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PaulTWinterton

Why Is My Tach Oscillating?

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For the last few months my tach needle has been erratic. Not dropping below actual rpm, but jumping higher.  Much higher.

 

Firstly, this is a 100% stock tii with near new coil, wires, condenser, plugs.  I have the Diagnostic Tool from Pat Allen as well, and it reads the same erratic fluctuation, like the tachometer.

 

My first thought is that it's a surge in current, but I'm not experienced enough to determine where more current would be generated.

 

It seems especially bad when the engine is cold, or when the actual rpms are 2500+.  It's less erratic when the engine is warm.

 

I've checked grounds and have added a ground to the back of the instrument cluster.  Unless I've missed a ground somewhere I'm at a loss.

 

Condenser, distributor, coil, short?

 

Any experience with this glitch or engine electrical knowledge that could cause this would be appreciated.

 

Thanks for reading and replying.

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Be sure the spade terminal at the coil or dizzy is tight, if loose it isn't making a good contact. Same thing happens with water temp sensor on the water divider.

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Be sure the spade terminal at the coil or dizzy is tight, if loose it isn't making a good contact. Same thing happens with water temp sensor on the water divider.

+1

 

The additional symptom that you have also points to the possibility that this is the cause.  As the engine (and engine compartment) warms up, it's possible that the male spade is expanding, as well as the curled edges of the female spade, thereby making better contact and stopping the oscillation.  Just a possibility.

 

Regards, Maurice.

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After you've eliminated loose/dirty/corroded terminals between tach and dizzy...

 

From the old BMW CCA Tech Tips book--and actual experience:  presuming you're still running actual points in your distributor...a worn point plate--the thing that the mechanical and/or vacuum advance unit moves and that holds your point set--will cause tach fluctuations.  That part is hardly ever replaced, but it does wear.  It was once available as a spare part both from Bosch and BMW; not sure if it still is.  

 

 

I had a flickering tach on my '69 many years ago, and replacing the point plate solved the problem.  And that one's still going strong after 25 or so years.

 

mike

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I checked all my connectors and they are super clean and tight. Water temp sensor connection is good as well.

Mike, I was hoping for some experience with this exact issue. Thanks. "Tech Tips Book"! Cool.

I have been suspicious of the distributor and was thinking that the condenser (or connection) was faulty. Or the last guy that was in there (me) screwed something up.

Diagram item #1?

post-38825-0-05221800-1395277196_thumb.j

Any chance that plate is just loose?

I'll jump in there soon.

Cheers.

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Yep, it's item #1.  When you bought the part from Bosch, it usually came with a new set of points.  

 

Easy to check if it's loose as it's held in place with two or three small screws.  

 

The plate is in two parts:  the lower piece is fastened to the distributor housing, while the upper piece (that holds the points) is free to rotate, fastened to the centrifugal weights linkage and (f your dizzy has a vacuum advance unit) the vacuum advance.  When the grooves in which the two parts move get worn, it'll cause the points to move slightly, thus affecting when they open, and the signal they send to the tach.  

 

One more thought...those tachs, while very reliable, do go bad...

 

Happy troubleshooting...let us know whatcha find.

 

cheers

mike

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When I was running points, tach jumping like that usually meant that something was wrong with the points-

 

gap closed down, once I had a spring break, or the rubbing block wore way down.

 

On a distributor with a lot of radial play, it seemed to happen a lot quicker.

 

Maybe it's that easy?

 

t

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To recap: this morning the car was running fine, but the tach was oscillating upwards.  I removed the cap and inspected the plate.  It's tight.  I was hoping for a loose screw. 

 

While I was looking I noticed the spade connector inside the dizzy was loose.  I cleaned and tightened the fit.

 

I decided to clean the electrodes and the rotor.  I used emery cloth and compressed air to rid the cap of dust.

 

I re-assembled the dizzy and THE CAR WOULDN'T START!!!  Not even fire.

 

I re-opened the dizzy and cleaned the electrodes a little more and tried again.  Car would start but run on 3 cylinders.  Cylinder #1 was not firing.

 

More cleaning and air and the car will run smooth for a minute and stall out.  Like it would stop firing on one cylinder and then 2 cylinders.  Then quit.

 

Good news - Bad news.

 

The good news is that the tach is steady when its running.

 

The bad news is that I can't get the car to idle reliably. My only guess is that there is carbon debris inside the dizzy that is causing arcing. Everything was working fine before I cleaned the electrodes and rotor.

 

Thoughts?

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Put the dizzy cap on backwards? Wrong plug order?

 

I wish it was that easy. 

 

I borrowed a cap and rotor from a buddy and tried that.  No firing at all.

 

Therefore it must be the points, wire connection, spade connector, or condenser.  Remember, just 2 days ago it was running fine until I went into the dizzy.

 

I'm going to change out points and condenser and try again.

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It was right in front of my face, and I still spent hours on the problem.  Live and learn, and re-learn, and re-learn.

 

 

When I was running points, tach jumping like that usually meant that something was wrong with the points- gap closed down, once I had a spring break, or the rubbing block wore way down. On a distributor with a lot of radial play, it seemed to happen a lot quicker. Maybe it's that easy? t

 

Absolutely correct.  Pivot - Running Block.

 

post-38825-0-87996900-1395524002_thumb.j

 

New points in, dwell at 60, tach is back,runs smooth.

 

Thanks everyone for all the valuable info.

 

Cheers.

 

 

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