Why would a bad Tachometer cause engine to run rough

[George Benson]

My 1999 340 Sundancer with twin Mercruiser 7.4s ran rough several weeks ago and then cut off completely (Starboard engine). The tach had jumped to 3000 from 1500 without the engine revving up. Checked compression/good, checked plugs/replaced, Distr cap/good, fuel filter/replaced, rotor/replaced, still running rough. Pointed out to the Mechanic that the tach would jump up significantly 1500-2000 RPMs without engine revving. He said "your tach is shorting out and that is what is causing the engine to rough". Is this right? How can something that measures something (RPMS) cause problems? The shop manual also indicates in the trouble shooting section that this can be a problem. Does anyone know why? :huh:

 

[cmount1]

Yes it can, as you said it measures engine speed which is done by reading the duty cycle the ignition coil is running at. If it shorts to ground it will shut the engine down just the same as if you switched the key off.

 

[Mike Blake]

Swap it out and see if the other side quits

 

[Bt Doctur]

simply unhook it.see if the problem goes away

 

[mwph]

simply unhook it.see if the problem goes away

.......at the engine, right?

 

[JimG]

Yea, what they said.

Welcome to your first post after nearly 3 years of lurking! :thumbsup:

 

[Saverio]

I belive its a grey wire on - side of coil

 

[Westie]

Swap it out and see if the other side quits

Bad advice.
Never move a suspected bad component to something that is working.
There was a case not so long ago of moving a bad IAC valve to an different motor and possible damage to a good ECM as a result.

 

[JimG]

Never move a suspected bad component to something that is working.
There was a case not so long ago of moving a bad IAC valve to an different motor and possible damage to a good ECM as a result.

Yes, great advice... but you can take good components from the running motor and try them on the faulty one, in most cases.

 

[CliffA]

.......at the engine, right?

easiest thing to do is just unhook the wires from the back of the tach if you don't know which wire it is at the coil.......

cliff

 

[mwph]

easiest thing to do is just unhook the wires from the back of the tach if you don't know which wire it is at the coil.......

cliff

Doing that might not remove the problem. Depending where in the system it is grounding out at. Thats why I was wanting BT Doctur to clarify for the OP where to "disconnect" it from. Hope Im not missing something here.

 

[mistercomputerman]

ABYC color codes call for tach to be gray.

 

[CliffA]

Doing that might not remove the problem. Depending where in the system it is grounding out at. Thats why I was wanting BT Doctur to clarify for the OP where to "disconnect" it from. Hope Im not missing something here.

very true and thanks for clarifying.....it is possible the wire could be grounded before it gets to the tach, but based on the threads i have read about this the problem is more likely to be bad internal wiring in the tach causing the grey wire to ground via the tach ground wire....

cliff

 

[tbirdvet]

Any thing is possible but I have never had any tach. cause an issue like this and I have had several bad tachs. over the years. However could be bad wiring (short) somewhere in the tach. circuit outside of the tach. itself.

 

[George Benson]

Thanks, I appreciate everyones reply