Testing Tachometer circuit and gauge?

2002 Mercruiser 5.0L with Thunderbolt V ignition and Faria combo Tach, Oil and Temp gauge. Boat sat all last season but given the shutdown, I've been pretty busy upgrading and catching up on maint. Finally started the engine today and everything is working great except the tach. Circuit should be a simple single wire pulse from the ignition. Can I use a function generator to test the gauge? I have a couple that are sine or square wave with hopefully enough output to drive the gauge. Got an O-Scope I could dig out and check what is coming from the engine but suspect it is the gauge.

Figured I'd ask first before attempting. Also, if it is the gauge, what are my options to replace?

it would be easier to buy a inexpensive stand alone tach, even borrow one and simply connect it, by itself to the coil and power.

Not sure on the combo tach but on a marine stand alone tach there is a selector switch for 4,6,8 cylinders. Soemtimes moving selector switch from one selection to another and back again will let gauge read properly agian

kghost said:

it would be easier to buy a inexpensive stand alone tach, even borrow one and simply connect it, by itself to the coil and power.

Not sure on the combo tach but on a marine stand alone tach there is a selector switch for 4,6,8 cylinders. Soemtimes moving selector switch from one selection to another and back again will let gauge read properly agian

Click to expand...

Thanks Jack,

I'm thinking it's the needle itself that is stuck. I didn't use the boat last season but the season before the needle stuck at about 1200 rpm. I'll give the selector switch thing a go. I just went through the house electrical but the engine wiring is separate. I did also just pulled the manifolds and risers and replaced the gaskets so was initially a bit paranoid. I checked all the connections at the engine. Wishing now I didn't throw out my old tach and dwell meter years ago. Who knew I'd own something this old school.

I did a bit more searching online and found a few YouTube videos showing how to test using a square wave generator. I was in the electronic repair business years ago and still have a decent test bench though like the tach and dwell, I've often been tempted to get rid of it all. Just don't know if either one I have will have enough output to drive the tach. I'm about 25 miles north of you and it is a bit chilly this morning but when it warms up later, I'm going to give it a go and if that doesn't work, I'll pull the gauge and get it on the bench. I'd have to order something otherwise and UPS is so stacked up with deliveries, they only drop off once a week here at the house.

I'd be willing to replace all the gauges but there is not a ton of room and these are odd ball gauges as far as combinations go. I'd probably have to redo the entire dash panel. So nothing to lose trying to repair it. Figure I used to fix tiny camcorders, a tach is a simple divider circuit and meter.

Where are you located?

I have a couple electronic components that could use some repair....

Private message me the info....

Pulled the gauge after attempting to drive the tach with a square wave from my function generator. Only can get 7 volts out of the generator but needle wasn't budging. I drilled a small hole in the side case just below the bezel and poked the tach needle a bit with a thin piece of mig wire and it is pretty stiff and sticks in any position I move it to.

Looks to be toast. May take a Dremel and cut the edge of the bezel to disassemble it completely so I can test the coils of the meter movement but probably need to start shopping for 3 new gauges. Only question is how do you match oil pressure and water temp senders to the correct gauges?

most everything in the states uses the American standard for the gauges; for the two in question, the sender provides 240 ohms and the gauge will show its MIN value. With the sender providing 33 ohms, the gauge should go to MAX value...

So you can either measure you existing sender units or if you gauges are working, test them with a couple fixed resistors or a variable one...

my Faria tach sticks every spring. About 3 or 4 good raps on the bezel with my knuckle fixes it - probably 5 years in a row now.

hystat said:

my Faria tach sticks every spring. About 3 or 4 good raps on the bezel with my knuckle fixes it - probably 5 years in a row now.

Click to expand...

That worked last time but it sat an extra year. Didn't even get to launch the boat last year. I cut the back ring of the bezel off and pulled it all apart. The shaft is stiff and a little light lubrication hasn't helped loosen it up any. Thinking of replacing it with a 5 inch tach and 5 inch 4 way multi gauge. Not nearly as many as 4 inch but this way I won't have to redo the whole dash.