small GPS unit in place of it and set it to display speed
and see how far off your speedometer really is.
if you get lost set it to map and find a pub where the Food is good

[Puffs]

Following a biased and flawed moderator choice (someone posting aggressive personal attacks is rewarded, while my technical posts are removed - behind my back & without any justification!), I have withdrawn from this forum.

[TheFlyingPolak]

That's right, an electronic RPM counter, takes pulses directly from the ignition and no longer gives a readout due to the electrical failure I've mentioned earlier. Interestingly, it didn't even blow the backlight bulb.
I tried taking the tachometer apart but it's proving to be almost impossible without breaking it. It's also very hard to find one for sale. It's the rectangular gauges which, I believe, only came on saxon models.
I've been living with the dead tachometer for several months now but just wanted to give it a go anyway.

[breakwellmz]

mount a small GPS unit in place of it and set it to display speed
and see how far off your speedometer really is.
if you get lost set it to map and find a pub where the Food is good

A PUB?!!! YOU HAVE PUBS OPEN??!!

[nice2day]

That's right, an electronic RPM counter, takes pulses directly from the ignition

If the tacho is any thing like the later Saxon Models ( and I'm pretty sure the wiring is similar ) or any MZ with electronic tacho then as far as I can see the pulses derive from the unrectified output of the alternator, not the ignition system. This is easier and safer. The alternator thus provides an AC trigger voltage /signal to the S terminal of the unit which is directly Frequency vs Rev related. The other 2x connections are just Negative and Positive supply. Knowing this makes fault finding easier.
1) First check with a DC voltmeter (multi-meter) that you have the full battery voltage at the + and - terminals at the tacho when the ignition is switched on.

2) Then remove the S wire from the unit. Set your meter to AC VOLTS around the 30 volt range and connect the test leads (anyway round) to the S wire and ground. You should see a voltage (I would suspect around 12-20 volts) depending on how much you rev the engine. Stopping the engine should see the reading go down to ZERO.

3) Note if you have a better meter which has frequency in Hz you can then select this setting and see for yourself the frequency rise as you rev the engine.

Doing the above test will at least give you confidence that you are not wasting your money by trying to source another tacho as it will check the wiring needed to drive the tacho.

These tests will take only a few minutes so worth doing....Les

[TheFlyingPolak]

Thanks a lot for the suggestion nice2day!

Here you can see that the tachometer takes pulses directly from the ignition contact, which is located in the alternator.

Probing the + and - gives me about 0.45V less than battery voltage. Probing the signal gives -0.2V DC and the voltage does change when I rev the engine. Interestingly, the voltage is -0.08V when probed where the tacho signal wire (green) branches off from the ignition pulse wire (blue).

I'm guessing this means that the tacho is dead

[nice2day]

Thanks for showing the correct Circuit diagram which indeed is different from the older MZ ones.. . I was taking a guess with the circuit but was wrong. So yes, you are perfectly correct in that the Rev counter signal (green wire) DOES come from the ignition trigger in this model. The voltage variation is probably an effect caused by the way the meter reads the spiky AC signal. Most meters read RMS values (0.707 peak value of a sine wave) but because the ignition trigger spike is probably a square wave of sorts, it is causing some variation but this effect is not important anyway now. The best way would be to use an oscilloscope to see what the signal comprised of but not to worry. I would go back to the original suggestion of substitution or take a gamble and buy another one. Let us all know what you end up with. Best of luck....Les

[TheFlyingPolak]

Update: I got inside the tachometer! Looks like there are multiple problems to find there.

Here's what the whole thing looks like on the inside:

The first problem I found was a resistor that somehow became de-soldered, so I re-soldered it and tested it. The needle now moves but there is another problem. When the ignition is turned on, the needle starts from slightly above 0 and slowly creeps upwards. When the engine is started, the needle never comes down until the ignition is turned off. Looks like whenever rpm increases, the needle rises but never falls when rpm drops. So by the time I start the engine from cold the tacho already makes almost a full revolution!

As to how to get inside the tachometer, the whole outer shroud is held in place with some sort of glue. It needs to be pried off all around, with plenty of force, and it will come off. The three brass screws at the back only hold the motor to the circuit board.

P.S. somehow there were a few little flies between the gauge and the glass

[nice2day]

Well done. looks a very simple circuit. I would first replace the diode and zener diodes and the 2x electrolytic caps. Total about £5 and then if no change, replace the Lm1819n about another £5 (NB: This chip is obsolete and slightly hard to find a supplier apart from China via Ebay) and to make replacing the chip less risky, solder in a 14 pin dil socket so you can simply plug the new chip in. I reckon I would have a go if a replacement tacho was expensive...

[Puffs]

Following a biased and flawed moderator choice (someone posting aggressive personal attacks is rewarded, while my technical posts are removed - behind my back & without any justification!), I have withdrawn from this forum.

[Blurredman]

That does look like an extremely simple bit of work, electronically..

You might not think it's worth re-flowing all joints first? If that doesn't work remove resistors one at a time and check? And also the diodes.. there isn't much there, so it's just a process of elimination - and it won't take long because the board is so small.

[Steve razor]

I recently had tacho problem. Resistor was unsoldered. Resoldered, still didn't work. Changed diode & all is OK now.. Diode was cheap ..there is a tacho\speedo combination on eBay for £25 plus postage from Florida (£28).. Would have done later if I couldn't have repaired it..cheers.p.s.mz was recent purchase ( barn find of 20yrs in garage).. Tank needed rust repairs, flow master internal coating, re spoke 2 wheels, repaint frame, swing arm, shocks & everything else, strip chrome from exhaust, handlebars, rack, replaced all cables, brake master & slave rebuild, new grips, new brake switch..list goes on and on,,,but I can see light at end of tunnel !!

[Puffs]

Welcome Steve!

and that wets our appetite for pics of your barn find...