Tach problem - 1980 Merc 115

[imported_scott_r]

Just installed a new tach with a 1980 Merc 115. Tach is set to 6 (12-pole). When I turned the ignition on the tach needle pegged at max. Turned ignition off, then on again; the tach needle pegged again, then dropped to zero. Now no needle response with ignition on and when cranking (haven't started engine). First question: Did I blow something in the new tach? Second question: What would cause this? Any help appreciated.

Thanks,
Scott

 

[jddenham]

Re: Tach problem - 1980 Merc 115

Check it with the engine running, if it works then don't worry about it. If it doesn't then check your bridge rectifier. if that's not the problem check your stator.

Maybe check the wiring from the motor to the control box before checking the stator. You are connected to the three wires on the control box right?

 

[imported_scott_r]

Re: Tach problem - 1980 Merc 115

I used the three wires coming from the MerControl control box. The previous tach had connectors for "+12V" (white wire) "GND" (black wire) and ALT (gray wire). New tach has connectors for "+", "-", and "P". I connected the white wire to "+", black to "-" and gray to "P".

 

[jddenham]

Re: Tach problem - 1980 Merc 115

Have you started the motor yet?

 

[imported_scott_r]

Re: Tach problem - 1980 Merc 115

Not yet. I hope to run it soon and also check the charging (rectifier) at the same time. Thanks for the help and pls check back if you can.

 

[Texasmark]

Re: Tach problem - 1980 Merc 115

6P (6 Pulse) should be the correct setting for a 12 pole stator which Merc likes to use.

Merc (Quicksilver) have a self test mode where the needle goes to 0 or no higher than 50 rpm when the key is first turned on.

I'm sitting here looking at the back of a QS tach right now and there are 3 wires to hookup IGN (your +) GND (Your -) and SEND which is your (P) where your gray tach input wire should connect.

Not familiar with the setup on '80 engines. On later engines, the rectifier module is the source of the gray wire's signal. If you have one (rect module) check for the gray wire (guess they haven't changed the color as it is gray on recent engines).

If you have the wiring hooked up correctly and it zeroes when you switch the ignition to on then the tach is supposed to be good.......according to Merc.

Then go to the rect/regl or interconnect wiring for your problem.

If you dig around the archives, Willy or someone has posted what the ac voltage should be from the rect at specified rpm's.

A bad rect/regl can impact the tach reading.

Best way to do that is substitution with a known good one.

Best I can do.

Mark

 

[diaric]

Re: Tach problem - 1980 Merc 115

what brand of tach? I'd have to go check mine but it had settings from 1 to 6 and they wern't the pole numbers.
If I remember correct, 5 set it at 6 pole and when i had it on 6 which i thought was 6 pole, it just burried itself when i turned on the key or ran it. just a thought, as mine had me baffled till I e-mailed the company and got it sorted.

 

[imported_scott_r]

Re: Tach problem - 1980 Merc 115

I don't know the brand. It's an OEM product I bought online and I have no instructions for it. It might be a good idea to call the supplier so I'll try that. It has a switch with settings for 2, 5 and 6. A 1980 Mercury 115 outboard should be 12-pole and my understanding was that the setting should be 6, which is what it was set to when I received it, but I'll check on it. Thanks.

 

[diaric]

Re: Tach problem - 1980 Merc 115

that sounds correct to me

 

[hkeiner]

Re: Tach problem - 1980 Merc 115

As a coincidence, I just recently replaced a tach gauge on my boat and it did not work correctly at first. When I turned on the key, the needle did move to zero OK but when I started the motor, the needle shot up to maximum at idle and stayed there until I turned off the motor.

I was sure that I had the pole setting on the back of the gauge correct, so I initially thought I had a bad gauge or wiring. Before I did anything else, I took a small screwdriver and moved the pole selector on the back of the gauge back and forth through various settings and then back to my 20 pole setting. The tach started working fine after that. I can only guess that the pole selector was really not exactly in the correct position (i.e. somewhere between two positions) or the contact inside the gauge was a little corroded and moving the pole selector back and forth a few times cleared it up enough to work.

Your results may vary

 

[imported_scott_r]

Re: Tach problem - 1980 Merc 115

I ran the engine today and the needle pegs at max (8K RPM) when the ignition is turned on and stays there until the ignition is turned off, regardless of engine speed. I tried different pole settings on the back of the tach but the result is the same. The charging system appears to be working as the voltage at the battery increases at higher RPM. Does anyone know where the tach lead is on a 1980 Mercury 115 so I can test there and see if there's a signal? Then I'll know if I should start tracking down a wiring problem. Thanks!

 

[imported_scott_r]

Re: Tach problem - 1980 Merc 115

Never mind. I found that the tach lead was miswired at the engine. Everything looked OK and was nicely tie-wrapped but the tach lead was on the terminal box and not the rectifier. Thanks for everyone's help.

 

[Texasmark]

Re: Tach problem - 1980 Merc 115

Hey......successful conclusion.....great.

On the tach; there are 5 positions. If you have position marking, 5 is correct. If it has 5 positions and the 5th position is marked 6P, that is correct.

The OEM says that you have to insure that you are centered on the marking to insure accuracy.

One quick check for correct switch position, is that if your engine is setup properly, in F gear at idle, your rpm's should be around 600-800 rpm's. If you peg the meter at low rpm's you are definitely on the wrong switch position......as long as switch position is the only problem.

Mark