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  New Old Stock Airguide Tachmeter and 1974 Mercury 50 HP Motor

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Author Topic:   New Old Stock Airguide Tachmeter and 1974 Mercury 50 HP Motor
Buckda posted 05-31-2009 09:26 AM ET (US)   Profile for Buckda   Send Email to Buckda  
I am restoring a boat for my brother and have re-installed the 1974 Mercury 50 HP 4-cylinder 2-stroke motor. Part of the restoration includes a vintage stainless steel airguide tachometer (New, old stock, still in the box).

Yesterday, I installed the tachometer per the instructions on the box and the motor will not start. I disconnected the tachometer and the engine will operate as normal.

The tachometer came with a harness that installs to the three-prong connector on the Mercury control box. When attempting to plug in the tach to the control box while the motor was running, the motor instantly died, and the wiring on the harness became very hot.

The motor starts and operates as normal without the tachometer.

There are no "pole settings" on the back of the tachometer housing - it has two wires (Black and Green) that come out of the back of it and need to be connected to the wiring harness adaptor.

One other factor - the box says it is good for Mercury engines with CDI ignition. I don't have manual information for the engine, but the decal says "Thunderbolt" ignition. I believe that this is their branding for capacitor discharge ignition, but want to be sure before moving forward.

Thank you in advance for your help.

Buckda posted 05-31-2009 10:25 AM ET (US)     Profile for Buckda  Send Email to Buckda     
Some more information, and perhaps the solution.

The harness has three wires - black, green and white. On the wiring diagram, the white goes to a "light" and then ground.

I believe I have grounded the circuit out, as I installed the white directly to ground, as there is no light available. My thinking is that the white lead is a "positive" lead, and by grounding it, I'm causing the problem, which would also account for the heat - as all that voltage would be rushing to ground. Correct?

I'll give this a try later this afternoon.

Anyone with electrical savvy is welcome to chip in.

Thanks.

Dave

L H G posted 06-03-2009 01:13 AM ET (US)     Profile for L H G    
Generally, the Mercury color code for tach installations is black-neg, purple-ignition, and grey-sender. I would guess this older code to be white-ignition, and green-sender. White, if ignition, should not be grounded.

On all of my gauge installtions, I wire the gauge lights to be always on when the ignition is on. The heat keeps condensation down. Do this by installing a short jumper wire from the ignition post of each gauge to the light post.

jimh posted 06-03-2009 01:28 AM ET (US)     Profile for jimh  Send Email to jimh     
Dave--Scan the wiring diagram that you are talking about and post it.

If the WHITE was supposed to feed a lamp circuit, you should not connect it to ground! The WHITE is a feed from 12-volts (intended for the lamp), so it would get very warm if hooked to ground.

BLACK is almost universally a battery negative lead in DC wiring convention.

That leaves GREEN--it must be the tachometer signal.

Buckda posted 06-03-2009 12:32 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Thanks for the responses. I have NOT had a chance to tinker with it, as I am making some needed repairs to my Whaler trailer right now.

I believe JimH and LHG have accurately described the situation and the problem in my original attempt to connect this instrument.

There is no light. The back of the tachometer is sealed with just a green and black lead coming from it. The green, black and white wires are bundled in the harness that is made to connect directly to the Mercury control box where the throttle, shift, choke, fast idle and keyswitch are located. This harness connects to the control box via a three-hole receptacle that is covered with a hard plastic cap.

Jim's description is correct, that the diagram indicates the green to green, black to black and white to a lamp, with a black ground from the lamp. This is depicted in B/W, as color diagrams were even rarer in 1975 than they are today.

Buckda posted 06-16-2009 10:17 AM ET (US)     Profile for Buckda  Send Email to Buckda     
Over the weekend, I had a chance to tinker some more. I disconnected the white wire (it is now connected to "air", until I figure this all out). Now I have black to Black and green to green between tachometer and harness. Plug harness in to the control box and the engine turns over but does not spark. Remove harness and the engine fires immediately.

So, something is shorting out the ignition function.

I'm stumped. While the tach sure looks sweet, for the money I paid for it (short money), I'm ready to remove it!

Jeff posted 06-16-2009 05:17 PM ET (US)     Profile for Jeff  Send Email to Jeff     
Dave,

Let me dig up the vintage Mercury Tach I just pulled off the 13 with the 1974 Merc 65. I still have the wires attached and may be able to add some info.

RSP95 posted 09-24-2009 12:21 PM ET (US)     Profile for RSP95  Send Email to RSP95     
Buckda, were you able to fix your tach [unclear]? If so, what was the answer? Thank you.
Buckda posted 09-24-2009 01:20 PM ET (US)     Profile for Buckda  Send Email to Buckda     
I have not resolved this problem. Well, I have, sort of. The boat has been delivered to my brother with the tach not functioning.

...so it's his problem now(!). (Hey, I bought and restored a boat for the guy, no complaint's right!?).

We're working through a fuel pump problem right now, and then will refocus our attention on the tachometer. It's been raining a bunch in Alabama lately, so he's not been working very hard on either problem.

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