I purchased a new tach for my 1987 Mercury TOP 115. Hooked it up as per the directions, with the 12 pole setting. All connections are new and clean. The tach operates as i would think it should until it gets to 4200rpm, and then goes to zero where it remains until the RPMS drop below 4200
I tested the rectifier and it tested OK. I tried replacing the 'send wire' from the rectifier to the tach with a temporary new wire to rule out something funky in the control box. No difference. I tried jumping the ground connection to my ground connection to make sure i had a good ground. no difference.
I put my meter on the + and - of my forward fuse panel, which is a direct connection to my battery. I measured a little over 12V. Interestingly i didn't measure much voltage increase at all when i revved up the motor. It stayed basically the same.
I then tried measuring the AC voltage on the send terminal of the tach, which comes off of the rectifier. It behaved as it should. The AC voltage ramped up in parallel with the tach from around 10v up to 60 or 70 volts. Interestingly, it kept climbing even as the tach went to zero. That confused me as i thought that there would be a voltage drop at the point where the RPMs drop.
I think i've hit the wall of my logic. Does anyone have any obvious things i'm missing or ideas of things to try?
Thanks in advance
Ian
I tested the rectifier and it tested OK. I tried replacing the 'send wire' from the rectifier to the tach with a temporary new wire to rule out something funky in the control box. No difference. I tried jumping the ground connection to my ground connection to make sure i had a good ground. no difference.
I put my meter on the + and - of my forward fuse panel, which is a direct connection to my battery. I measured a little over 12V. Interestingly i didn't measure much voltage increase at all when i revved up the motor. It stayed basically the same.
I then tried measuring the AC voltage on the send terminal of the tach, which comes off of the rectifier. It behaved as it should. The AC voltage ramped up in parallel with the tach from around 10v up to 60 or 70 volts. Interestingly, it kept climbing even as the tach went to zero. That confused me as i thought that there would be a voltage drop at the point where the RPMs drop.
I think i've hit the wall of my logic. Does anyone have any obvious things i'm missing or ideas of things to try?
Thanks in advance
Ian