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Discussion Starter #1
I've wanted a tach in my Amazon ever since I first got it, but original Volvo Smiths tachs are like hen's teeth so I settled for one pulled out of a (negative ground) MGB. The trouble I'm having is they seem to have disparate instructions for installing. The MG wiring instructions say the male bullet plug on the body of the tach should be connected to a switched +12V supply from the ignition switch, whereas the Volvo instructions say the male bullet plug should be connected to the side of the distributor in place of the wire that connects the distributor to the coil (and the female bullet connector on the tach body goes to the coil). So which is it?

Here's a picture of the back of the same model tach I got, with the pinout according to the MG instructions:




And here's what the Volvo instructions say it should look like:




Any help is greatly appreciated. I would love to be able to fit the tach in this weekend, but I don't want to risk frying it...
 

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The Smiths tachs use the current flowing between the coil and distrib to induce a readable signal in the tach...No direct connection to the tach circuitry inside the case.

All other (electric) tach types take their signal from the (-) coil (or distrib), and electronically convert to RPM readings. That's what I think you have...

I believe the connector you marked "+12 volts" is actually the ground, easy to test with continuity to case.
One of the others is +12 volts, and the third is for the signal from the coil OR distrib...I'd be surprised if there wasn't a clue about which connector is which...
 

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Try it one way and see if it works. If not, try it the other way. You won't fry anything -- the engine will run either way, but the tach will only work one way. You probably know this, but just to be sure... the tach is in series with the distributor and coil, so you're moving the existing connections, not adding two more wires.
 

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Discussion Starter #4
I believe the connector you marked "+12 volts" is actually the ground, easy to test with continuity to case.
One of the others is +12 volts, and the third is for the signal from the coil OR distrib...I'd be surprised if there wasn't a clue about which connector is which...
The connector I marked as +12V is the power connector. There is another spade spot-welded directly on the case for grounding. I've already successfully bench tested this tach by feeding it +12VDC on the power connectors and giving it a very faint pulse of ~ .25V and I could see the indicator moving.

Try it one way and see if it works. If not, try it the other way. You won't fry anything -- the engine will run either way, but the tach will only work one way. You probably know this, but just to be sure... the tach is in series with the distributor and coil, so you're moving the existing connections, not adding two more wires.
Yeah I know, the Volvo instructions say to remove the wire that goes from the coil to the side of the distributor (not the HT lead, duh) and connect the coil to the female terminal and the distributor to the male terminal.
 

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I think "ignition" and "distributor" in this case are synonyms. The MG instructions don't mean "ignition" as in the key switch, they mean it as in "the sparking device".

This is my guess.
 

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That particular type of Smith's tach senses the switching of the coil primary current by the points. In the ignition circuit, the primary current flows from the ignition switch to the coil + terminal, through the primary winding of the coil, out the coil - terminal to ground through the points (when they are closed). Except for the point when the ignition points open and you have a decaying oscillation as energy is exchanged between the inductance of the ignition coil and the capacitor, the current flowing into the + terminal of the coil is the same as the current flowing out of the - terminal. The sensing element of the coil just has to be connected in series with the primary circuit somewhere. Since it is an MG tach, connect it as per the MG instructions. As Phil notes, you won't damage anything.
 

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Discussion Starter #11
Ok so I guess when I tested it, I wasn't paying much attention. I finally wired it up "properly" and it seems to only be reading half the revs (says I'm idling at 500 when I know it's closer to 1000). Per the MG diagram I posted (the tach came from a negative earth MGB) the signal wires from the tach should be going to the positive and negative low tension terminals of the coil, except on the 122 with the OEM coil one of those terminals is the armoured cable. Am I out of luck until I replace my coil with a Bosch blue?
 

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Discussion Starter #12
Well, never mind. I guess when the car is warming up, it does idle at around 750~800rpm. Drove it home and when I parked it, the needle was resting comfortably around 1000rpm.

 
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