new tach for '87 inline Merc. goes to 4200rpm and back to zero

[ianmoore]

I purchased a new tach for my 1987 Mercury TOP 115. Hooked it up as per the directions, with the 12 pole setting. All connections are new and clean. The tach operates as i would think it should until it gets to 4200rpm, and then goes to zero where it remains until the RPMS drop below 4200
I tested the rectifier and it tested OK. I tried replacing the 'send wire' from the rectifier to the tach with a temporary new wire to rule out something funky in the control box. No difference. I tried jumping the ground connection to my ground connection to make sure i had a good ground. no difference.
I put my meter on the + and - of my forward fuse panel, which is a direct connection to my battery. I measured a little over 12V. Interestingly i didn't measure much voltage increase at all when i revved up the motor. It stayed basically the same.

I then tried measuring the AC voltage on the send terminal of the tach, which comes off of the rectifier. It behaved as it should. The AC voltage ramped up in parallel with the tach from around 10v up to 60 or 70 volts. Interestingly, it kept climbing even as the tach went to zero. That confused me as i thought that there would be a voltage drop at the point where the RPMs drop.

I think i've hit the wall of my logic. Does anyone have any obvious things i'm missing or ideas of things to try?

Thanks in advance
Ian

 

[ianmoore]

Does it sound like the charging system is working correctly?
The voltage from the battery seems pretty consistent regardless of the RPM(right above 12v), but the AC voltage is climbing as would be expected. Anything else i could test?

 

[Bill kubiak]

I had a similar issue with a tach made by Faria and they sent me a new tach and told me to toss the old one. Mine bounced around and was not steady above 3500 rpm

 

[ianmoore]

I've contacted Teleflex. They are sending out a new tach. Hopefully it is simply a defective one!
Thanks

 

[ianmoore]

I got the replacement tach and unfortunately am still having the same problem.
For the sake of clarity, let me go through the issue, what I have, what i've done, etc.
Basically I bought a new Teleflex tach for my '88 Mercury TOP 115. It is the correct tach for the motor.
-I set the switch to 6 poles as per the directions.
-When i get to about 4200 RPM, the tach goes back down to zero.
-all connections are new, tight, and solid. I have rechecked for any loose crimps.
-My battery connections go to a fuse panel with a + and -. These are solid connections.
-I am taking my ' ignition' from my wiring harness, as well as my send. I tried subbing in another wire to see if the send wire was somehow compromised, but got the same result.
-I have tried temporarily clipping both my ground and my ignition wires to my + and - to see if there was a faulty ground with no difference in results

I would assume that if the ground is good, then the only thing that could be doing this would be the send wire, but when i put a meter on it while ramping up the RPM's it continues to climb in AC voltage past where the tach drops to 0..

My logic is hitting a dead end. Any other suggestions?

Thanks
Ian

 

[mlbinseattle]

The AC voltage of 60-70 vac may be too much for that tach. Do you know what the tach spec for this is? It may be a matter of installing a limiting resistor in series with the signal line to the tach input. AC amplitude to the tach shouldn't be an issue since the tach is only counting pulses.

 

[ianmoore]

I think you may be right! I just found this on the Teleflex tech info:

1. tachometer pointer will go to full scale when engine is started.
2. Evinrude/Johnson "Specials" (48, 88, 112 HP, etc.): If the pointer falls back to "Zero" when operating at about 4,000 rpm or higher, you may have to add a 2,000 ohm, 1/2 watt (or larger) resistor, connected from the "SEND" to "GND" terminal.

 

[mlbinseattle]

I think you may be right! I just found this on the Teleflex tech info:

1. tachometer pointer will go to full scale when engine is started.
2. Evinrude/Johnson "Specials" (48, 88, 112 HP, etc.): If the pointer falls back to "Zero" when operating at about 4,000 rpm or higher, you may have to add a 2,000 ohm, 1/2 watt (or larger) resistor, connected from the "SEND" to "GND" terminal.

There ya go.

 

[Chris1956]

Ian, I ran Teleflex Tachs on my TOP for years. No issues at all. It is a very common combination, as TOPs are/were popular and Teleflex is a large supplier.

I am thinking that there is something wrong with your rectifier or stator. If you can find a Radio Shack store, you can buy a 25A full wave rectifier for $5 and it is very superior to your Mercury rectifier. It is worth it as an experiment.

You might ohm out the stator. Also, the stator contains multiple coils for ignition as well. Maybe some electrical engineer can explain the coupling between separate coils (ignition and charging) wound on a common core. Maybe some signal from the ign coil is interfering with the tach signal?

 

[ianmoore]

Chris,
Thanks for the comment! Duly noted..I am going to try the 2k resistor. I am thinking it is probably this because my AC voltage continues to climb in concert with the RPM's. There doesn't seem to be any voltage connection between that and the tach falling to zero. The behavior sounds exactly like what the Teleflex guide talks about.
I'll try the resistor and report back.
Is the radioshack rectifier a drop in? that sounds great. I'm sure it's quite a bit cheaper as well. If it has the same mounting holes i may buy it just to have it.
Ian

 

[Chris1956]

The RS rectifier has four male spade terminals and a hole in the center to bolt it on. Your motor has 3 ring terminals. You need to ground the -DC terminal to the cowling support and change the ring terminals to female spade terminals. Then bolt it on.

 

[ianmoore]

Good news is that the resistor solved the tach problem! I'm now getting proper tachometer readings.
I will look into using this regulator. Is it this one?
http://www.radioshack.com/25a-50v-full-wave-bridge-rectifiers/2761185.html

 

[Chris1956]

That looks to be the one, although the picture is not the best. The rating of 25A is correct.

I still cannot believe the correct fix was to add the resistor. I consider that a design flaw on the part of Teleflex. Just for fun, after you install the new rectifier, disconnect the resistor and retest. I would be interested in the result...

 

[mlbinseattle]

That looks to be the one, although the picture is not the best. The rating of 25A is correct.

I still cannot believe the correct fix was to add the resistor. I consider that a design flaw on the part of Teleflex. Just for fun, after you install the new rectifier, disconnect the resistor and retest. I would be interested in the result...

I consider it to be a design flaw by Teleflex, as well. They should have used a coupling capacitor with a clipping circuit at the input to block any DCV and allow only the pulses from the rectifier to pass as an AC signal. The peaks of any AC signal would get clipped after it exceeded a certain amount. There's several ways they could have designed that tach to accompany any increases voltage pulses. (I was an ET1 in the Navy in a previous life. )