Mercruiser 4.3mpi Tach

[Superjetjim]

Hi my Temp Gauge and Tacho are both not working / moving on 2005 Regal with 4.3MPI motor, both are Faria branded. The tacho stays at 1,000RPM with ignition on or off / motor running or not. The Temp gauge moves a tiny bit when ignition is switched on but stays at bottom even when motor is warm.

How would you go about diagnosing if it’s the gauges or the senders please?

Would a faulty alternator affect those gauges?

 

[Superjetjim]

Assume that’s the temp sensor right?

 

[Scott Danforth]

Trace the grey wire from the ignition coil to the tach. trace the tan wire from your temp gauge to the sender

make sure ground wire on both gauges has continuity to ground. Make sure purple wire on both gauges has battery voltage when the key is on

If grounding the tan wire makes your temp gauge peg high, then you have a wiring or sender issue.

 

[Fun Times]

Try tapping on the RPM gauge to see if it moves back down to 0 both engine on and off…
There should be a engine cylinder selection switch with an arrow on the back side of the gauge…With a small flathead screwdriver, exercise the arrow switch back-and-forth a few times landing on your engine cylinder number and see if it works more normal.

The sensor in your photo is the temperature reading for the ECM… Though can’t see it there should be another sensor on the other side of the housing with one wires attached (Tan) that’s used to read the temperature gauge.

Be sure the engine is truly getting up to temperature of 160 degrees because if the thermostat is broken/stuck open the gauge would read really low to where it looks like the gauge is broken.

 

[dubs283]

Try tapping on the RPM gauge to see if it moves back down to 0 both engine on and off...

This.
A gentle tap or nine on the frame/edge of the gauge with the non-lighted end of a flashlight at times gets a sticky gauge to respond/behave appropriately.

If no response it's time to inspect the gauge/associated hardware and wiring

 

[achris]

How would you go about diagnosing if it’s the gauges or the senders please?

If you have access to a few resistors (or better still, a potentiometer), you can check the check the temp gauge. It's a very standard 240-33 ohm gauge. The tacho is a little more difficult to check...

Would a faulty alternator affect those gauges?

No. But what it sounds like to me, if the other gauges are working, is a wire may have fallen off on the back of one of the gauges, probably the power wire...

Chris.......

 

[Superjetjim]

Trace the grey wire from the ignition coil to the tach. trace the tan wire from your temp gauge to the sender

make sure ground wire on both gauges has continuity to ground. Make sure purple wire on both gauges has battery voltage when the key is on

If grounding the tan wire makes your temp gauge peg high, then you have a wiring or sender issue.

Hi Scott, I’v resolved the temp gauge (stuck thermostat) - just tacho to go.

With this being an MPi motor how would I locate the grey wire, the coil (according to Google at least) looks like this has move of a sealed unit approach (rather than round coil and exposed grey wire with the carb motors)

Any ideas where this box is located on the motor and if we have a different testing approach if MPI please?

 

[Scott Danforth]

Hi Scott, I’v resolved the temp gauge (stuck thermostat) - just tacho to go.

With this being an MPi motor how would I locate the grey wire, the coil (according to Google at least) looks like this has move of a sealed unit approach (rather than round coil and exposed grey wire with the carb motors)

Any ideas where this box is located on the motor and if we have a different testing approach if MPI please?

regardless of induction. the coil still has a ground wire. look at the coil connections.