RPM Gauge died

[Wingover]

I noticed that my RPM gauge started to "fog" (see picture) on my last two flights, something it never did before.  Today after 5 minutes of flying is stopped working but sometimes you could see the needle come back to life for a second.  I use the JPI 830 and backup RPM.  Plane is 1987 M20J 205.

Before starting to troubleshoot, is this a mechanical or electrical sensor?  Also, anyone else had this issue and can say what it is?

[toto]

It’s electronic. My ac had the same tach, and I ended up replacing it with an EI R-1 after wrestling with problems for a while. 

I believe the factory tach used a tach generator, and the R-1 just uses the p-lead. 

The fogging is pretty strange - mine was just intermittently erratic, but no fogging

Hope others will have ideas about the fog ..

 

[toto]

By the way - here’s the R-1 tach. This was a pretty easy replacement for the factory tach. 

I don’t really like the look of the R-1 (someone here on MS said it looked like a pinball machine), but honestly I thought it looked pretty nice when installed. 

I used the R-1 for less than a year before replacing it with a G3X+EIS, so I don’t know about longevity, but I had zero problems with it. I wish I’d done it sooner. 

https://iflyei.com/product/r-1-rpm-tachometer-instrument/

[PT20J]

If you want yo replace it, Skyman Avionics has the one I removed for sale.

info@skymanavionics.com

[Wingover]

2 minutes ago, PT20J said:

If you want yo replace it, Skyman Avionics has the one I removed for sale.

info@skymanavionics.com

I saw it on their website.  The issue is that I don't know if it is the gauge that is bad or something else.

[PT20J]

Just now, Wingover said:

I saw it on their website.  The issue is that I don't know if it is the gauge that is bad or something else.

It’s either the gauge, the tach generator, or the wiring. I’d check the wiring first. If that isn’t the issue then I’d replace the gauge because it’s fogged anyway. If that doesn’t do it, then that leaves the tach generator. This tach also drives the hour meter if you are still using that. 

[Wingover]

I will test it all next week when I get back to the hangar. It is not foggy all the time, it starts to fog during flight only, fine on the ground and only started doing that a week ago.

[Aerodon]

56 minutes ago, PT20J said:

It’s either the gauge, the tach generator, or the wiring. I’d check the wiring first. If that isn’t the issue then I’d replace the gauge because it’s fogged anyway. If that doesn’t do it, then that leaves the tach generator. This tach also drives the hour meter if you are still using that. 

Do you remember what the tach generator looks like?  And where it was attached?  Don

[Aerodon]

I have a spare one out of a 252 with the same markings for you.....Should have the tach generator too, but will have to search.

Aerodon

[PT20J]

1 hour ago, Aerodon said:

Do you remember what the tach generator looks like?  And where it was attached?  Don

[Aerodon]

My electrical drawing shows a sensor attached to the left magneto (1986 M20K).  I will go look today.

Aerodon

[PT20J]

2 minutes ago, Aerodon said:

My electrical drawing shows a sensor attached to the left magneto (1986 M20K).  I will go look today.

Aerodon

Forgot to mention it is attached to the accessory case where the mechanical tach cable would attach.

[Aerodon]

So my 1986 M20K has the RPM sensor on the left magneto, inboard side.  Still installed, but I can remove it.  Then the matching tachometer.

No tacho-generator.

Aerodon

[moosebreath]

I just despised the R-1 that the previous owner installed. It looks like a cheap pinball machine.  A dim low tech lcd display is a horrible replacement for a traditional tach.  UMA makes a certified 2” tach with an electronic sensor for a reasonable price. 

[moosebreath]

My UMA sensor goes to the mag also.  No idea if it is compatible with yours though.

[Aerodon]

Did you resolve your problem?  

 

 

 

[Wingover]

2 hours ago, Aerodon said:

Did you resolve your problem?  

Not yet.  Did have a free hour so went flying and the gauge worked fine for the first 5 minutes.  It started fogging after and stopped working so my guess is it is heating up enough to cause the fogging.  I am going again tomorrow to try and see where the wires are going to (i.e. magneto P lead or have a tach generator).

I will PM you.

 

 

 

[N201MKTurbo]

2 hours ago, Wingover said:

 

I think I have one of those tach's in the hangar.

[Wingover]

So finally had time to look at the setup.  I also went flying twice and the gauge doesn't fog anymore but also works for two minutes and dies.

I don't know how to tell if it is the generator or the gauge 

Here are pictures of the location: