Tach problems GM HEI ignition

[Bone fish]

Hi, new here but not to boating. I have looked at lots of threads regarding bouncing tachs and could not find an answer to my problem.
Motor is a Marine Power 454 with GM high energy ignition, inboard shaft drive.
I have had problems with erratic tach readings since the motor was installed (1994). Replacing the tach (with a Medalion brand guage) mad it better but it would occasionally read high, a tap on the lense would bring it back in line.

I have recently upgraded to Faria guages and I can't get the new tach to read close to correct. I fiddled with the switch on the back to no avail. I ran a new wire from the tach output at the engine harness, I ran a new ground. I sent the tach back, it is OK. I tried providing power from a completely independant power source to rule out some interference.

While installing a Crane ignition kit in my wifes 66 Galaxie I noticed they recommended installing a resistor in the signal line if the tach was erratic. This did not help my boat.

The boat has an auto pilot with the drive motor built into the helm. Its pretty close, could the motor magnets be causing interference? There is no difference if the Autopilot is on or off.
Is there something different about GM HEI I need to know?

I'm stumped, any ideas out there?

Thanks!

 

[JustJason]

Re: Tach problems GM HEI ignition

I ran a new wire from the tach output at the engine harness

Have you tried going to the coil itself? The tach itself only needs signal and ground. Any power wires going to it are for lighting. Pull the tach out of the dash and go back to the motor. Ground it to the battery directly and grab the coils - terminal. If it works you have a wiring problem, if it doesn't you have a bad tach.

 

[Glastron_V210]

Re: Tach problems GM HEI ignition

In HEI equipped cars this is a sign that the ig. module in the distributor is giving up.

Try a new GM module. Use the DELCO, they are by far the best. Google "megasquirt HEI" for more info on the type you have. They are easy to change. Remeber to use heat sinking compund with it (The white stuff).


Chay

 

[Fun Times]

Re: Tach problems GM HEI ignition

Do you have resistor spark plugs? Make sure the IGN modual box is ground good, Some times the paint on the engine in that area will not let it ground good, And make this happen. And the alt, going bad might do this also, One guy got it to stop for a min as a test only by unpluging the purple wire off the back of his alternator. These are just some more ideas for you. Good luck with it.

 

[Bone fish]

Re: Tach problems GM HEI ignition

Thanks for the thoughtfull responses. I'll give them a try this weekend.
TD

 

[AZSenza]

Re: Tach problems GM HEI ignition

Same issue with mine, I think its odd that tapping on the gauge face makes it work right...I just deal with it. I can tell right off if its reading correct or not, a gentle tap and im back in business usually for the rest of the day.