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Author Topic:   1987 Johnson: Tachometer and Trim Gauge Installation
jeremy posted 07-30-2006 06:51 PM ET (US)   Profile for jeremy   Send Email to jeremy  
Hi all, new here and I am sure you have heard this problem more than once. I have a 1987 Johnson 90hp, model CJ90TLCUR serial number C7490361. I am attempting to hook up the tach and power trim and tilt gauges. The part/id number on the OMC tach is 174052 and on the T/T gauge is 582965/2451.

First, what do I set the small black switch on the back of the tach at for this motor, 2, 4, 5 or 6?

Second, I have the violet from the tach harness to the I on the T/T gauge, then the black from the tach harness to the G on the T/T gauge and a ground lead from my fuse block also to the G on the T/T unit, I believe this is now grounding the T/T unit as well as the tach...?

I have the blue lead from the tach to the L (light) pole on the T/T unit which both are powered when the ignition is turned on.
I am then left with the white w/brown tracer and the black w/brown tracer from the power trim sending unit on the rear of the engine block. I have tried all the combinations of connecting these two and have had no luck getting a proper T/T reading.

If I put the black w/brown tracer to the S terminal and the white w/brown to the G terminal the reading spikes, with the same two the other way around I get no reading. Curious thing is that my T/T unit at the motor works with the ignition on as well as off. Is this causing the problem? I know before I got the boat the trim motor was replaced and I am sure the power trim and tilt on the control handle should only work with the ignition on? The problem as I see it is either I have the wiring wrong, the T/T gauge is broken, the tach setting is wrong and effecting the T/T or the wiring from the T/T at the engine to the control box, or even at the control box, is wrong.

I realize this is a long winded story but if anybody has any idea what I am talking about and might have a solution, please help, thank you very much for any input!

PS. I have searched the web high and low for a wiring diagram of this motor with no luck.

Joe Kriz posted 07-30-2006 07:42 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

You are close. But, I think you have way too many ground wires. All you need is one.

Let's forget about the black with brown tracer for the time being.

The White with Brown Tracer is the one that should be connected to the "S" terminal. The "S" stands for Sending Unit or Sender.

The Violet or Purple is correct. This is Ignition on and is connected to the "I" terminal like you have it.

The Ground wire is the "G" terminal like you have it.
Only one ground for now please.

The Blue wire is the light and should be connected to the "L" terminal like you have it.

Give that a try and leave off the Black with brown tracer. This might be another ground but you have too many already.... I will have to look at mine to see what is used for ground unless someone chimes in here before I get a chance.

jeremy posted 07-30-2006 07:48 PM ET (US)     Profile for jeremy  Send Email to jeremy     
Thanks for the quick response! I have been pulling my hair out all afternoon trying to sort this out.

So I will leave the black w/brown tracer off and hook up one groung. Which one, the black ground from the tach harness or the ground from the fuse block which is direct from the battery?

Also, any idea what I set the tach to? 2, 4, 5 or 6?

Thanks again.

Joe Kriz posted 07-30-2006 07:59 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

My instruction book for the OMC Trim Gauge says what I told you above...

The ground should come from your terminal block with the rest of the grounds.

(It is possible that the black with brown tracer is a ground but try using just the ground from the terminal block) Remove the ground wire from this gauge that came from the tach. Don't use the ground from the tach although it should be a good ground because that wire is used for the tach. Let's keep the wires separate....

jeremy posted 07-30-2006 08:02 PM ET (US)     Profile for jeremy  Send Email to jeremy     
OK, any combination of ground 'bottoms' the needle out and there is no movement when T/T is operated?

I tried the battery - ground, the tach ground and the black w/tan (which is the correct tracer color and not brown!) all seperatly as well as the tach and battery ground together, nothing. Coming to the conclusion the gauge is broken, sending unit is faulty or the ignition switch not being correctly wired is causing the problem. Not sure what to try next.

jeremy posted 07-30-2006 08:08 PM ET (US)     Profile for jeremy  Send Email to jeremy     
Perhaps I need to be clearer, the needle on the gauge goes to the bottom of the gauge when the ignition is turned on no matter which ground is hooked up.
gturner posted 07-31-2006 08:06 AM ET (US)     Profile for gturner  Send Email to gturner     
Jeremy, it sounds like the polarity is reversed on the guage that bottoms out when power (signal) is applied. Arnold
jeremy posted 07-31-2006 09:48 AM ET (US)     Profile for jeremy  Send Email to jeremy     
So if the polarity is reversed how might I fix it? Do I hook up + and - opposite?
gturner posted 07-31-2006 05:28 PM ET (US)     Profile for gturner  Send Email to gturner     
I can't in good conciense tell you to reverse those wires. I don't have enough info to do that. Digital stuff doesn't act like analog stuff, and meters are expensive. Is your meter digital or analog?
Joe Kriz posted 07-31-2006 06:06 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

Do not reverse the wires.

