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Tach Not Reading Below ~1200 rpms


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Hi All,

 

New to this forum and I have been searching for a couple of hours to no avail. All I found was someone had an erratic tach so they added a capacitor to the (-) terminal of the ignition coil and it made it work just fine but I am not sure if this would help me.

 

My original tach was stuck at like 2700 rpms so I replaced it with another original tach. Now, it reads just fine unless it's under 1200 rpms. Under that, it reads at 0. When the engine is cranking at startup, it does flutter under 1000rpms like it wants to read but then drops to 0 when the car fires on. I know it gets the signal from the ignition coil (-) terminal so my question is, is it not getting enough voltage from the ignition coil under a certain rpm to get the tach going? 

 

i did disassemble the rear of tach when I received it to remove the green light bulb filters (I like the white/yellow color better). Did I add nuts in the wrong order making a bad connection on the two little prongs on the back (they have tiny nuts on them)?

 

I am referring to 1 and 2 in this image: http://www.gwfweb.com/datsun/images/tach.jpg . If anyone has a better picture of that setup that might help. Thanks!

 

Car:

1978 Datsun 280Z 2+2

Edited by 78-280Z
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Edited... oops I typed that in a hurry. 

 

Yes, according to the seller it did. This honestly is not a huge concern to me except for the fact that I would like to check my idle speed (it idles a little rough) and verify that the idle screw is set correctly and then check my timing. However, I can't do this if I don't know what my idle rpm is. I'd prefer to not have to go buy an aftermarket gage to test this out. 

 

I have a new distributor cap, rotor, AFM intake boot and spark plugs on their way tomorrow. I am also hoping this helps with the idle. I also plan on changing every vacuum line in the engine bay to avoid having vacuum leaks be an issue. I have only had the car for about a week. 

Edited by 78-280Z
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If you have a decent meter that measures Hertz/frequency, you can get a number from your coil terminals and do some math.

 

Or just drop idle speed way down for setting timing.  Once you're below 1200 the mechanical advance mechanism should be fully retracted.  You could also watch the timing while you drop idle speed, until it stops retarding.

 

Many ways around it.  In the long run that "0" is going to bother you though.

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Can you elaborate on what I would be measuring at the ignition coil? I'm not too bad at the math part :) although electrical things are not my strongsuit. 

 

That's a good idea, I imagine it doesn't start advancing timing until higher up in the rpm range. I plan on setting the timing tomorrow once my parts are installed and I have replaced all the vacuum lines. I don't want to adjust it to an inaccurate idle speed caused by a potential vacuum leak. 

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You'd be measuring coil discharges.  Voltage changes that your meter will count.  In events per second, aka hertz.

 

Hertz will not directly equal RPM. Assuming the frequency meter reads each plug firing event as one cycle you can calculate RPM from the frequency (Hz) using this formula: Hz x 20 = RPM.

 

Hz      RPM

50     1000

75     1500

100   2000

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I measured  the frequency at the ignition coil yesterday across both terminals and got .46Khz = 460Hz. This number doesn't seem to make much sense using the formula above. Unless it was .046Khz... I'll have to check again when I get home. It was almost midnight when I was doing this last night. 

 

For those that see this later and dont understand the equation it's:

Hz x 60 to go to per minute x 2 (assuming you're firing every other revolution), dividing by 6 (cylinders) which would give you the RPM. 

 

[Frequency (Hz) x 60 (sec) x 2 (Firing every other revolution)] / 6 (cylinders) yields the equation of: Hz x 20 = RPM (for a 6-cylinder 4-stroke engine)

Edited by 78-280Z
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You might have a noisy ignition circuit, on your Hz reading.  Could be  sign of a failing ignition module.  Or you used AC instead of DC, or need a better meter.  Who knows.

 

People have reported sticky needles causing tach problems.  Could be when you had it apart you torqued something.  Maybe take it apart again again and twist things around a little until the needle moves more freely.  Could even be a piece of goo on the needle stop.  Or there's a zero adjustment that needs a turn, or got turned.  The other gauges have some adjustment screws on the back, some are usable and some are glued once set.  Worth another look.

 

Apparently the voltage tachs work like volt meters, averaging each pulse over time.  Maybe you've got a an offsetting ground loop on the tach ground.  That's just a WAG.  You could jump the terminals while it's out and see if it you can make it work right.  The diagram is in the FSM.

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If you have an oscilloscope use that to view the waveform at the coil. The period will give you the RPM indirectly. 1/T = Hz. Apply the formula above.

 

Have you check the resistor that is in-line with the signal to the tach? There is a 2.2k Ohm resistor buried in the wire harness in the passenger foot well area in series with the signal wire to the tach. It looks like an in-line fuse holder.

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  • 4 weeks later...

So just ran accross your thread. I have a '77. I did the 280zx alternator upgrade with no complications a year ago. A couple of weeks ago I Did the 280zx dizzy upgrade and am now experiencing the same problem you are having with your tach. Last weekend the 280zx alternator fried (internal voltage regulator failed). Well the minute it fried my tacH worked perfect.... Replaced alternator and tact is back to not working under 1200 RPMS. I haven't figured out what the problem is but it's obviously related to to much voltage coming from the signal wire. So an inline restistor in theory should fix the issue.

 

Hope this helps. Let me know if you figured out the problem on your end, I'm curious

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Actually, it might be voltage quality, not quantity, with the alternator clue.  An alternator is an inherently noisy electrical source.  Condensers and resistors can both suppress noise.

 

Not an electronics expert.  

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  • 2 weeks later...

I had the "Original" ignition coil. Or so I think. I switched to the Pertronix Flame-Thrower 3ohm for my 78 280Z. They say if you don't have the ballast resistor, to go with 3ohm. If you have it, go with the 1.5ohm ignition coil. The 78 does not have one.

 

See below:13524289_2933839582585_78925399952684432

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