[ddfour1]

Posting here as well as in the Diesel area as I am not sure the source of my issue, but I think it is voltage related.  I have CAT 3126 350hp MUI engines on an 03 3500 Open.  At startup and while idling out of the harbor, both tachometers work fine. At startup, my cranking engine volt gauge needle drops down and does not come back up until I hit about 1500 rpm.   (As an aside, It used to cycle up and down as the air intake heater cycled on and off, but that no longer happens so I am wondering if my intake heater is not working).

As soon as the cranking battery volt gauge pops up at ~1500 rpm, I simultaneously lose power to my port tachometer or it starts bouncing.  Also, some of the switches at the helm panel impact the port tach. For example, if I turn the panel lights on, the port tach goes dead. Using the trim tabs will also deflect the port tach needle. 

What I have tried:

1. Swapped port and stb tachs and the issue stayed on port side so the tach is not the issue.

2. I replaced the port tach sender. 

3. I cut back the ground wires going into the port tack and put a new spade on.

4. I inspected all of the ground wires behind the helm for contact and corrosion.

5. There is a relay associated with each tach. I replaced the port tach relay. 

I have externally regulated Balmar 100amp alternators that have been tested and are putting out. The voltage regulator is a Charles Next Step that does not appear to be showing any error codes. 

Any thoughts on where the issue might lie?

Thanks

[karibnsoul]

I had a very similar problem with our 1999 3500 Open. We have the 3116TA’s. When we purchased the boat about three years ago, both tachs would bounce around. Pushing the trim tabs or pressing the horn seemed to settle them down. Both tachs finally quit completely. Both cables were replaced and they now both work, however at around 2000 rpm’s they bounce back and forth about 300-300 rpm. Touching the trim tabs switch seems to settle them down once more. Our repairman had to get replacement parts from the engine synchronizer people (I’ve forgotten their name). Now I’m having the repair guy back. There’s obviously a short problem there somewhere. I’ll post if he figures it out.


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[ddfour1]

I had previously cut back and put a new terminal on the ground wires but not the positive wire. Last trip out I put a new spade on the positive wire and then I tapped into both the positive and ground wires and ran test wires to the helm so I could monitor voltage to the tach while underway.

The issue must have been a bad connection at the positive post because the new spade on the positive wire seems to have eliminated the problem.

After start-up and while idling out of the harbor I was measuring 12.1 volts going into the tach. As I slowly throttled up, the voltage increased correspondingly, likely because the alternator output was ramping up with the throttle. By 1500rpm I was measuring 14.1 volts going into the tach and it was not bouncing and no longer cut out when I turned the panel lights on.

[karibnsoul]

Thanks. I’ll give it a try.


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