How to test/replace/repair ammeter gauge in 1970 E-model?

[AndreiC]

I don't think the ammeter gauge is showing anything on my 1970 M20E. It never moves at all, even if I turn on the pitot heat without the engine running, which should be drawing a significant charge. I suspect it is the gauge itself, not the wiring. 

Can the gauge in the six-gauge linear cluster be replaced alone? Or are there whole clusters available for sale? Or does anyone refurbish these? 

The gauge is not needed for flight, and my EDM-700 shows me voltage, so I have pretty much the information needed about the electrical system, but it would be nice to not have an inop gauge in the panel.

[Andy95W]

It is probably not your instrument cluster.  There are 2 fuses between the shunt and the gauge.  They don’t normally blow, but the wires that attach to them often break.

[FlyingDude]

28 minutes ago, AndreiC said:

Can the gauge in the six-gauge linear cluster be replaced alone?

Can you post a picture?

[AndreiC]

1 hour ago, FlyingDude said:

Can you post a picture?

This is a pretty bad picture but it shows the instrument pack at the top.

 

[PeteMc]

It could also be a bad ground.

 

[N201MKTurbo]

5 hours ago, PeteMc said:

It could also be a bad ground.

 

I don’t think anything is grounded in the ammeter circuit. It is just a floating voltmeter across the shunt.

[AndreiC]

1 hour ago, N201MKTurbo said:

I don’t think anything is grounded in the ammeter circuit. It is just a floating voltmeter across the shunt.

This is correct. Do you happen to know what voltage range is it supposed to measure? I was going to try to hook up a 1.5v battery to it and see if it moves (after disconnecting one lead from it) but I don't want to fry it if it is a super-sensitive voltmeter.

[PT20J]

14 minutes ago, AndreiC said:

This is correct. Do you happen to know what voltage range is it supposed to measure? I was going to try to hook up a 1.5v battery to it and see if it moves (after disconnecting one lead from it) but I don't want to fry it if it is a super-sensitive voltmeter.

Many shunts are 50 mV at full scale current. 

[N201MKTurbo]

If you just use your ohmmeter on a low range, it should make the meter move.

[AndreiC]

11 minutes ago, N201MKTurbo said:

If you just use your ohmmeter on a low range, it should make the meter move.

I thought so, and I did try this a few days ago with one lead disconnected. It did not budge with my ohmmeter at any range. So my guess is the meter is toast. (Or maybe just jammed? Can this happen?)

[FlyingDude]

8 hours ago, PeteMc said:

It could also be a bad ground.

 

Ammeter is not grounded.

[N201MKTurbo]

8 minutes ago, AndreiC said:

I thought so, and I did try this a few days ago with one lead disconnected. It did not budge with my ohmmeter at any range. So my guess is the meter is toast. (Or maybe just jammed? Can this happen?)

It can. I’ve seen meter movements that had tape around the windings. The adhesive on the tape failed and the tape lifted and jammed the movement.

if you can get the movement out of the plane, rocking it back and forth in your hand will usually move the needle. You can try to manually move it with a toothpick. I use a toothpick because there is less chance of chipping the paint off of the needle or the face.

[Yetti]

    I believe Rochester is the manufacture of those.  It's probably the Black inline fuse holder up near the shunt on the passenger side firewall.

[PeteMc]

4 hours ago, N201MKTurbo said:

I don’t think anything is grounded in the ammeter circuit. It is just a floating voltmeter across the shunt.

@N201MKTurbo @FlyingDude   Okay, I'll have to dig up the schematic.  For some reason I thought those gauges did have a ground, but makes more sense that it doesn't. 

 

Edited Thursday at 05:09 PM by PeteMc

[DonMuncy]

My guess is most likely one of the fuses. Mine has two. Very different fuses. Located near the ammeter. Little short stubby ceramic cylinders. If you are diligent, you can find replacements at quite reasonable prices.

[cliffy]

Most of them are regular BUSS glass fuses about 1/4 inch in diameter  Available everywhere

Many times it has been so many years (40+?) that they have never been removed they will be all crudded up and have bad connections

Be prepared to clean the holder contacts and the fuse ends when you take them out. The wires may be very brittle.

Bonus question- 

Regs state that any airplane that has fuses installed should carry spare ones for replacement

Does this apply to these fuses?  :-)