Original Tachometer Wiring?

[sailingchance]

I have several wiring diagrams for the A4 but I can't seem to figure out the proper way to wire my Tachometer. It is the original tach. I have no idea if it works. I've been running the engine by sound and when necessary, using a laser tach.

I HAD it wired to what I thought was correct. The grey wire with the quick disconnect goes to the Coil + right? I've several variations with the larger posts. At one point, when I had it wired, if the grey wire wiggled at all, the engine would die. It was a loose connection, I'm guessing, but I've never so much as seen the tachometer move while the engine is running.

until I can figure out how this is wired, I wont know if it works. I also have to check the engine with e laser to confirm my RPMS.

Anyone know how to wire this thing?

I have an early model A4, points, condensor, 35amp motorola alt with external regulator.

Here are the pics

[edwardc]

Typically there are 4 wires on the tach.

One, is a post on the case and is connected to ground.
Another is the illumination lamp and goes to batt+ thru a switch.
The third is the power, going to the ignition +12, often on the coil + terminal.
Tha last is the sensor lead, and goes to the so-called minus terminal on the coil. It's not a ground. That terminal is connected to the actual ground by the points every time a cylinder needs spark.

In your case, the wire going to the diamond-shaped piece is definitely the lamp.

The gray wire is most likely your sensor wire and should go to coil -

The last two labeled "?" are your +12v power and ground. Can't tell which is which from the photo. Inspect them carefully. One will be insulated from the case with a nylon or plastic collar. That one will be the +12V.

[Loki9]

Cool looking tach, I wish those were available new!

[sailingchance]

Confusing

The last two labeled "?" are your +12v power and ground. Can't tell which is which from the photo. Inspect them carefully. One will be insulated from the case with a nylon or plastic collar. That one will be the +12V.[/QUOTE]

I thought so too, but they are both identical. There is no insulation and the screw terminals are the same.

So if the gray goes to coil -
Light goes to + (will wire to switch)

If I wire one large post to + and one to -
then I am turning the hole tach into a circuit... which wouldnt make sense...

[edwardc]

The last two labeled "?" are your +12v power and ground. Can't tell which is which from the photo. Inspect them carefully. One will be insulated from the case with a nylon or plastic collar. That one will be the +12V.

I thought so too, but they are both identical. There is no insulation and the screw terminals are the same.

So if the gray goes to coil -
Light goes to + (will wire to switch)

If I wire one large post to + and one to -
then I am turning the hole tach into a circuit... which wouldnt make sense...

If they both are connected to the case, then you definitely don't want to connect them across power and ground. That would produce a dead short. Those studs were probably intended for a "U" shaped mounting bracket to hold the unit in the panel.

I suppose it's possible that the tach is so old that it has no active electronics inside, and the meter movment is powered solely off of the tiny amount of energy it gets from the sense wire.

In that case, here's something to try. Connect the case to ground, and the gray sense wire to to the coil minus. No connection to +12 at all. Even if wrong, this should be safe to try.

[67c&ccorv]

Hang on to that puppy and get her rebuilt - that's a keeper!

[sastanley]

Cool!

SC, I like Ed's idea...kinda wonky to think there is no dedicated +12v connection for the tach. How many wires coming out of that boot..two?

Maybe +12v & ground is supplied thru the booted terminal that also appears to power the light...sending that to the ignition "on" post should be sufficient, but be aware the light is on whenever the switch is on. Be sure to ground the other wire if there are two..otherwise, the case may be ground.

The "sense" for the tach is almost always coil (-), unless you have a diesel tach, which would be fed a signal from the alternator.

Just FYI, typical tachs these days are 4 wires and that is NOT a typical tach.
red = +12v feed
black = 12v ground
green = sense wire to coil (-) (as others have mentioned, this should NOT be considered a GROUND!)
white = +12v to light

[hanleyclifford]

The diagram I have definitely shows the gray going to coil (-). I believe the two "terminals" on the perimeter are simply mounting studs. There appears to be a slot in one of the interior studs; could that be an adjuster?

[edwardc]

Jason,

Did you ever get this tach to work?

[sailingchance]

Nope

Not yet. But I am still trying. t this point, I think the tach might be shot.

[67c&ccorv]

There are companies out there that will rebuild the tach if you want to keep to the orgininal look.

[sglazebrook]

If you look here (http://faria-instruments.com/site_manuals/IS0013E.pdf) you will see that one of the mounting studs on Faria tachs for gas engines is also the battery positive. I would assume that your mounting studs are the same - one positive and one negative (although I would make sure that the two are not directly connected by the metal housing with a continuity test before trying with the battery).

Oohh! Maybe it "grounds" or powers through the light? That might explain the "+" and "-" writing on the housing and the single wire from the light…burned-out light = no tach?