Tachometer Issue

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bigdaddypt

Petty Officer 1st Class
Joined
Aug 10, 2009
Messages
233
Hi All,

I have an 1986 Merc 5.7. The motor runs great. I just replaced all of the gauges because most of them didn't work and the one's that did were on their way out. The tachometer was one that didn't work.

I got a set of Teleflex Amega gauges and the tachometer is not working properly. It goes to 0 when the power is turned on but after I start the motor it shoots up past 7000rpm. I confirmed that it is wired properly at the gauge and the ignition coil. I tried running a different wire from the coil to the "send" post on the tach and it does the same thing, so it's not the wire. I swapped out the Amega tach with another new one and it still does the same thing. Adjusting the calibration screw does nothing.

Voltage is 14.1 at the gauges like it should. I was getting a reading of 12v on the send post of the tach with the red probe from my voltmeter on it.

Anyone have a clue what is going on? Again, the engine starts right up, idles properly and accelerates like it should.

Thank you!
 
Bondo

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,525
Re: Tachometer Issue

bigdaddypt said:
Hi All,

I have an 1986 Merc 5.7. The motor runs great. I just replaced all of the gauges because most of them didn't work and the one's that did were on their way out. The tachometer was one that didn't work.

I got a set of Teleflex Amega gauges and the tachometer is not working properly. It goes to 0 when the power is turned on but after I start the motor it shoots up past 7000rpm. I confirmed that it is wired properly at the gauge and the ignition coil. I tried running a different wire from the coil to the "send" post on the tach and it does the same thing, so it's not the wire. I swapped out the Amega tach with another new one and it still does the same thing. Adjusting the calibration screw does nothing.

Voltage is 14.1 at the gauges like it should. I was getting a reading of 12v on the send post of the tach with the red probe from my voltmeter on it.

Anyone have a clue what is going on? Again, the engine starts right up, idles properly and accelerates like it should.

Thank you!
Click to expand...

Ayuh,.... Which coil post ya got it hooked to,..??
 
B

bigdaddypt

Petty Officer 1st Class
Joined
Aug 10, 2009
Messages
233
Re: Tachometer Issue

One more thing to add. Not sure if it is helpful or abnormal. While trying to test it. I had the gray wire off the tachometer with the boat running. I accidentally touched it real quick (without having any part of my body touching any other connections) and got a real nice tingle/zap.
 
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bigdaddypt

Petty Officer 1st Class
Joined
Aug 10, 2009
Messages
233
Re: Tachometer Issue

bump. Is it somehow possible that it is the ignition coil? I don't see how it could considering the boat starts right up and runs well.

I also can verify the setting is correct on the back of the tach.
 
04fxdwgi

04fxdwgi

Senior Chief Petty Officer
Joined
Jun 10, 2011
Messages
754
Re: Tachometer Issue

bigdaddypt said:
One more thing to add. Not sure if it is helpful or abnormal. While trying to test it. I had the gray wire off the tachometer with the boat running. I accidentally touched it real quick (without having any part of my body touching any other connections) and got a real nice tingle/zap.
Click to expand...

That is normal to get "bit" from neg side of coil. Called "inductive kick". Happens when points open.....

Tach.jpg

Here is a diagram for the tach. Is yours wired like this on on back of tach? If your coil wire is connected to the "IGN" connection, all kinds of strange things will happen. Seen it happen before and is a common mistake.

Selector needs to be on 4P / 8C position
 
B

bigdaddypt

Petty Officer 1st Class
Joined
Aug 10, 2009
Messages
233
Re: Tachometer Issue

04fxdwgi, thank you so much for that diagram. That helped me figure out the problem. The wiring was correct. However, my range selector was 180 degrees off because I didn't see the arrow. Even after looking at the diagram and holding the tach under a light, it's tough to pick up on the stupid thing. Poor design in my opinion. However, it is now working properly.

Thanks again for your help. Much appreciated!
 
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