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1976 Mercruiser 350 mie gm 454 - Tachometer issue

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clt1wjs

New member
My Thunderbolt 1 454 rus perfectly. My my 37 year old guages needed replacing. I replaced them with Faria guages and I cannot get the tach to work. It is absolute wired correctly to the guage. The old tach didn't work so I am trying to trace it back to where it gets the signal from. Google searches produces all kinds of BS. Does anyone know where the signal originates on my engine? The wiring harness for 1976 has a BROWN wire for the tach. Later generation it is gray. Please advice.
 
Run a jumper from the neg. side of the coil to the tach. If it works the original circuit is open. Look at the engine barrell plug pins. Ohm both directions from the plug.
 
Thanks for the suggestion. I ran a jumper from the tach to the "brown" negative side of the distributor. The tach slightly moves when the key is turned on and that is it?
 
OK your response makes NO sense.

1. in order for a tach to show rpm's the motor must be running. Was it? if so SAY so!

2. running a jumper is a troubleshooting tip, BUT did you remove the existing wire connection from the tach and coil? if so SAY so!

When turning the key on and engine not running = no tach readings............

try again with as much info as you can possibly spare so we are not throwing out info for NO reason.
 
Is there a good +12 VDC supplied to the "IGN" terminal of the tach?
Is the "GND" terminal of the tach grounded to a GOOD ground?
Is the small rotary switch on the back of the tach in the "8 CYL" position SOLIDLY? (hint: move the switch back and forth a couple of times and reset to "8 CYL"

If yes to all these questions and the temp wire is connected between the coil "NEG" (and the origional wire is off the coil) and engine is running, then tach should be operating properly.
 
Thank you for all your answers. I have good a good ground to the tach, the +12 vdc is good. I have moved the 8 cyl switch to all positions and it is correctly set on the 8 cyl position. I hooked up a jumper wire to the negative side of the coil with the existing connection in place. I started the engine and had no response on the tach. I then tried using the jumper wire without the original wire connected and the engine would not start an a screeching noise came from the coil. It would not stop with the key in the off position. Only when I disconnected the battery did the sound stop. I removed the jumper. Replaced the original connection to the negative side of the coil. I attempted to restart the engine and it would not start. The same screeching sound is coming from the coil. Any ideas? I had a strong running boat with a broken tach and now I have a boat that wont run. :(
 
A bad tach can and will mess you up big time..

So the tach is now completely disconnected ?

And the engine cranks but won't start ?

The dead mans switch is fully out. Cycle it a few times.

You have no spark at the plugs now ?
 
clt1wjs said:
Thank you for all your answers. I have good a good ground to the tach, the +12 vdc is good. I have moved the 8 cyl switch to all positions and it is correctly set on the 8 cyl position. I hooked up a jumper wire to the negative side of the coil with the existing connection in place. I started the engine and had no response on the tach. I then tried using the jumper wire without the original wire connected and the engine would not start an a screeching noise came from the coil. It would not stop with the key in the off position. Only when I disconnected the battery did the sound stop. I removed the jumper. Replaced the original connection to the negative side of the coil. I attempted to restart the engine and it would not start. The same screeching sound is coming from the coil. Any ideas? I had a strong running boat with a broken tach and now I have a boat that wont run. :(
Click to expand...

Are 100% sure the coil / ignition is connected exactly as it should be? If all connections are indeed correct, something surely doesn't make sense.

On Tach:
+12 VDC supply to the "IGN" term of tach
Ground to "NEG" or "GND" term of tach
Coil sensor wire to "SEN" term of tach.

On coil:
+12 VDC to the + terminal (if there is a resistor wire in there, is the +12 VDC start bypass energized all the time or only during start?)
Points / ignition module on - (neg) side of coil (Tach wire should be disconnected while troubleshooting to eliminate it from problem)

There are only so many things that can cause symptoms like you describe.
1. Incorrect wiring
2. Shorted points in std ignition system
3. Shorted condenser in std ignition system
4. Bad electronic ignition module, if equipped.
5. Bad coil.

Sounds like there is a dead short from the neg side of the coil to ground, the coil is shorted internally or the +12 VDC start bypass is energized all the time, not during just the start cycle (screaching noise w/ ignition off makes one think this).

Disconnect the neg side of the coil and see it it "screaches" again. If it makes noise, coil is deffinately kaput. If not, then you need to be troubleshooting the neg side of the circuit. Try disconnecting everything (including the output from the alternator) and start from scratch again, starting from the basics, checking voltages and ground connections as you go.

Try seeing what the resistance is in the tach between the "sen" teminal and ground.

Does that system have have the shifter / ignition interupter circuit in it? If so, it may be out of wack in that circuit.

You said it was a TB1, is that a Thunderbolt 1 electronic ignition? Don't think Merc used electronic systems before the early 80's (like '84).
 
Last edited:
clt1wjs said:
Thank you for all your answers. I have good a good ground to the tach, the +12 vdc is good. I have moved the 8 cyl switch to all positions and it is correctly set on the 8 cyl position. I hooked up a jumper wire to the negative side of the coil with the existing connection in place. I started the engine and had no response on the tach. I then tried using the jumper wire without the original wire connected and the engine would not start an a screeching noise came from the coil. It would not stop with the key in the off position. Only when I disconnected the battery did the sound stop. I removed the jumper. Replaced the original connection to the negative side of the coil. I attempted to restart the engine and it would not start. The same screeching sound is coming from the coil. Any ideas? I had a strong running boat with a broken tach and now I have a boat that wont run. :(
Click to expand...

did you every resolve your issue? I’m having the same problem with my thunderbolt ignition and new Faria tach
 
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