Tach discrepancy

[svaron]

I have a 1990 54 ED with 892ta's.

While coming back from vacation this past weekend the port tach was hovering between 600 and 800RPM's. While under way I noticed that the tach was jumping around a little so I decided to return to the dock and replaced my primary Racor's.

I soon found out that that was not the problem, same symptom. What I did notice was that while the port engine was running and the starboard engine was off there was no deviation on the port tach. Once I started the starboard engine there was an immediate affect on the port tach. The port engine throughout all of this sounded fine. The engine was not ramping, it was running smoothly.

We decided to leave for out trip home. Once we were cruising at 1900RPM's with the synchronizer on there was a 100-200 rpm difference between the 2 tachs. However, the engines sounded perfectly in sync. The starboard tach was reading 1900 and the port tach read anywhere between 1700-1850.

I also should indicate that the engines exhaust are perfectly clean. No black smoke.

Would appreciate any insight to this.

 

[spartonboat1]

If you are running the digital tachs along with Glendinning synchros, then the wandering tack may have a gear going bad in the sender on the tail stock. I did not always hit the zerk annually, and one went out. Easy to get and replace.

 

[Liquid Asset]

I have a problem too with my tach. My port tack jumps all over the place up to 1500 rpm's, but after that it settles down. I think it is the sender. you should try that first.

 

[Genesis]

Sender, coupler or cables. Old cables will do this too. Fix is to replace them.

 

[Captcoop]

Your tach has nothing to do with the engines being synchronized because the synchronizer is mechanical and the tach is electric driven by a pulse generator. You most likely have a ground problem at the tach. If this is not the problem check the drive tang that turns the pulse generator. Good Luck

 

[Genesis]

Cable whip will cause tach fluctuation.... the senders are on the front of the Glendinning..... they are electric but driven by the long cables.

 

[MikeP]

I had an intermittent fluctuation in the port tach a few weeks ago. It would work fine at cold idle and then jump around as the engine warmed up. By the time it was at normal op temp, the tach wouldn't register at all. Engines would stay in synch just fine. Turned out it was a poor crimp-connector connection in the tach wiring in the port engine room. I replaced the wire-to-wire crimp connectors with the approved method - terminal block/ring connectors. That fixed the problem.

I had a similar tach problem a few weeks prior to that caused by a faulty connection at the tranny pressure sending unit. Again - terminal blocks/ring connectors replacing the sleeve connector at the sender solved the problem.

Yes, if the tranny pressure sender is disconnected, the tach won't work.

No, I don't know why it's wired that way! Looks OEM but might not be - I didn't check the Hatt wiring diagrams.

 

[Nonchalant1]

On my 8V71TI's I had a "jumping tach" problem that was caused by the little 3" stub cable that drives the long tach cable (a right angle cable drive). The stub cable originally had a square end which went into a square hole, but the cable had been rounded off over the years and was slipping in the square hole. Bottom line, $12.95 part from DD (really great service as noted earlier - the service manager delivered it to my boat same day, on his way home, which saved me because we were in the middle of a month long cruise to the North Channel). It can be replaced in 10 minutes. It has worked for 1,000 NM since.

On 8V71TI's the Glendinning sender is at the engine BEFORE the cable, but after that little stub shaft, so the sync's won't work if your tach doesn't work because of that little stub cable.

Good Luck!

Doug Shuman

 

[svaron]

Thank you for all of your comments. I will go to work on the problem this weekend...