Testing tachometer Pulse

[ttriebler]

I have a 1976 Mercury in-line 6 outboard 115hp and a 7,000 rpm VDO tachometer.<br />The tachometer is not working and I need info on how to test the tachometer and the pulse going to it.<br />The tachometer is wired to the throttle hand piece with three wires :+12v, Earth and pulse. <br /><br />The 12v works correctly when ignition is on (checked with multimeter). I have also checked the continuity of the pulse wire from the throttle hand piece to the tachometer to be ok.<br /><br />Tachometer is set to 6cyl on the back with the dip switches 1 and 5 engaged. Have tried the pulse wire on both the "hi" terminal an d "low" terminal.<br />Any clues?

 

[The Marine Doctor]

Re: Testing tachometer Pulse

I would test the rectifier on the engine and make sure the problem does not originate from there.<br /><br />TMD

 

[ttriebler]

Re: Testing tachometer Pulse

Thanks Marine Doctor, you are a wealth of knowledge.<br />This may well be part of a larger problem. The motor is not charging the battery and I was just about to start trying to work on that problem.<br />Next time I put the boat in the water I will test that I have 12v AC coming out from the flywheel/magneto wires and whether or not there is 12V DC coming out of the rectifier.<br />Is there any way to test the rectifier itself and is it a serviceable part?

 

[The Marine Doctor]

Re: Testing tachometer Pulse

If the rectifier is bad you will have to replace it.<br /><br />To test it.<br /><br />Use an ohm meter and test for continuity only one way through it. <br /><br />There should be continuity from +>- but not ->+<br /><br />So if you put the red lead on the +, the black lead where the yellow lead goes...there should be continuity.<br /><br />When you reverse the leads there should be NO contiunity.<br /><br />The engine wires must be removed from the rectifier to do the test.<br /><br />The yellow wires run to the rectifier...so you can find it.<br /><br />TMD

 

[ttriebler]

Re: Testing tachometer Pulse

Hi Marine Doctor (and others)<br />I carried out your tests and found no continuity at all in the rectifier so replaced it.<br /><br />I took the boat out for a drive and the guage showed 12.5 volts at the dash (same as what my multimeter showed, and about 0.5 volts less than what my multimeter tested across the battery terminals). Unfortunately the tachometer showed no signs of life.<br /><br />After a half hour of driving the dash guage gradually climbed to be over 16 volts (top of it's range). Nothing started to smoke and I headed back towards the boat ramp. Using the trim pump to trim the motor up a resulted in an increase in engine revs (approx 500 rpm) and the voltmeter back down to 13.5 for about 15 seconds before returning to 16. (maybe just the load of the trim pump)<br /><br />With the engine switched off, the voltmeter shows 12.5 volts again, about 0.5 less than the multimeter.<br /><br />I tested the wiring and rectifier for damage but everything came up OK I think....<br />Rectifier :<br />Continuity from each + terminal to the - terminal but not in the reverse direction.<br />No continuity between both - terminals<br />Wiring :<br />Reading 0.3 ohms resistance between the two yellow/black AC wires coming from the stator.<br />Neither AC wire shorting to earth.<br />When wires are re-connected to rectifier my multimeter shows 12.5v from the red wire to earth.<br /><br />Here's the trick - my engine has Thunderbolt ignition, the CDI type with a distributor. I read in some other posts that most tachometers will not work with thunderbolt ignition. Mine might be of this type. If indeed the ignition pulse is messing up the tachometer, then is it possible that it messes up the voltmeter reading (they share the same + and - power wires).<br /><br />I will disconnect the tachometer altogether and see if this improves it. Will also test for 12v AC going to the rectifier.<br /><br />Is it possible to take the tachometer pulse wire off the switchbox and connect it to one of the AC wires? I read in another post that this may work on this type of tachometer. Wonder if that will all work with the tachometer powered by 12v DC and piggybacked onto the voltmeter.....