Tachometer pegs when connected

[country cadillac 48]

Hope someone can give this old timer some suggestions with my 1979 70 hp MERCURY as to why when I connect my tachometer the needle pegs. The boat did not have a tachometer when I got it, so I went to the local boat repair shop and they set me up with a used Air Guide Sea Speed Tach. It has a green and a black wire out the back and the letters 2GC printed on casing and 999 in a little white tag. I also got the plug in connector that goes to the control box only it has a white wire in addition to the green and black and I would guess that's for a lighted tach. So I thought I would connect the tach to the wires out of the box connector to see if it worked. Started the engine and it pegged the tach, which I immediately disconnected. I read the posts and I don't know what these different pegs mean. I also found the charging circuit not working and had to replace the rectifier and that fixed that problem and gave the control box power to the tach plug in.. I would really appreciate some help and thanks for your time and sorry about rambling on.

 

[Texasmark]

Re: Tachometer pegs when connected

I'm going to hit this to bump it back to the top. I know just enough to be dangerous. The tach runs off 12v and ground plus the tach input from the engine. White is probably for the light as you surmised.

The numbers on the back with the slotted switch are there to setup the tach to the number of poles on the engine. As I recall, Merc 2 strokes uses something like 6GC. If you had it on 2 you were running the dial 3x faster than you should have and yes it would peg.......but I'm guessing at part of this.

Mark

 

[glenn property of pam]

Re: Tachometer pegs when connected

from what i have read the Air Guide Sea Speed Tach proberly has a converter thats meant to be installed at the motor.

"So I thought I would connect the tach to the wires out of the box connector to see if it worked" what box is that?

"I also got the plug in connector that goes to the control box only it has a white wire in addition to the green and black" control box?

a more modern tach may be a better option unless theres a reason for wanting the old school type

 

[gbrowne9]

Re: Tachometer pegs when connected

the white wire will be the alt pulse ,the green wire is power and the black wire is ground.check it with a test light with the ign on

 

[country cadillac 48]

Re: Tachometer pegs when connected

from what i have read the Air Guide Sea Speed Tach proberly has a converter thats meant to be installed at the motor.

"So I thought I would connect the tach to the wires out of the box connector to see if it worked" what box is that?

"I also got the plug in connector that goes to the control box only it has a white wire in addition to the green and black" control box?

a more modern tach may be a better option unless theres a reason for wanting the old school type

I've tried and tried to respond but I guess I got the question wrong in the box......here goes again....the box is the shifter,gear lever and choke...there is a tach plugin on the front...they didn't give me the converter so I guess I should get a newer one as you suggested. What should this tach have with it or on the back ??? Thanks .......Ray

 

[country cadillac 48]

Re: Tachometer pegs when connected

I'm going to hit this to bump it back to the top. I know just enough to be dangerous. The tach runs off 12v and ground plus the tach input from the engine. White is probably for the light as you surmised.

The numbers on the back with the slotted switch are there to setup the tach to the number of poles on the engine. As I recall, Merc 2 strokes uses something like 6GC. If you had it on 2 you were running the dial 3x faster than you should have and yes it would peg.......but I'm guessing at part of this.

Mark

Mark.....my tach has no switch,just a brass screw with a slot, I don't see any adjustment.......Ray

 

[country cadillac 48]

Re: Tachometer pegs when connected

the white wire will be the alt pulse ,the green wire is power and the black wire is ground.check it with a test light with the ign on

thanks for the response but I'm not sure how to do that.....Ray

 

[country cadillac 48]

Re: Tachometer pegs when connected

I've tried and tried to respond but I guess I got the question wrong in the box......here goes again....the box is the shifter,gear lever and choke...there is a tach plugin on the front...they didn't give me the converter so I guess I should get a newer one as you suggested. What should this tach have with it or on the back ??? Thanks .......Ray

Glenn....took your advice,brought back the Air Guide and they told me this Medallion tach was what I needed. This does have a 4 position switch with a grd post, power post and S post. Have power to the P & S post coming out of the tach connector at the control. So I put the switch on 3, 3 cyl. correct, put the black wire on grd. green to power post and the white to the light. I'm not sure what the S post is for......the light works but the tach doesn't. My ignorance is really showing today........ I figure I've done something wrong, can you steer me straight...............................Ray

 

[mercurymang]

Re: Tachometer pegs when connected

I think the first thing you need to do is determine for sure which one goes to the light. From there, the other should be the signal wire and the black should be ground. Looks like you hooked it up opposite of what gbrowne recommnded?

 

[glenn property of pam]

Re: Tachometer pegs when connected

Glenn....took your advice,brought back the Air Guide and they told me this Medallion tach was what I needed. This does have a 4 position switch with a grd post, power post and S post. Have power to the P & S post coming out of the tach connector at the control. So I put the switch on 3, 3 cyl. correct, put the black wire on grd. green to power post and the white to the light. I'm not sure what the S post is for......the light works but the tach doesn't. My ignorance is really showing today........ I figure I've done something wrong, can you steer me straight...............................Ray

follow the wiring on post#4 as im not use to the old wiring code for mercs. S is sense (alt pulse)and it will look at the magnets under the flywheel so you set the switch depending on how many there are. Set it to the highest number to start with for testing until someone tells you the correct number

 

[country cadillac 48]

Re: Tachometer pegs when connected

I'll try your suggestions as soon as the thunderstorms pass and I really appreciate all the info you've given me. Hopefully I'll get this thing installed correctly. Thanks again for your time and I'll let you know what happened a little later......Ray

 

[Texasmark]

Re: Tachometer pegs when connected

Teleflex-Morse has a service bulletin out. Name is "Outboard Engine Tachometers, 4 through 12 pole alternators". My sheet has no www for reference. My Merc '02 90 hp has a 12 pole alternator. It explains about the setting on the tach that has to correspond to your alternator poles.

I have one in front of me. The brass screw you mentioned is probably the zeroing adjust. The TM tach has a second screw with range numbers on it to set it to your corresponding poles. If your tach has no second adjustment it must be a special purpose designed for one engine application.....I guess.

Best I can do,

Get one of the old salts, mechanics on here to help you. Just keep bumping your thread up to the top till one of them sees it.

Mark

 

[Timestep]

Re: Tachometer pegs when connected

Outboard Engine Tachometers, 4 through 12 pole alternators

Here it is :

http://www.v3instruments.com/trouble_shooting_guides/o_b_tachameter_ts.pdf

 

[country cadillac 48]

Re: Tachometer pegs when connected

Finally got it working thanks to you guys. I set the newer tach on the highest setting...#4...located the light wire...white...those two went to the power pole, black to ground and green to the S pole. Now there's a few more things I'll be able to do knowing the RPMs.It's great to see that there are lots of folks out there that are willing and knowledgeable enough to help others that need the help. This old Vermonter really appreciates everyone who helped and hopefully I'll be able to return the favor...........................Ray

 

[Texasmark]

Re: Tachometer pegs when connected

Outboard Engine Tachometers, 4 through 12 pole alternators

Here it is :

http://www.v3instruments.com/trouble_shooting_guides/o_b_tachameter_ts.pdf

Thats exactly the same picture I have from the TM site. Only difference is the verbiage added to this. I made a copy for future reference.

Thanks,
Mark