#1  
Old 10-19-2020, 02:11 PM
masterhussey masterhussey is offline
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Default Yep, still can't find the solution...

I did search the forum and didn't see anything.
1974 SD. that runs very well. Rev's nicely, def has power. Its a single points distributor factory. yes I know i could just put a new one in or convert it to the petronix (going to do the petronix later, I just want the tach to work correctly first)
Need some help: My tach is very jumpy not the motor, motor sounds amazing, idols perfect. So here are the things I found wrong, and tried.
1. I found the lead from the distributor cut and spliced back together. It was done terribly. So i replaced the lead with a new one.
2. I disconnected the factory tach and hooked up a different tach and it is the same issue. I even fully disconnected the factory tach when testing the aftermarket one so that eliminates the actual gauge.
3. It is a new coil, I tried a different coil and it was the same thing.
4. I'm going to replace the condenser tomorrow so i'm not sure that will fix it.
5. I did have a loose ground from the head to the firewall so i cleaned and tightened that up.
6. the points, cap & rotor appear to be in good shape.
I know its probably something stupid so what am I missing? i'm starting to get stumped. so all ideas welcome. I would like to keep the original one in there.

  #2  
Old 10-19-2020, 03:13 PM
"QUICK-SILVER" "QUICK-SILVER" is online now
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Bad condenser will make a tach needle jump around. Although, most everytime I've seen it happen the engine is acting up from misfire.

Has your coil got a radio capacitor hooked to the (+) terminal? Just make sure it's not hooked up to coil (-). That could possibly make a tach act up.

Clay

  #3  
Old 10-19-2020, 03:54 PM
masterhussey masterhussey is offline
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The Radio Capacitor is on the negative side of the coil. is it supposed to be on the positive side?

  #4  
Old 10-19-2020, 03:58 PM
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Yes it goes on the (+) side.

On the (-) side it would be like trying to run with two condensers.

Why I asked about it
Clay

  #5  
Old 10-19-2020, 04:24 PM
masterhussey masterhussey is offline
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I will be doing this first thing when I get home tonight or tomorrow morning when I get up in the morning. thank you and I will keep you posted

  #6  
Old 10-19-2020, 05:42 PM
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george kujanski george kujanski is offline
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Yep, also the tach wire goes to the (-) terminal.

George

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  #7  
Old 10-19-2020, 05:45 PM
masterhussey masterhussey is offline
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yes sir, I got that one in the correct spot.

  #8  
Old 10-20-2020, 03:38 PM
masterhussey masterhussey is offline
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Update: I disconnected the factory tach and temporary hooked up an aftermarket tach; I disconnected the radio capacitor (radio suppressor as per the service manual) which was connected to the negative (-) side of the coil. The tach seemed much much better but was still a bit jumpy. I hooked back up the radio capacitor to the correct side of the coil which should be the positive side (+) Thank you Quick-Silver for that advice. The result of that was back to the tach being very bouncy. So I started back at square one only using the factory tach with the same results. So as of right now It appears that my factory Tach works better without the radio capacitor hooked up. My next step is to check the condenser inside the distributor and to check some of the grounds. after that i'm all out of ideas.

  #9  
Old 12-16-2020, 07:23 PM
masterhussey masterhussey is offline
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Default Update: Moving closer.... still not right yet.

So here is a quick update. I replaced the points and condenser. The one on it was a one piece condenser/points so it was replaced with the correct 2 piece. voltage coming to the coil is about 10 volts not sure if its too much or not enough. Next up is to see if there is a way to test the ignition switch on the column to see if its putting out the correct voltage so if you know how to do that please let me know as I do not and will be researching this. Thank you all for the help and guidance.

  #10  
Old 12-16-2020, 07:49 PM
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Bill Hanlon Bill Hanlon is offline
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To determine "not sure if it is too much or not enough" check voltage at the - side of the coil with the points closed, the key on, but the engine not running. Record the voltage. Now open the points using a plastic tool and record the voltage again. What are the two voltages?

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  #11  
Old 12-16-2020, 09:22 PM
alleyCat alleyCat is offline
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I have exactly the same thing going on except I have Pertronix. I talked to their tech today and they recommend to try a 10k, 15k or 20k ohm 1/4 watt resistor placed in the tach signal wire. I have them on the way to try. Not sure if this pertains to the stock pounts setup but if it works I will let you know. BTW you probably have a resistance wire from the ignition switch to the coil which will affaect voltage to the coil when in the “run” position to help keep the points from burning . This wire is bypassed to give full voltage when the ign switch is in the “start” position to allow full voltage during start up. I have been reading for days trying to figure out how to stop the bounce ! If it works I will let you know

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Old 12-20-2020, 01:23 AM
alleyCat alleyCat is offline
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After going nuts with a bouncing tach I got it done today. I used a 10k ohm 1/4 amp resistor in the tach signal line and that took out all except a 100 rpm bounce. I was going to leave it at that but I had a few old coils on the bench so I tried one. Boom....zero bounce and all is right with the world. I may try taking out the resistor but rather leave it. My luck it will start bouncing again.

  #13  
Old 05-10-2021, 07:38 PM
masterhussey masterhussey is offline
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So as you can read, I have done a ton of diagnostic for the tach and ended up replacing the Tach. (don't you worry all your purist I'll be sending my original one out to get restored/repaired) I replaced it with an OER. The Tach works perfect and even the clock works great.

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