WOT Tach Problem

I have a 1993 Stratos 274 with a 120 Johnson 2 stroke model # VJ120TLATS. Earlier on I had a charging problem and after testing the diodes in the rectifier I found that it was bad and replaced it. I did and still do have a problem with the Tach which I was hoping was related to the bad regulater but have found it was not. It will gradually and correctly increase to approx. 4300 RPM but after that it will either drop to 0 and stay there or it will go down to approx. 3300 RPM and gradually increase again. This is happening as I gradually increase the engine RPM's to WOT. The engine runs fine too.

I have checked the charging status of the rectifier again and found that it does get 13-14 volts to the battery as the engine increses in RPM. I used an actual battery tester to do this and also the battery tests fine when I put a load test on it. I believe my charging system and battery to be in good condition according to these tests.

My next step after determining the charging system is working fine was to check the wires going to the Tach. I checked the purple wire and I am getting a 12V signal. Also my ground is sound as I did a voltage drop test across the purple and black wire and found it to give me a 12V reading.

After that I did another voltage drop test across the grey wire and black wire and found that at idle I was reading around 2.3V and as I gradually increased RPM's to WOT the reading went to 0. Telling me that it was reading full voltage at WOT. The reading did not bounce around and gradually transitioned as I believe it should.

I believe the problem I have is in the Tach internal workings but before purchasing a 70+ dollar Tach I would like to have a second opinion as to whether or not my tests were correct and/or the problem most likely is the tach. Any help would be greatly appreciated. Thanks.

Re: WOT Tach Problem

First of all I think you were doing a simple voltage test and not a drop test. When you had your meter on the gray wire did you have it set for AC volts? The tach runs off an AC pulse from the stator and should increade as RPMs increase. Does your charging voltage stay steady when the tach drops off? If it does then it does sound like a bad tach.

Re: WOT Tach Problem

I had it set to read DC which would have only given me the voltage reading in one direction on my meter, correct? Even though I had it set wrong, the pattern was smooth. Yes the voltage remained smooth and even (voltage increased and decreased 13V-14V) throughout the RPM range while testing the battery, even when the tach dropped off. Also, as far as I can see, all the wiring is clean and in good condition. No breaks, corrosion at terminals, etc.

On another note, i think you were correct. I was not doing a voltage drop test. I got to thinking about that after you replied and you are correct. I had one lead on the ground and one on the grey. Voltage drop would be one lead on the wire and one on it's respective terminal and measuring the difference, right?