Something else is shorted out...

The description of where to hook up the wires is according to the book..

A ground is a ground.
It is easy to check for power using a test light.
Hook the test light to the violet and the ground and turn on the key. If the light comes on, then the ground and power are OK..

Then, trace down the White/ Tan stripe back to the engine. See if there is a problem with this wire shorting out somewhere.

At this time, who cares about the blue wire for lights. Disconnect it temporarily until you find the problem.

jeremy posted 07-31-2006 08:51 PM ET (US)     Profile for jeremy  Send Email to jeremy     
All good points and I have been listening. I have not changed anything as far as polarity, there is power to the ignition/violet wire, ground from the fuse block is hooked up, I assume the problem is with the white w/tan tracer or the black w/tan tracer or the gauge itself.

The black w/tan is attached as a ground to the block behind the T/T relay box mounted on the engine block between the top two cylinders, the white w/tan appears to be connected to one of the relays?

I have still not been able to find out what the small black dial on the back of the tach should be set at.

I appreciate all the input.

Joe Kriz posted 07-31-2006 09:48 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

I can't find those part numbers you listed for either of the gauges.

Do they look like any of the ones on this page here?

My OMC tach manual states to set the tach to #6 for OMC engines 70 HP and above.

I also checked another OMC trim gauge manual that I have.
Black with Brown stripe is ground. "G" terminal.
White with Tan stripe is signal and should be connected to "S" terminal.
Key on power should go to "I" terminal.....

It sounds like you have it hooked up correctly so it is either your gauge or something is wrong with the White wire with the tan stripe.

jimh posted 07-31-2006 10:15 PM ET (US)     Profile for jimh  Send Email to jimh     
We need an OMC gauge expert (which I am not). But, there may be two types of gauges, and they may have opposite "sense" of scale.

The trim gauge is really just a current meter inserted in a circuit where the tilt sender is the rheostat. The circuit looks like this:

[+12-volts]---(violet)---[I]---[resistor]---[S]---(white/brown)---(to sender)

---(white/brown)---[sender variable resistor]---(black/brown)---(to gauge)

---(black/brown)---(isolated ground)---[G]

The gauge has to be matched to the sender so that the resistance of the sender limits the current to match the meter's full scale reading and calibration.

jeremy posted 07-31-2006 11:00 PM ET (US)     Profile for jeremy  Send Email to jeremy     
My gauges are the ones in the top pic, the T/T just doesn't have the writing on it and just arrows. I think we have all come to the same conclusion, wiring is correct so gauge must be hooped or wiring at the relays is wrong. I will source another gauge and try it, failing that I will have the wiring at the T/T box looked at closer.

Thank you all for your input, I do appreciate it. Despite this wiring problem, I do know a little bit about outboard motors (been playing with them for 20+ years, only wiring was never my thing) so I hope I can help somebody in the future with a problem I know something about.

Joe Kriz posted 08-01-2006 12:00 AM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     
I have had almost every OMC gauge that was made from the Tech Series back to the Traditional Series including the Classic Series.
I also have many of the manuals still in my collection.

I will let anyone out there be the judge whether I am an expert or not.

I believe this is the gauge Jeremy has according to his description.

The above gauge is the Traditional Series by OMC... This is the early Traditional Series gauge as it later changed to the one in the link above that Jeremy refers to. I prefer that later version as shown in the link that has the writing or description on it rather than the one with just the arrows.

Jeremy, that is an older gauge you have. Is it New Old Stock?
Or is it old and used?

I would definitely get a new gauge. You can buy new old stock Traditional Series gauges on eBay from time to time. Or a good used one. Otherwise, you can buy the Tech Series which isn't too much different except for the internal lighting which pipes the light through the needle.

jeremy posted 08-01-2006 01:39 AM ET (US)     Profile for jeremy  Send Email to jeremy     
Yes, that is the right gauge. I will try another gauge that I know works and see where I stand. After looking at the wiring numerous times, and listening to all your feedback, I am almost 100% that it is all correct, leaving the gauge itself as the culprit, thanks again and I will update.
jimh posted 08-01-2006 08:35 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the polarity in the circuit: If the positive and negative were exchanged, the meter movement would be in the wrong direction, and the indicator would be pulled against the stop or rest when the current flowed through the meter. As long as you have the violet wire connected to the [I] terminal, you likely have the polarity correct.

What I was attempting to describe above about the meter sense was the layout of the meter face and dial pointer. The normal orientation on a meter is for the resting position to be at the left and for the pointer to rise or move to the right with increased current. On some OMC TRIM gauges, for example the one I have, this direction is reversed, and the resting or no-current position of the gauge is at the right hand side. When more current flows the pointer moves to the left. (These directions are based on a reference of having the pointer pivot at the bottom of the meter face, which is also the usual orientation of a meter.) I do not really know if there are variations among OMC gauges with respect to this, but if there were, it would affect how the meter operated. The result would be the indicator would be opposite of the actual engine position, that is, it would show up when the engine was down and vice-versa.

