1995 Mercruiser 5.7 TBI - tach not reading

[Devil_Inside]

I have 1995 Mercruiser TBI/EFI engine with a Delco distributor, engine serial number 0F293683.
I started the boat for the first time this season and I noticed that my tach is dead - it is not reading anything. The gauge itself is only 3 years old so I suspect the culprit is elsewhere. Where does the tach get its signal from on this model? Is there a sensor or a module of some sort? Does anyone know the proper troubleshooting procedure for this?

 

[Scott Danforth]

Grey wire connected to negative side of coil

 

[Devil_Inside]

I am assuming you are referring to ignition coil itself and not the coil inside the distributor, correct? I was looking at both connectors earlier and I did not see a gray wire, but maybe I missed it; it is hard to distinguish between gray and white sometimes. At any rate, can the coil fail in such a way that only the tach would stop working?

 

[Scott Danforth]

can the coil fail in such a way that only the tach would stop working?

NO

the coil is either working or not working. if its not working, the motor isnt running.

also, 99.999% of the time it is never the coil. yet back-yard mechanics replace almost a million perfectly good coils a year. I wish I was in the coil business.

your coil should have a purple wire on the + side (purple is ignition), a grey tach wire on the - side and the wire to the distributor which should be black

how are the connections at the dash?

 

[Devil_Inside]

Scott, please take a look at the attached diagram - I think we are talking about two different ignition systems. Mine is quite different....

From the distributor's built-in ignition module I have a total of 6 wires:

WHT, PUR/WHT, TAN/BLK and BLK/RED - all go to the ECM

PNK and WHT go to the Coil

From the Coil, I have only two more wires, RED and WHT, which go to the "harness connector to starting/charging harness"


The only two places I see a GRAY wire is between the Manifold Absolute Pressure Sensor (MAP) and the J1 connector on the ECM, and between the Throttle Position Sensor (TP) and the J1 connector on the ECM.


IF the Gray wire is in fact for the tachometer, which I believe it is, it sounds like I need to inspect the MAP sensor and TP sensor connectors. The question is, if either of these sensors are bad, could that be causing my issue? What are some other symptoms of a bad MAP or TP? I do seem to have a misfire of some sort at fast idle, which is causing quite a bit of vibration, which I did not mention before because I didn't think it was related... I though it might be a dirty or worn-out cap/rotor which I was planning on inspecting as soon as possible

 

[Devil_Inside]

The site doesn't allow me to upload a large image so please see the link below for a copy of the diagram:

https://imgur.com/a/FkOQMsU

 

[Devil_Inside]

I called Mercruiser support and it turns out the gray wire from the dashboard gets converted to a white wire at the starting harness, and it goes to one side of the ignition coil as suspected. This gives me a starting point at least. I will test the tach itself first since that's easy to rule out.