Is the RPM gauge being off an engine thing or a wiring thing?

[LAC_STS]

I bought a brand new RPM gauge about a month and a half ago and when I have my digital timing light hooked up I can see that the gauge is way off.

At idle the gauge shows 1K RPM but the tach gun (digital timing light) shows 650. And it just gets worse from there up.


The gauge is brand new and the place I bought it from tests them on the bench before selling them so I'm pretty sure its not the gauge.

Are tachs usually this off? Is this a wiring problem or something else.



Thanks

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

I bought a brand new RPM gauge about a month and a half ago and when I have my digital timing light hooked up I can see that the gauge is way off.

At idle the gauge shows 1K RPM but the tach gun (digital timing light) shows 650. And it just gets worse from there up.


The gauge is brand new and the place I bought it from tests them on the bench before selling them so I'm pretty sure its not the gauge.

Are tachs usually this off? Is this a wiring problem or something else.



Thanks

The gage probably has different settings for 4,6,and 8 cylinder engines. Break out the gage paperwork and check the setup. I'm betting it's set for a 4 cylinder if you have a 6, or 6 cylinder if you have an 8. Past that it is most likely a gage problem.

 

[Aloysius]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

a tach is a pulse counter. sometimes there's a switch on the back for 4/6/8 cylinder.

Wanna quick, cheap, and extremely accurate way to check your tach? Wait until dark, shine a 120v fluorescent light on the damper. It will "stand still" at 1800 and 3600 rpms. Nothing more accurate than 60 cycle line voltage.

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

a tach is a pulse counter. sometimes there's a switch on the back for 4/6/8 cylinder.

Wanna quick, cheap, and extremely accurate way to check your tach? Wait until dark, shine a 120v fluorescent light on the damper. It will "stand still" at 1800 and 3600 rpms. Nothing more accurate than 60 cycle line voltage.

Awsome parlor trick. 35 years in the automotive industry and I never heard that one. I can however see how it would work. And they say you can't teach an old dog new tricks.

 

[LAC_STS]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

The gage probably has different settings for 4,6,and 8 cylinder engines. Break out the gage paperwork and check the setup. I'm betting it's set for a 4 cylinder if you have a 6, or 6 cylinder if you have an 8. Past that it is most likely a gage problem.

I have checked and it is set to 8. It is also the original Donzi gauge that goes to the boat. Donzi is local to me and when they re tooled they sold a bunch of parts to the Marine Surplus that I got the gauge from.

a tach is a pulse counter. sometimes there's a switch on the back for 4/6/8 cylinder.

Wanna quick, cheap, and extremely accurate way to check your tach? Wait until dark, shine a 120v fluorescent light on the damper. It will "stand still" at 1800 and 3600 rpms. Nothing more accurate than 60 cycle line voltage.

What is the damper?

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

That being the case, and assumeing your using the correct setting on your digital tachometer I can only assume there is a problem with the Donzi tach. I would be more willing to trust my diagnostic equiptment than the unit equiptment. To be sure I would check it with a different piece of diagnostic equiptment for the same results. Then your 100% sure the unit tach is the incorrect tach.

It's definitly not an ignition problem, and most likely not a wireing problem. A wireing problem would be more of an all or nothing deal. either it works or it doesn't. Your tach switching for cylinder settings may not be working properly internally.

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

What is the damper?

The bottom pulley, or behind the bottom pulley on the engine. Where the timing mark is.

 

[LAC_STS]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

The old tach would work but would not go over 3200 RPM's. When they tested it on the bench it would go to 1K 2K and 3K but when they put the machiene at 4 or 5 or 6 the tach would sitck at 3200. So maybe I should see about the old tack if it is off too.

I was thinking a wiring problem because when I changed the tack I had for forgot it was not installed yet and I tried to start the engine and it would not start. It would turn over like it wanted to start but it would not start. I realized the tach was not in yet and I put it in and it started right up.

I was told this is wrong and the engine should start with the tach disconnected.

The timing light I am using is a digital Actron light that uses one of those things you clamp around a spark plug wire.

Do you guys think the timing gun could be off because I had it connected to the #1 plug wire? Should I try it on the coil plug wire?


The tach gauge I have is the correct gauge. It matches the original I had in the boat and it matches the one on the exact same boats I have saw on Youtube.

In my manual it says there should be 4 or 5 (forget) wires hooked up to the tach. My tach has 10 wires hooked to it.

 

[LAC_STS]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

a tach is a pulse counter. sometimes there's a switch on the back for 4/6/8 cylinder.

Wanna quick, cheap, and extremely accurate way to check your tach? Wait until dark, shine a 120v fluorescent light on the damper. It will "stand still" at 1800 and 3600 rpms. Nothing more accurate than 60 cycle line voltage.

So how do I do this? If I shine a flor light at the damper it should stand still at 1800 and 3600 RPMs? But what does this mean? When it stands still the gauge should say 1800 or 3600 RPMs?

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

10 wires? I can see 4. I'm not familiar with Donzi wireing, and can't imagine what they have running into it. A tach is a pretty simple, cut and dry system. Do those wires go directly into the tach, or into the dash panel that the tach is in? Might be something you have to check out with someone who knows Donzi. With all those wires it might have stuff to do with rev limiters, or ignition controle.

 

[LAC_STS]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

It looks like someone added some wiring or did something to it. I can see the OEM wire connectors. They are bigger and nice. Then I can see the regular wire connectors like you buy at an auto parts store.

The lights to the gauges are daisy chained (?) together. So that may account for 2 other wires but that still leaves 4 more.

I forget right now, Ill have to look at it again but when I was looking at it last time with my manual wiring diagram, I saw that two of the posts on the tach that are supposed to have 1 wire going to them have 2 or 3.

I'll look at it sometime today or tom with my diagram and maybe take a pic of it and post it.

If its something simple Ill just fix it, but if all of the gauges are all wired like crazy and there is a bunch of stuff messed up I might just re wire it all this winter.




Thanks so much everyone

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

Well you do have more gages in your dash. temp, oil pressure, tilt, amp, etc. They would each require at least one more wire other than illumination. Like I said. The tach should require only three or four at most. If the wireing were bad it would most likely just result in it not working at all. If your setting is correct for the engine. my best guess would be a problem with the tach.

Good luck sorting it out.

 

[Aloysius]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

a timing light is simply a strobe light..flashes each time it gets a pulse. Put it on the coil wire, you'll see 4 pulses, each 90 degrees apart. But wait you say..it's an 8 cylinder. True..but the damper/crank goes around twice for each complete cycle.

3600 rpminute = 60 rpsecond...60 cycle current

 

[LAC_STS]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

???

Huh?

So if I put it on the coil wire it will show higher rpm then it actually is?

 

[Adirondack]

Re: Is the RPM gauge being off an engine thing or a wiring thing?

???

Huh?

So if I put it on the coil wire it will show higher rpm then it actually is?

Yes. About 8X. Correction exactly 8X.