Tachometer reading erratic

[dmcnicholl]

My aeroplane has a Westach tachometer of the type which picks up an electrical impulse from the P-lead. Not sure of the exact model number but it's at least similar to the WS3ATU32, 3-1/8" diameter with a 240° needle sweep.

It has been in the panel for nearly a couple of decades but lately it's reading has become intermittently erratic. Sometimes in flight the needle will start ranging down the scale, sometimes it fluctuates, sometimes it just drops to indicate maybe 70-75% of the RPM.

The engine runs fine, the magneto, harness and plugs are in good order, but can anyone suggest what might be causing this behaviour? Might it be the P-lead deteriorating?

[C Rule]

Donald
I had a similar problem on an 0-320 powered RV6. The Westac Tacho started over reading by a non linear quantity varying between 300 and 600rpm. we checked all the connections to no avail. The only solution was a new Tacho from Westac via LAS/Aircraft Spruce a ten minute job to change and re-calibrate with a strobe.
The only problem was the supply chain took 10 weeks to deliver.
Cheers
Colin

[dmcnicholl]

Thanks for the reply, Colin. I did consider that it might just be that the instrument is at the end of its life but thought I'd investigate other possible causes and try to pick the brains of others. Fortunately Westach still sell the same model, or at any rate Aircraft Spruce still list it.

I know the thing is easy to swap out, but calibrate? I'm pretty sure there is no means to do that on a WS3ATU32 and there is nothing in the instruction sheet to say that it can, or how. Does your's rely on a sender or is it the simple kind, like mine, that connects to the P-lead at the switch?

10 weeks? Blimey, you'd think FEDEX or UPS could do better than that.

Aye,
Donald

[mikehallam]

Donald,

It might not help, but after years of running well & out of the blue, mid summer my Tacho started reading oddly and erratically far too high at the top end rev's.
I too looked at the £100 cost of a new one for my Rotax 912.

Then googled for ideas, and found a[u] dodgy earth was one possibility.[/u]
Removed gauge, cleaned the three feed leads socket ends to it & the three pins & in hope reconnected - not knowing a) if it was and Earth problem & b) which of many earth points !

I was lucky that time & it's been good ever since.

FWIW, mike hallam.

[dmcnicholl]

Mike,

Thank you for your reply too. Before I do commit to a new gauge, and I will if I have to, I'll have a look at my P-lead. The switch connection appears to be sound but it's rather more fiddly to get into the mag end, so I have not yet investigated, but I think that's the next item on my agenda.

It's hard to understand though what could be going wrong there. The switch kills the mag, no problem, but perhaps if the contact is getting slightly worn that would be enough to give the erratic behaviour while remaining good enough to ground the primary. Time to get the screwdrivers and spanners out, I think.

Aye
Donald

[C Rule]

When I said recalibrate I perhaps should have said Check the calibration.
As you say there is no adjustment on the instrument.
As has been said before worth checking all the connections for any corrosion as that will act as a diode and can effect the pulse counting.

[dmcnicholl]

Thanks again, Colin, but I think I may have discovered the cause.

Disconnected and removed my P-lead today. Continuity seems fine for both the core and the screen connections but the little jumpers from the switch to the tacho look highly suspect.

Being a smarty-pants, when I fitted that tacho, replacing an earlier 2-1/4" 90° scale Westach with the 3-1/16" 240° scale instrument I used that 'approved' single core stuff with the thin white insulation. Those jumpers and the P-lead all pass through a hole in the panel mounting frame behind the mag switch and the jumpers have clearly been fretting on the frame exposing the cores in several places.

So I'll switch to the original Westach supplied jumpers, which I still have, and I'll probably also heat-shrink-sleeve the bundle for extra protection from chaffing. I haven't done this yet but purely on looks alone, this is the probable cause. I'll update when I know how it has gone.

[dmcnicholl]

Haven't flown it yet but ground runs are encouraging. I'm still seeing a little hunting, no more than a hundred RPM, but much better that the huge swings I was seeing before.

[Rob Swain]

Sounds like you've found the issue, but in the interests of completeness...

I believe that this can also be caused by the Mag needing servicing.
The points inside may have become worn and pitted and/or the capacitor (condenser) is on it's way out so you no longer get nice sharp electrical blips on the p-lead for the tacho to 'read'.

A test for this might be to try the Tacho on the other mag.

[dmcnicholl]

Thanks Rob but my mag was overhauled a couple of years back. Note 'mag' singular. It's an RF3 and there is only one.

I got to try it in the air yesterday and I'm not quite there yet. Much improved but, as on ground runs, I'm still seeing some hunting. Wetsach instructions say that for stability on some installations the two leads can be swapped on the back of the tacho.