Tach problems(erratic readings) help

[bigeasy1]

My Tach on my Tsunami,Mercruiser I/O 5.7 350 V8 is acting up.The needle swings wildly from the mid 3 to 5 thousand Rpms. when under power.This tach(a Teleflex) was installed new last season, along with all new gauges and electronics.It has less than 50 hours on it.
When it's idling,it seems to function fine,but when the engine is above 3,000 rpm it starts with the erratic readings with the needle jumping back and forth between 3 and 5k.
It worked fine for the first few months,but started acting up at the end of last years fishing(September)
I haven't had time to dig completely into it,but I did check the wiring at the gauge and it is solid(it's all tinned marine wire in perfect cond.).All the other gauges work fine,and are problem free.
I took a quick check on the engine end and all wires and connections seem to be fine.

Anyone have any ideas of what would cause this(ground looks good)or something that I should check first,before I tear it out.The gauges are all jumped from one to another as were the originals,and It's all run through the Mercruiser harness.It's a thunderbolt IV ignition around 1991 vintage.The engine is in perfect running condition.

Any thoughts or comments are appreciated.Thanks all.

[oldfielder]

The first two things that come to mind are whether the gauge itself is amy good and then whether the selector switch on the back of the gauge is set properly.

[bigeasy1]

It's has to be something in the wiring or a electrical issue of some sort,as I set the gauge to 8 cyl when I installed it,and it had been working fine.The gauge is only a year and a half old with very few hours.
The odds that the gauge is bad I think are very low.I'm going to have to check out all the wiring and follow it back to the engine and harness.

[76Red18]

A bad rectifier/regulator on my outboard causes that. The tach is fed from it. What feeds the tach on that Mercruiser?

[eggsuckindog]

A bad rectifier/regulator on my outboard causes that. The tach is fed from it. What feeds the tach on that Mercruiser?

good question - mine also runs from 1 of 2 recifiers

[bobbert]

Had similar problem, tracked down to find a spider had gotten into the tach and built a web. Removed by sucking out with a shop vac,
Good luck

[bigeasy1]

I'm not sure what feeds it,and unfortunately I can't get back up to the boat for a couple of weeks.
Hope to get up there and install my new(to me)radar unit in the next few weeks,and plan to give the tach problem a going over at that time.

Quite a few people have told me to move the selector switch on the back of the tach back and forth several times.They claim it's common to get corrosion on the internal contacts at this switch point which causes erratic readings.
I only hope it's that simple.I'll llet you know what I find.

[oldfielder]

wouldn't it be nice if it was just the switch. That seems awful soon for it to get gunked up, though. I had a few issues inside the black shrink tubing at the motor end of a few connectors in the harness that I thought were tight but had come loose inside from getting hit with the boeshield a few too many times.

[Fr. Frank]

Quite a few people have told me to move the selector switch on the back of the tach back and forth several times.They claim it's common to get corrosion on the internal contacts at this switch point which causes erratic readings.
I only hope it's that simple.I'll llet you know what I find.

Use a good electrical contact cleaner just before trying this. If it's a mild film of corrosion, the contact cleaner will break up the surface of the corrosion, and then leave a protective lubricant film behind. I use Radio Shack's "Control/Contact Cleaner and Lubricant", item #64-4315, on electrical cable connectors, switches, potentiometers, etcetera. I then spray the outside with Fluid Film to provide a light moisture/salt barrier.

If you don't use a cleaner, you may just scratch a temporary connection through the corrosion, that will then fail again fairly soon.