The outcome; 1980 Johnson 55. Tach or rectifier problem?

steviecops · May 7, 2004

I fitted a new tachometer to day and got erratic readings. It is a Smiths Tach, (a good make, made in the U.K), made for outboards. I wired it to the plug in the control box. It seemed to work O.K at low revs, up to 2-3k, but when I increased the revs, the tach needle dropped first then gradually climbed again. But it never seemed as if it was reading high enough. Could this be a rectifier problem, or maybe a loose or dirty wiring connection in the control box? I did take the control box apart and found the ground connection screw was loose and the terminals on the wires were dirty. I've cleaned and tightened the connection, but haven't tried the tach since then. Is the dropping needle consistent with any common faults? I'd appreciate any help on this. Many thanks Steve

steviecops · May 7, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Any ideas anyone? Just read through all the fitting instructions again, just to make sure I hadn't overlooked anything. The tach is for outboard engines with electronic ignition and a tachometer output terminal. Aparently it is suitable for 2 stroke and 4 stroke engines, just the settings differ. The switch on the back has settings for 2,3,4 0r 6 cylinders and there is a note which says,: "A number of outboard engines have a tachometer output terminal which provides a signal derived from the alternator. This will be revealed by the tachometer reading being exceptionally high and can be corrected by setting the adjuster as for a 6 cylinder two stroke engine." I take it from this note that the tach should be suitable for my engine, so there must be some other problem. Thanks Steve

steviecops · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Well I've eliminated the rectifier as the problem. I tested it today and it checks out fine. Anything else I should check, or do you think the loose and dirty earth terminal in the control box could have been the problem? I won't get a chance to run the engine properly until tomorrow or Monday. Thanks Steve

mcginleyc · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Tachometers have a "pole" switch on the back that must be ste for specific engines. A tach works by counting the revolutions of the flywheel "poles". The poles refer to the stator on the flywheel. Different engines have different numbers of poles depending on the number of charge and ignition coils. If it is set incorrectly it will give a false reading. Check the owners manual to find what your engine has

VistaMan · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

I beleive thge setting should be at 6. Also there is a note about Johnson 50 HP and unstable readings in the Teleflex tech notes. http://www.tflx.com/pdf/obtach~1.pdf Maybe this will help.

DJ · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

As I recall, you posted a while back that you had reversed the battery leads. That will pop a rectifier for sure.

steviecops · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Thanks for the info. My engine is a 2 cylinder. It has CD2 ignition. From what I can make out from my manuals, under the flywheel on the stator assembly there are 8 coils wound around an iron core, one charge coil, one sensor coil and three AC coils. Then of course there are the two ignition coils, one for each cylinder. Now all this doesn't mean much to me, but I believe that a 6 pole setting would work from what I have read on here. My tach doesn't have a setting for 6 pole. Instead it has settings for 2,3,4,or6 cylinders, although it says on the box and in the instructions that it's for outboard engines with electronic ignition. The bit on the teleflex page about fitting a resistor between the send and ground terminals was interesting. Maybe I need to do that? Thanks again for the info. Steve

steviecops · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Hi DJ Must have been someone else, I don't recall reversing the battery leads ;o) My rectifier checked out alright today when I tested it. No reading in one direction on all leads to ground, then infinity with test leads reversed. Then the same when tested from the red lead.

VistaMan · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Did you measure the battery voltage before and while motor running? If no change then carging circuit is bad.

steviecops · May 8, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Hi MarkHP I haven't checked the battery voltages yet, but will do tomorrow. So even if the rectifier is O.K, I could still have a problem with my charging circuit? Is that what you are saying? What would be the probable cause? Faulty charging coil maybe? Is there anyway I can test it? Also, would a fault in the charging system cause the problem I described with my tach? Thanks Steve

rickdb1boat · May 9, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

steviecops Did you disconnect all the leads going to the rectifier when you checked it? Even if you get some increase in voltage just above idle, does not nessesarily everything is good above that. But it is true that generally it is a good test, as MarkHP said. Also, the link the MarkHP gave is very interesting about the resister. Can you borrow another Tach and check it against that one? Also, as you said, you found loose connections and have not tried it since then. That could be it in a nut shell... What were your readings at idle? BTW- 6 pole is the correct setting for your motor..

CorB · May 9, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Hi, I have a '78 Johnson 55 HP with a Faria tachometer. Until 5500 RPM it works just fine but from that point on it jumps to 6000 and back to 5500 without the engine making more or less revolutions. That bogs me for a time now. But anyway, it's strange that you don't have a pole setting, but a cylinder setting on your tachometer. In this document http://faria-instruments.com/site_manuals/IS0086J.pdf It looks like your outboard has 10 poles. I found another document on the Faria site which has some explanation of tach's and could help you a bit? http://faria-instruments.com/site_manuals/IS0100D_9.pdf

steviecops · May 9, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Rick I did disconnect all the leads from the terminal strip before I tested the rectifier. The info about the resistor was interesting, and I might well try that if all else fails. I'll try to borrow another tach first though. I was hoping that the loose ground terminal screw and dirty connector might be the problem, but just wanted to know if that would cause the tach to behave as it did. The tach showed 1000 rpm at idle in neutral and seemed to increase smoothly up to about 3000, but anything past that and it would drop back to 2000 then climb slowly. CorB Thanks for the lnks, I'll read through them. The Teleflex PDF document also says that all OMC outboards under 75hp have 10 poles and need the 5 pole setting on the tach? Doesn't sound right to me though. Thanks for the input, it's appreciated. Steve

steviecops · May 10, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Took the boat out today and the tach seemed to be working fine. Must have been the loose/dirty connection in the control box. I also tested the battery voltages. 14.7 volts at about 3000 rpm. I discovered that I'm only getting 4500 rpm at WOT, but I'll start another thread on that problem. Thanks for all the advice Steve

VistaMan · May 10, 2004

Re: The outcome; 1980 Johnson 55. Tach or rectifier problem?

Glad to hear it is fixed, and for cheap too

The outcome; 1980 Johnson 55. Tach or rectifier problem?

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