Bad Rectifier and Tach response

1975 135HP Johnson
Last night, I found out I had a bad rectifier. There was no voltage change between engine off and a high idle as read across the battery. I ran through the rectifier test procedure comparing forward and reverse voltage passing over each diode (Y-Gnd, Y/G-Gnd, Y-Red, Y/G-Red) and got infinite resistance over each combination except one (Y/G-Gnd or Gnd to Y/G, I don't recall which direction).

So I replaced the rectifier and started the engine. it ran rough so I thought now I may have to redo my fine tuning now that the new rectifier was in. Then I noticed my tach was running at a higher number. Instead of showing about 650-700 rpm it was now showing about 1200. But it didn't sound like it was revving much higher than before the new rectifier, to my very untrained ear.

I can assume that maybe now the tach is seeing twice as many pulses now that the rectifier is working correctly (aledgedly) an possibly my earlier tach readings were wrong due to the bad rectifier. All I know is that if I lower the rpms down to less than 1100 the motor just dies. So perhaps the reason my engine was working fine at low idle was because it wasn't really at low idle. Would it be obvious when shifting with an idle at 1200 that the idle was too high? I didn't notice a big clunk or other evidence of a hard shift.

Re: Bad Rectifier and Tach response

There should be a dial on the back of your tachometer. It should be set to number six (6). Is it?

Re: Bad Rectifier and Tach response

Joe Reeves said:

There should be a dial on the back of your tachometer. It should be set to number six (6). Is it?

Click to expand...

Yes it is

Re: Bad Rectifier and Tach response

On a flushette (garden hose), the rpm should be set to 1000 rpms. The rpms would drop to normal when the rig is in the water.

The sensor terminal of the tachometer, either a terminal marked "S" or a "Gray" wire leading from the tachometer, should be attached to the Yellow/Gray lead of the rectifier OR if both yellow leads of the rectifier are just plain Yellow, attached to the Yellow wire that is attached to the stator Yellow/Gray wire. If there is no Yellow/Gray wire on either component, the Gray wire can be attached to either Yellow wire.

Initially you indicated that the tachometer was reading 650-700 rpm when in the water BUT the tachomer shouldn't have read at all with a faulty rectifier. Perhaps I am reading something wrong there...... and you meant that in the past, the 650-700 was the reading with a previous good rectifier.

If all is as it should be (wiring, setting, etc), I cannot imagine why you would obtain a double reading UNLESS the tachometer was damaged internally. If possible, temorarily connect a known good tachometer to compare.

If the compression, ignition, or fuel system was flawed whereas the rpms had to be at a higher rate to keep the engine running, that would be another matter, but you haven't indicated that. You have indicated otherwise as per your mention of the shifting function appearing to be normal. However, it might be a good idea to check the compression and spark to eliminate that possibility.

Re: Bad Rectifier and Tach response

Joe Reeves said:

On a flushette (garden hose), the rpm should be set to 1000 rpms. The rpms would drop to normal when the rig is in the water.

The sensor terminal of the tachometer, either a terminal marked "S" or a "Gray" wire leading from the tachometer, should be attached to the Yellow/Gray lead of the rectifier OR if both yellow leads of the rectifier are just plain Yellow, attached to the Yellow wire that is attached to the stator Yellow/Gray wire. If there is no Yellow/Gray wire on either component, the Gray wire can be attached to either Yellow wire.

Initially you indicated that the tachometer was reading 650-700 rpm when in the water BUT the tachomer shouldn't have read at all with a faulty rectifier. Perhaps I am reading something wrong there...... and you meant that in the past, the 650-700 was the reading with a previous good rectifier.

If all is as it should be (wiring, setting, etc), I cannot imagine why you would obtain a double reading UNLESS the tachometer was damaged internally. If possible, temorarily connect a known good tachometer to compare.

If the compression, ignition, or fuel system was flawed whereas the rpms had to be at a higher rate to keep the engine running, that would be another matter, but you haven't indicated that. You have indicated otherwise as per your mention of the shifting function appearing to be normal. However, it might be a good idea to check the compression and spark to eliminate that possibility.

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Sorry I left out so much info in my description. I was running my motor submerged up to the cav plate in a tub of water. I think I blew out my rectifier last year but do not know for sure. All I can say is that the tach was operating and howing an idle in gear at 650-700 and a WOT over 4000 the last time I had my boat in the water. I do not recall why I decided to check my rectifier this weekend as I really did not suspect it was bad up until that point. It could have been that I saw a post over the weekend about something unrelated (I'm working on timing issues) and decided to check my rectifier.

I agree that it seems odd that I was getting any signal from the rectifier. But then when I look at my testing results, I did see that one diode was working and the other three were open. It is possible that this diode was between ground and the Y/G (I think it was this path, but did not write down my test results, once I saw everything was coming back bad). So does that mean it was possible for the tach to see half the signal from the stator?

Compression is great (150+/-), spark was good last time I checked ,and the carbs were just rebuilt by a "pro". I have already discussed the timing issues with you in PM's and that is what I was working on as I figured I finally got everything else in order.

Re: Bad Rectifier and Tach response

Randyg123 said:

I agree that it seems odd that I was getting any signal from the rectifier. But then when I look at my testing results, I did see that one diode was working and the other three were open. It is possible that this diode was between ground and the Y/G (I think it was this path, but did not write down my test results, once I saw everything was coming back bad). So does that mean it was possible for the tach to see half the signal from the stator?

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I found that the rectifier on my 78 115 had all 4 diodes open and the tach worked only between 1500 and 3000 rpm. weird.

maybe someone can chime in a tell us how a boat tach really works. do they actually count half rectified sine pulses or integrate to DC a voltage? what does the switch really do, and why '6' for a 4 cy 2 stroke?

Re: Bad Rectifier and Tach response

77_starcraft said:

I found that the rectifier on my 78 115 had all 4 diodes open and the tach worked only between 1500 and 3000 rpm. weird.

maybe someone can chime in a tell us how a boat tach really works. do they actually count half rectified sine pulses or integrate to DC a voltage? what does the switch really do, and why '6' for a 4 cy 2 stroke?

Click to expand...

I know the stator has 12 poles on this era of Johnsons and that is why the number six is used. I suspect the number of poles is more related to the size of each coil and the amount of space along the circumference of the flywheel. Among many other things. But I am sure I am probably wrong. Can't wait to see this one argued out.