Tach Reading Faster Than Tach/dwell Meter?!

[motivealloy]

Okay here is a wacky tach issue. Not really a must fix issue bit annoying non-the-less.

 

So when I hook up my tach/dwell meter when tuning up our 1970 L24 engine I've noticed that the tach in the dash is reading much faster than my tach/dwell meter hooked directly to the coil. We have stock coil and vac adv dist. I don't know if the tach is original with the car. I assume it is, but we haven't torn into the dash yet.

 

Here is the issue:

When my tach/dwell meter reads 700rpm the tach in the dash is reading about 1100rpm. When I slow the rpm down to say about 500 rpm the dash tach reads about 850rpm. When the engine rpm goes up to about 3000rpm the inside tach reads closer to 3200rpm-ish. Could it be that the inside dash tach is out of calibration some how? 

 

Has anyone else seen this issue with their dash tach? Get a new (used) one?

[240260280z]

Same issue that Johnc and I discussed a few days ago. Have a look at the post with all the meter photographs here:

 

http://www.thesamba.com/vw/forum/viewtopic.php?t=508798

[Zed Head]

My 76 tach shows a steady 200 RPM too high, when compared to my meter.  It's a Fluke 115, I use DC hertz to measure, then do some math.  I've had my tach dash jump around and misbehave with a new ignition module,even though the engine ran fine.  An extra condenser on the negative wire brought it back to life.  The tachs seem sensitive to noise.  They don't need much "signal" to work, there's actually a 2,300 ohm resistor inline with the coil, on the 76.  Your 240Z tach uses the positive side though but may still be sensitive.  Maybe a new condenser or an extra on the circuit would help damp some noise.

[motivealloy]

Thanks Zed and Blue,

 

I'll try a new condenser. We haven't changed that out yet - could be a culprit. It's pretty grungy so swapping for a new one wouldn't be a bad idea.

 

I did notice that a lot of the blade connector leads on the wires in the engine bay are pretty brown with oxidation. i'll go through and spritz them with Lime-a-way to get back to clean brass again. This should cut some of the resistance from the wire leads as well.

 

Definitely sure my tach/dwell meter isn't broken. It registers RPM correctly on my V6 Mustang and 4cyl Honda. So i knew that wasn't the case.

[240260280z]

There is a calibration adjustment on the tachs.

[djwarner]

My on board tach reads a consistent 200 RPM high. This is compared to a top level Sears timing light and a CenTech automotive meter from Harbor Freight.

 

A post on another thread stated the CenTech unit inductive pick-off was useless. This is not true but it is used differently than those used on a timing light. The CenTech inductive probe is placed on the HT lead between the coil and the distributor. You then select the number of cylinders on the dial and the readout is the RPM/10. Thus, when my idle is set on 800 RPM, the coil fires 3x800/60 seconds or 40 pulses/second. With the Dial set on 6 Cylinders, the microprocessor in the meter converts it to 80 (800RPM/10).

 

I imagine there is a calibration adjustment on the back of the tach, but I have yet to see a procedure on calibration.

[Zed Head]

I am fairly certain that I tried to turn those adjustment screws and they wouldn't budge.  I've also tried to turn the adjustment screws in my oil pressure gauge and they wouldn't move either.  The only adjustment I've been able to make on any gauge is on my voltmeter.  I wonder if Nissan didn't use some kind of threadlocker on them, since they should only need adjusting one time.  Maybe a drop of acetone or MEK on the screw would break them free, or some heat from a soldering iron.  A future project maybe, for a spare meter.

[beermanpete]

As Blue said, the factory tachs have an adjustment. It is a trimmer on the back of the tach. Use a thin flatblade screwdrive to make the adjustment. You might need to remove the tach to get good access. The wires are long enough to run the car with the tach loose. Use a known good service tach and adjust the factory tach to match.

[djwarner]

I have an upcoming project that may require a tach and speedo removal. Will have to attempt a calibration at that time.

As for frozen calibration controls, I also had a problem adjusting the voltage regulator on the oil/temp gauge. It was a matter of 40 years of corrosion at the pivot point. Once broken free, the control moved as expected.

[240260280z]

Having problems putting images in line???

 

Below is a 280z tach module on left and 240z tach module on the right.

 

The adjuster is at ~ 12:30 "o'clock".

 

You can access it from the back of the tach's "can" enclosure.  The access hole is between the two lamps.

[240260280z]

It looks like the top assembly that holds the needle bearing has drop tabs connected to springs may be "user-rotatable" too.  I have never played with it so I can't say for sure.

[motivealloy]

Okay, so the adjuster on the 280 tach is the aluminium thing with the slot that has 30K stamped on it, and the 240 is the brass nut.