As for grounds, the trim gauge has a separate ground lead, the black and tan conductor, and this is all that needs to be used. If you have other grounds on the [G] terminal of the gauge it will not help, and the isolated ground is provided in order to increase the accuracy of the TRIM gauge.

In another article ( ) you can see the wiring of an OMC TRIM gauge. Note that the gauge has a resistor connected on the back between the terminals [I] and [S].

Joe Kriz posted 08-01-2006 11:21 AM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

There is also one last thing I can think of to look at.

Before, I asked you to follow the White w/tan stripe wire.
Do this again.
But, this time, you should find a connector somewhere under the engine cowling that has both the White w/tan stripe and the black w/tan stripe.

Pull this connector apart and see if the connector and wiring is OK.
This connector may be located under the carbs and may be oily.
I have seen this before.
Clean off the connector and when putting the connector back together, make sure the wires are plugged all the way in.
When this connector gets oily, the wires want to pull out of the connector and not make contact.

Let us know what you find out.

jeremy posted 08-01-2006 02:53 PM ET (US)     Profile for jeremy  Send Email to jeremy     
Well problem solved. We were all in the wrong ball park. Turns out the tilt trim switch above the tilt trim motor at the mid section was stuck all the way open, hence the reading looking like it was bottoming out every time power was supplied. I removed the switch, cleaned it up and lubed it up and everything works great, thanks to everybody for their input!
jimh posted 08-01-2006 08:21 PM ET (US)     Profile for jimh  Send Email to jimh     
Sounds like your problem was mechanical, not electrical.
jeremy posted 08-01-2006 10:44 PM ET (US)     Profile for jeremy  Send Email to jeremy     
Not really just mechanical, the switch operates by movement but sends an electrical signal, bottom line, it is fixed. Strange that none of us thought about that unit that actually almost always sits underwater and is subject to corrosion, especially here in the salt water--good little bit of info to pass on for similar problems.
jimh posted 08-02-2006 09:36 AM ET (US)     Profile for jimh  Send Email to jimh     
The trim sensor on the outboard mounting bracket is not a switch; it is a variable resistor. It sounds like the electrical circuit was fine. You had a mechanical problem; the sensor was stuck mechanically, not electrically.
Joe Kriz posted 08-02-2006 12:13 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

I will never assume anything after this.

I assumed that when you said you were trimming the motor up and down that you also knew that the arm on the sending unit was going up and down also.

When I check a trim gauge and sending unit, I take the engine all the way up and manually move the sending unit arm up and down and watch to see if the gauge moves. This way I know that the lever is moving and is not stuck.

Thanks for opening my eyes, and again, I will not assume someone knows about checking this lever in the future.

jeremy posted 08-03-2006 01:11 AM ET (US)     Profile for jeremy  Send Email to jeremy     
I agree, never assume, I looked at this as an electrical problem and presented it as such. The symptoms showed that [it was an electrical problem]. The 'sending' unit was never brought up or mentioned. I knew it was there but thought the problem was wiring or a gauge problem because of the response when power was supplied.

It was never talked about to check the sending unit, and now I know, after consulting a good friend who is an OMC wizard, over the phone who told me within 30 seconds to check the culprit and we had the problem solved.

MarineLife posted 06-16-2008 10:15 PM ET (US)     Profile for MarineLife  Send Email to MarineLife     
I just read this whole thread because I have the same problem. I am going to go through the same things everyone suggested here to find out the problem. I just took out a Faria T/T and installed a OMC Tech series and I cannot get it to work. Before this I had installed a Tachometer and a fuel gauge both of the same series and they work fine. So it cant be a wiring problem.

Should I look at the sensor? I have no idea where it is. I will also look that up to see. And, again, I thank everyone for their input on this subject from before, awsome job guys.

Semper Fi

jimh posted 06-16-2008 10:57 PM ET (US)     Profile for jimh  Send Email to jimh     
On an OMC motor the tilt sensor is typically located on the fixed portion of the engine mount, that is, the part that does not tilt up (called the port stern bracket). It typically has a spring loaded arm that bears against the moving part of the engine mount (called the swivel bracket). As the engine is tilted up, the moving arm of the tilt sensor moves with it.

The sensor is a variable resistor which controls the flow of current through the dashboard meter, thus indicating the engine trim or tilt position.

If you tilt your engine to the full-up position, you typically can see the tilt sensor if you view the engine from aft of the transom and below, looking upward into the tilt mechanism.

The tilt sensor usually is mounted with two screw fasteners, and some play is allowed in its mount in order to provide a small range of adjustment for calibration of the sensor and meter to match the engine position.

If you have an OMC motor, but you do not have the factory service manual, you can see exploded views of the motor on

by going to the electronics parts catalogue and locating your engine by year, horsepower, and model, or, if you have a service manual, you can find the location of the sensor shown therein.

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