Bouncing Tach Needle

[FlatRacer, aka BarrelBack]

My family skiboat is a '67 Donzi 18' 2+3 V-Drive repowered with a PCM 5.0 Liter Ford w/ conventional points ignition. It runs great except for this one problem: The tach needle reads erratically at all engine speeds. The original tach did this and so does the new Stewart Warner tach that I replaced it with. I also replaced the tach trigger wire with the same result.
So logically, it's not the tach and it's not the wire (or a bad ground, tried that too). Do I have a faulty distributor (the engine seems to run fine)? Do I need a "tach filter" (whatever that is)???

Eric

[Walt. H.]

Since you replaced everything that would be a possible cause for a erratic bouncing electrical tach needle, and i'm guessing it's working ok at idle but it's dancing at high rpm as if your ign point set was floating?
I would try another ign coil just to see if it's not the secondary windings breaking down and causing the tach flutter even though your engine is not noticing it, if not that then the other possibility would be a worn distributor shaft but then you would know it with a wobbling dwell angle while setting your points and the engine would be barking at you instead of running fine?

[mattyboy]

Ford Marine setups have some different variations for electrical setups.

I ran into an issue with the original wiring setup and the tach when I switched to a newer alt with an internal voltage reg not an external reg. how is yours setup?
an amp meter or volt meter?? I seem to recall some difference in wiring with a backfeed circuit.

also have you verified the ground on the tach itself. SW did some different things with grounds some gauges actually have a ground lug and some pick up the ground from their back bracket which means if the bracket is not grounded the gauge isn't grounded. this also means you can't use a teleflex plastic back bracket on a SW gauge looking for gnd on the bracket ( don't ask How I know this but it is a lesson I will never forget just like "you don't need a LH hinge and a RH hinge just reverse one)

[FlatRacer, aka BarrelBack]

i'm guessing it's working ok at idle but it's dancing at high rpm as if your ign point set was floating?

No, sometimes it bounces between 5,000 and 6,000 rpm at idle. Once on plane it seems to toggle between reading a steady 4,000 rpm (wrong), and pegging itself and bouncing off the peg.

I'll try another coil to see if it might be that. Thanks.

Eric

[silverghost]

In addition to what Walt has already told you ~
You might want to clean-up -all connections to the Tac. & engine ignition system.
Some Tac units have a switch on their back cases to select beween 4, 6, and 8 cylinder engines.
These selector switches sometimes get corroded and cause the needle to jump around also.

[Planetwarmer]

If I suspect a bad ground on something, I take an alligator clip on a piece of wire and run the wire to the (-) neg batt terminal. If you dont have a piece of wire that long, use jumper cables to cover a majority of the distance. The jumper cables are really too awkward to use by themselves and achieve a good ground on a small connection.

[Walt. H.]

Eric,
Since this is the second tach doing the same thing I just have to ask, you do have the tach trigger wire attached to the neg side of the coil and not the positive?

Sorry I know you know but I had to ask just in case it might be one of those brain fart moments.

[FlatRacer, aka BarrelBack]

I'll have to check that. I'm going to make a list of these suggestions and check them out next time I get up to the lake (I don't have the Donzi here at home). Thanks everyone. Keep those ideas coming!

Thanks,
Eric

[Tidbart]

Here is what Livorsi told me for a similar issue..."Bob, it sounds like you are picking up “ignition noise” through the tachometer wiring. There is a external filter to eliminate that problem for your tachometer that just connects to the wiring terminals on the back of the gauge"

Haven't gotten around to getting one from them yet, but it is on the list. I would assume it is a capacitor or something along those lines. Cost is about $6.

B

[gcarter]

Here is what Livorsi told me for a similar issue..."Bob, it sounds like you are picking up “ignition noise” through the tachometer wiring. There is a external filter to eliminate that problem for your tachometer that just connects to the wiring terminals on the back of the gauge"

Haven't gotten around to getting one from them yet, but it is on the list. I would assume it is a capacitor or something along those lines. Cost is about $6.

B

I like this answer!

[FlatRacer, aka BarrelBack]

Thanks, fellas. I'm going to buy a tach filter, as well as make a list of your suggestions (I'm "pretty" sure the tach wire is connected to the correct side of the coil, but stupider things have been known to happen) and bring the list with me the next time I get up to the lake.
If anyone has any other suggestions for things I should to check out, I'd appreciate it immensely.

Thanks again,
Eric

[silverghost]

Since you replaced everything that would be a possible cause for a erratic bouncing electrical tach needle, and i'm guessing it's working ok at idle but it's dancing at high rpm as if your ign point set was floating?
I would try another ign coil just to see if it's not the secondary windings breaking down and causing the tach flutter even though your engine is not noticing it, if not that then the other possibility would be a worn distributor shaft but then you would know it with a wobbling dwell angle while setting your points and the engine would be barking at you instead of running fine?

Do as Walt said~

In addition.
While you are checking out your ignition distributor, assuming you are still using the old points & condensor set-up,~
Why not replace the ignition points & condensor.
Old points can cause these Tacometer issues; as can a failing condensor.
Points can go out of allignment wear and wobble.
Contacts can be pitted.
Condensors are a big weak link in any point ignition system.
They fail often in a boat.
Make sure your point set is sitting on a bright clean ground surface on your distributor's breaker plate.
The points work to ground to actually trigger & fire your ignition coil.
See that the distributor itself is getting a good engine ground - contact also . Roust & corrosion is often an issue here.
Use a dwell meter to set-up your points properly if possible.

Good Luck & keep us posted on what you find.