1987 Johnson 70hp Tach

[BlazerSS]

I am installing a Faria tach on my motor. I bought a new tach and a new Johnson/Evinrude harness that plugs into the control box. Earlier tonight I got everything hooked up and the needle goes to zero on the tach when I turn the key on, but when I start the engine the tach doesn't register anything. I would greatly appreciate any help and direction.

 

[emdsapmgr]

When you turn the ignition off, the tach needle can come to rest about anywhere. When you power it up, it should zero out. Your tach does that. The fact that it does not respond to a running engine probably indicates there is a problem either with the new tach, or with the charging system of the engine. Before you get into real troubleshooting, you can do a simple test: Sometimes the tach switch on the back of the tach gets corrosion on the inside terminals. Put a screwdriver on the tach switch and turn it through a few switch positions, then return the switch to the setting that matches up for the number of poles on your stator. That simple maneuver can clear out any corrosion on the switch contacts, causing the tach to begin working again. If that does not help, you need to verify that the charging system (including the combination rectifier/regulator) is functioning normally.

 

[BlazerSS]

Ok thanks. I will check that today. How do I go about testing the rectifier?

 

[emdsapmgr]

A functioning charging system will provide extra voltage to the battery. If you run your engine for 20 minutes, put a simple voltmeter across the battery terminals. Normal battery voltage is around 12.5 to 12.7. If the battery is not being charged by the engine, the observed voltage on the meter will be close to the normal battery voltage. If the charging system is working, you will see the battery voltage creep up from 12.5/12.7 up to perhaps 13.5 volts. After running for a while you want to see the 13.5 volt reading. If it's stuck on the low figure, likely the charging system is not working-it's that system that provides the electrical signal for the tach to work. So-no charging, likely the tach won't work.

 

[BlazerSS]

Update on the tach. I got it installed and working and I took it to the river today and the tach worked fine for about 2 hours. Now it will only read up to 4000 rpm, above that and it falls back to 0. And if I'm running along and pull the RPM back down to 4K or under it will immediately start working again. What is going on?

 

[flyingscott]

Rectifier is bad.

 

[emdsapmgr]

Agree. When rectfiers fail, you can see numerous odd readings on the tach. Not all fail the same. Your description I've seen before on one of my rectifiers a few years ago on my 1990 70 hp engine. Tach did the same thing.

 

[BlazerSS]

What can cause the rectifier to go bad? Because this one is about a week old. It is a Evinrude/Johnson brand. Are there any better brands?

 

[David Young]

A functioning charging system will provide extra voltage to the battery. If you run your engine for 20 minutes, put a simple voltmeter across the battery terminals. Normal battery voltage is around 12.5 to 12.7. If the battery is not being charged by the engine, the observed voltage on the meter will be close to the normal battery voltage. If the charging system is working, you will see the battery voltage creep up from 12.5/12.7 up to perhaps 13.5 volts. After running for a while you want to see the 13.5 volt reading. If it's stuck on the low figure, likely the charging system is not working-it's that system that provides the electrical signal for the tach to work. So-no charging, likely the tach won't work.

Check your battery volts the way he said.

 

[mlaajanen]

What can cause the rectifier to go bad? Because this one is about a week old. It is a Evinrude/Johnson brand. Are there any better brands?

A short circuit could cause to to fail but very unlikely.

Easiest is to disconnect the rectifiers yellow wires, thus leaving the tach as the only user of the charging coils you do not need charging for the tach.

But, you could start by checking that the tacho receives 12V all the time via the purple line from the Johnson Control Center, also use the ground signal as the reference from the control center.

The best way to test cables harness is using a proper load so a few amps goes thru the wires. Take a 12V ~20W light bulb to load the purple wire and black ground from the control center when the ignition is on, that would draw about 2A so if the wires are bad you will see that then.

BTW, could it be overvoltage caused by a bad battery since it happends at high RPM?

Measure the 12V at 4000+ RPM and also try loading the 12V battery when running at high RPM.

A last idea, the tacho wire from the engine could be bad due to salt corrosion, I have replaced a few of the harness wires for that reason!!

 

[emdsapmgr]

The rectifier will smoke itself if the battery leads are not completely tight. To the battery and to the engine. If one is loose, the circuit is not complete.

 

[mlaajanen]

The rectifier will smoke itself if the battery leads are not completely tight. To the battery and to the engine. If one is loose, the circuit is not complete.

Hi.

If it was a AC generator like in car then I would agree but not the the one he or I have.

 

[flyingscott]

Hi.

If it was a AC generator like in car then I would agree but not the the one he or I have.

That is not true loose/bad connections will smoke a rectifier. Cars do not have generators they are both A/C Alternators. Cars are regulated and your particular outboard is not. That means you cannot run maintenance free batteries on your motor. This is because the battery is the regulator for the system and that generates heat so the acid boils off so you need to be able to re-fill. You may have just gotten a bad one because everything is getting cheaper. You could try the CDI Electronics rectifier/regulator combo for your motor and have the best of both worlds.

I have just had the same problem on my 1986 Evinrude 70 hp replaced last year failed this year. My stator and timer base and my battery are brand new rectifier is new chalk it up to a junk new rectifier. Doubtful you have a short in the motor would probably see the battery go dead. Was it an actual BRP rectifier or some cheaper version some dealers are buying cheaper ones and charging BRP prices.

If this one fails I am going buy the CDI Electronics combo for mine.

 

[emdsapmgr]

We have had a number of mid-80's V6 engines with the large 35 amp alternator come in for repair after they caught on fire. The battery cables to the battery were loose. The combination rectifier/regulator got so hot, it caught on fire. And these are the water cooled ones. Customer paid for a new rectifier/regulator, new stator, new flywheel and for a wiring harness. Not to mention the cleanup to the block. Fortunately, the fire did not extend to the carbs, as the engine was repairable. The cowling had a 14" black spot burned in the top, so it was either scrap, or needed serious cosmetic work. While your engine does not put out the kind of amperage that the bass 150 did-this is just an example of why you need to keep the battery terminals tight.