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Tach hookups

Rooney77

Rooney77

Bearded lady
Joined
May 20, 2020
Member Number
494
Messages
305
Loc
TX
Found a cool looking old tach in a junk store and would like to test it. According to Google the tach is from a vintage mercruiser of some sort but no info on what post does what. The top hole is obviously the light. The top middle post and bottom two posts elude me though. Anyone have any ideas?

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Gently scrubbed the back with some water. Not sure why there's a $ sign on it. :confused:

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Signal likely what comes from an GM HEI distributor.

The mfg looks like a Stewart Warner but usually they are really proud of their instruments even if they did move to mexico and would have their name emblazoned all across the front.

But seeing seeing the back its like their gauges.

What your calling a big S or $ stands for signal.
 
PAE said:
Signal likely what comes from an GM HEI distributor.

The mfg looks like a Stewart Warner but usually they are really proud of their instruments even if they did move to mexico and would have their name emblazoned all across the front.

But seeing seeing the back its like their gauges.

What your calling a big S or $ stands for signal.
Click to expand...

So you're saying the one insulated post is the signal wire?
 
Yes..Ill put an email in to my ex boss and ask his opinion if you like.

The large with insulator under it either goes to the negative side of coil or to HEI dist or field pole of alternator.

The other connections wont work if at least one of the power wires isnt insulated.

If the other two connectcons arent insulated and you put 12vdc to them they will short out.

Are B and C insulated and or do they have continuity with each other ?
 
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PAE said:
Are B and C insulated and or do they have continuity with each other ?
Click to expand...

They don't appear to be insulated so I assume they have continuity through the housing. I'll have to check tomorrow.
 
Im pretty sure that needs a tach module
looks like its just a needle/guage and requires something to control it
 
Wire up a neg coil term to the insulated post and ground anywhere else.
 
It sais 1969 so I would guess it's for a points system, not HEI. If that's the case it should be able to handle 12 volts going to the signal terminal without harming it. I wonder if it could run with only ground and a wire to the coil (-) post which would be +12 before dwell
 
larboc said:
It sais 1969 so I would guess it's for a points system, not HEI. If that's the case it should be able to handle 12 volts going to the signal terminal without harming it. I wonder if it could run with only ground and a wire to the coil (-) post which would be +12 before dwell
Click to expand...

It does have a faint 12v printed on the back.

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In your pic above just above made in USA it says AC..

Thats a manufacturers mark.

As in AC delco.
 
Talked to a buddy who's into old boats and he said that the insulated post is signal and everything else is ground.

He was right. It works. Isn't accurate though. Reads 1500rpm at idle on the LS in my cherokee. I'll need to figure that out eventually.
 
Rooney77 said:
Talked to a buddy who's into old boats and he said that the insulated post is signal and everything else is ground.

He was right. It works. Isn't accurate though. Reads 1500rpm at idle on the LS in my cherokee. I'll need to figure that out eventually.
Click to expand...

Maybe was a 4 cyl tach, so is reading 2x?
 
Rooney77 said:
Talked to a buddy who's into old boats and he said that the insulated post is signal and everything else is ground.

He was right. It works. Isn't accurate though. Reads 1500rpm at idle on the LS in my cherokee. I'll need to figure that out eventually.
Click to expand...

All the merc tachs that style I found were for a 4cyl.
so 750 at idle on a v8 sounds about right.
 
Rooney77 said:
Talked to a buddy who's into old boats and he said that the insulated post is signal and everything else is ground.

He was right. It works. Isn't accurate though. Reads 1500rpm at idle on the LS in my cherokee. I'll need to figure that out eventually.
Click to expand...

They sell signal converters to fix that problem. Harley guys use them to run Autometer tachs on their drag bike engines. The name of it is slipping my mind right now though...... I'll edit this post when I remember. :laughing:
 
SLOWPOKE693 said:
They sell signal converters to fix that problem. Harley guys use them to run Autometer tachs on their drag bike engines. The name of it is slipping my mind right now though...... I'll edit this post when I remember. :laughing:
Click to expand...

Dakota Digital.

Lots of money for some rather poorly constructed hardware, IMHO. But it works, assuming you're doing something like this.
 
Know nothign about Tachs:

The only signal here is the rubber grommet insulated post.

The rest is just ground.

The light requires it's own insert socket to ground against the tach case.

A. Signal
B. case bolt
C. case bolt
 
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