tach has the jitters

[mikeandanne]

The port tach on the bridge will read ok for a while then jump around,go upscale and read way off then return to normal----I have checked the grounds and cleaned and tightened all connections----maybe just a bad tach,this stuff was installed by PO and the temp gauges are dead as well,there are two temp sending units on each engine,have not got into them yet but the tach is annoying and hard to sync engines----any ideas???

[larryeddington]

Assuming you have 318s, I found a pin in the plug from engine to boat harness was badly corroded. As I was scrapping one boat I cut a pigtail with good 8 pin plug from the donor boat and tech spliced it in and used it. Now no erratic tach. Don't know if it helps but you can check the plugs.

[TC]

Hey Mike. I agree with checking connections at both ends, both the terminal and the wire into the terminal as well. Does your system have a signal wire only or a "modual" at the coil. Sounds like maybe someones getting hot. If no "modual" and connections are good you may try swapping coils. The tack may be picking up a protentionally bad coil. Could be a weak solder joint inside the tach housing and as the unit warms up gaps may appear. A job for a nice sunny day. Keep us posted.

[Big D]

There should be a rotary cylinder selector switch on the back of the tach. The contacts on these tend to corrode after a while. I find gently turning these back and forth several times is usually enough to correct eratic readings unless of course it's due to a bad connection somewhere as mentioned above. Remeber to set the switch back to the proper position when done.

[larryeddington]

good procedure tip big D

[mikeandanne]

Well I had not checked the selector screw on the back of the tach,but following big d's tip I checked ---- like everything on this boat so far it was set wrong and the stbd tach was also wrong so now they are selected to 8 cylinders and seem stable so far and both now match the lower ones --- : Thanks Mike