930 tach false reading with MSD

[Pesch]

Hello all,

I've got a problem with the tach of my 87 930 using MSD 6AL CDI.
It reads quite consistently a wrong low value for the RPM.
For instance at around 6000 RPM, it reads 5000. Idle reads 600 for 900.

I've read many topics about MSD 8920 tach adapter and none of them really describes the same problem I'm facing. Some had erratic readings some not working but none had wrong readings.

Does any of you faced the same issue?
Would you think that the MSD 8920 can be the cure to my problem?

Thanks
Arnaud

[flightlead404]

Do you own an oscilloscope or can you build one with a Pi https://bitscope.com/pi/? or https://www.raspberrypi.org/blog/build-oscilloscope-raspberry-pi-arduino/

It would go a long way to figuring out the problem.

See also:


* Tachometer Compatibility Chart
* HELP! tach died and I need to know what can replace it!


Alternately you could simply start rebuilding or swapping parts at random and see if the problem goes away. Start with the a tach rebuild.

[Pesch]

Thanks, I'll get my oscilloscope back soon and do some more measurements.

I've checked again my MSD, it is properly set with one cylinder selection loop cut and the rev limiter set to 6500.

Checking the documentation, I found a nice feature of the digital 6AL, is the Rev limiter verification.
At each "key-on", it will generate a frequency to display the set rev limit on the tach.

Here below is the video I took of this feature on my car:

https://www.youtube.com/watch?v=CRMfknLT3eU

Seems that the needle sweep to 6500 then stabilize at 5100.
I guess then that there's something wrong inside the tach as my innovate data logger reads properly the RPMs.

Time to open the tach for visual inspection for any burnt component.

[Pesch]

Quick update, looks like the culprit is the tach, here’s a video taken with another one:
https://youtu.be/3Nu4gE6aLI8

Time to open up the original one...

[Pesch]

Diagnostic update:
I should have open my eyes before:

It was written quite obviously 8cyl on it as confirmed by the inside...

How in the hell a modified 930 tach (with te right date) ended up in my car...

Hope the little adjustable resistor is the key to adjust the RPM display.

[flightlead404]

Can you configure your oscope to output a wave pattern?

If not, a 555 circuit or something similar switching 12v through a transistor to generate a 12v square wave. Pretty sure its once per ignition event, or 3 times per rev, so a frequency of 150 Hz should show 3,000 rpm.

[Pesch]

Hello all !
I finally made some progress on this issue
Here’s the little test rig I set to calibrate the tach and the final result:







I used a 3€ square wave generator, a salvaged 12v power supply and a USB oscilloscope I bought many years ago...

Funny to observe that the response of the tach is not perfectly linear, when 1000 and 6000rpm are aligned, 2, 3, 4 and 5000rpm aren’t....

[ELZ's930]

That's interesting. My tach in my 84 930 reads consistently low. Same MSD key on feature sends it to about the same low reading as yours. Maybe I have an 8 cylinder tach too.....but I doubt it.

[Pesch]

The setting is made by the little adjustable resistor we can see on some of my previous pics.
You have 2 options to do the setting:
- use the key on feature of you MSD
- invest in a 3$ square wave generator, the one I have is accurate, the oscilloscope isn’t needed.

To make things easy, I made a quick research on Amazon US, here’s a link to the same unit I bought:
https://www.amazon.com/uqiangbao-Adjustable-Rectangular-Electronics-Accessories/dp/B07PHB9VNY/ref=mp_s_a_1_9?keywords=square+wave+generator+pwm&qid=1573766458&sr=8-9

Shipping from China wasn’t very fast but it arrived anyway...

[ELZ's930]

I'm down for adjusting the tach myself. So...just turn it until the correct value is reached in terms of the MSD, seems logical. I did have the face changed to 10K?

and thank you very much for the link! How nice!!

Leonard