Tach for 1993 75hp Mariner doesn't work!!

[Big Buck Buster]

I recently purchased a used boat,motor,trailer. It came with the speedometer,fuel gauge. I wanted to add a volt meter and a tachometer. I got the volt meter to work, but can't get the tachometer to work.

I was told to hook the purple wire out of the controller to the ignition terminal, the black wire to the ground, and the gray wire to the signal on the back of the tachometer.

The back of the tachometer has different settings on it depending on what motor is being used. I was told to set it to the (6) pulse setting.

I did check for A/C voltage at the gray wire, and that seems to be fine. I did also make sure that there was a good ground and power with key on.

Note: This is a used tachometer. This is the second one that I purchase that they said worked. The first one I returned.

Do any of you Mercury/Mariner owners know what might be wrong??

[boatfixer]

You didnt mention as to whether or not you found voltage on the gray wire while it was running or not. If you found voltage while the engine is not running, then I would suspect your rectifier may be the culprit.

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.

[Big Buck Buster]

I didn't check the gray wire with the key on and engine off. I checked it while it was running. It did have a/c voltage that would increase with the increase of engine rpm.

I will have to check it the way you described. The way I was told how to check it varies a little. I was told to check the red lead with the yellow leads one way, than reverse the the meter leads to see if there was resistance only one direction(checking the diodes). I didn't check it with the case ground of the rectifier. I will have to do that.

Weird thing about today before going fishing on the river. Checked to make sure it was charging before I left. It was charging before we left, but when we got to the river, it seemed like it wasn't charging anymore. The volt gauge was showing that the battery voltage was dropping, and my depth finder voltage indicator did the same.

I just hope that the stator isn't going out. If the stator was going out, you would think that I would have running problems with it.

I will do some more testing in the morning.

Thank you for the information!!! Greatly appreciated.

BBB

[Big Buck Buster]

Thanks boatfixer.

I finally got around to checking the rectifier. I have infinite(open) resistance from the studs to case. Looks like I will be replacing the rectifier.

BBB

[marine_man]

Sounds like you found the issue...

marine_man

[Phred52]

BBB, ANY idea how what may have caused it? I had a guy stop by with a mid '90's Jonny. He had the same complaints, only in addition he said his battery kept going dead. After talking a while, he did mention that he'd installed the starting battery "backwards" about 3 weeks before, and with the one side hooked up, he found that the cable lug for the other side wouldn't fit over the stud. I asked "Did it spark some?" He said "Yea, but just a little, couldn't hardly see it." I said "That's when your rectifier went bad." A new rectifier and a 'little' bit for labor, He's happy and his motor runs great. I'd bet he'll watch polarity from now on, too!! Phred52

[marine_man]

On a Johnson / Evinrude switching the cables will almost always fry the rectifier, which eventually leaves you with a dead battery... but the tach will work.

Merc's aren't really known for toasting rectifiers because of that... but on this era of 75hp engine it sure seems they had a reliability issue with their rectifiers.

marine_man

[boatfixer]

You can defintly smoke a rectifier hooking up the battery backwards on pretty much most of them. All? no. Diodes would just prefer to flow one way if at all possible.

[marine_man]

True... maybe I should revise my statement slightly... OMC Products seem to be most sensitive and have the shortest amount of time to resolve the issue. The rest seem to have a little more time to watch the sparks fly before you smoke it and figure out what the problem is.

marine_man

[boatfixer]

100% in agreement my friend!

Could be Merc owners are a little quicker too!!!

I just stepped in it didnt I?

[Phred52]

Marine_man, that's what I had thought, the tach shouldn't be involved. But, low and behold, After I finished with the repair and started the motor on the muffs, the tach was spot on. I guess you just can't ever say never. Phred52

[Big Buck Buster]

Couldn't help laughing at the last couple statements. LOL

I will get the rectifier put in soon and let you know if it solved both the charging, and tachometer issues.

Thanks again for the info.

BBB

[Big Buck Buster]

Marine_man, that's what I had thought, the tach shouldn't be involved. But, low and behold, After I finished with the repair and started the motor on the muffs, the tach was spot on. I guess you just can't ever say never. Phred52

On my Mariner, the tachometer gets it's signal from the gray wire at the rectifier.

Hope it all come together after putting it in.

BBB

[boatfixer]

Its not the tach that gets smoked, the rectifier can and thats where the signal comes from.

[Phred52]

The tach on the Jonny also takes it's signal off the 'grey wire from the rectifier'. In this case, one way of checking the rectifier is, with the motor running, there should be 9 volts at the grey wire for the tach to operate. OR, without the motor running, and the key in the 'on' position, if 12 volts is present at the grey wire, the rectifier is bad. Boat_fixer, I understand the tach doesn't get 'smoked' (never said it does). I'm stating that on this Jonny I replaced the rectifier on, One of the symptoms WAS that his tach didn't work. And by all rights, his VRO shouldn't have been working either, as that ALSO takes it's signal from the grey wire. BBB, I wish you good luck. Phred52

[Big Buck Buster]

Thanks Phred52.

This is the same thing I am hoping for with the replacement of the rectifier: Charging again and the tachometer working.

Will see soon.

BBB

[boatfixer]

Ok, I see what you are saying now. My bad, I misunderstood

[Phred52]

boat_fixer, No Problem. I (without all the fancy technology) have to diagnose the old fashioned way sometimes a lot of head scratchin's involved.

[boatfixer]

Something to keep in mind when troubleshooting though in the future is that the gray wire has to do with the warning system and the actual pumping is still done mechanically.

I should have said the gray wire in relation to the VRO

[Big Buck Buster]

Something to keep in mind when troubleshooting though in the future is that the gray wire has to do with the warning system and the actual pumping is still done mechanically.

You just mentioned about the warning system.

When we were at the Miss on Saturday, I heard a very faint chirping noise coming from the control box.

Would this be from the rectifier being bad, and the lower voltage?

[boatfixer]

Just at idle?

[Big Buck Buster]

Yes,

It seemed that if I would increase cruising speed, it would go away. Then while idling, it would start again.

The water pump seemed to be working. It was squirting out.

Another tech told me that sometimes the warning siren sometimes make this noise when they are wearing out.

I hope that I am not looking at bigger issues with this. I just purchased it used.

[boatfixer]

Thats why I asked, because its symptomatic of a worn impeller. Even though you see a good stream, it may not be pumping quite enough and the increase in RPM's increases water flow. Its can also be the temp sending unit on the back of the block. I use a lazer thermometer to varify block temp.

[Big Buck Buster]

I have one at work. I will have to see what it does in the driveway with the muffs on it.

If it is the impeller, would I be able to diagnose with the muffs, or do I have to be in the lake/river?

[boatfixer]

You should do it in the water. The hose/muffs will help a weak pump at an idle.