Erratic Tach

[Doug Rohrer]

I was returning from Chapel Hill to my home in western KY yesterday when my tachometer started bouncing around with erratic readings. This is the standard Van's electro-mechanical small diameter tach in an RV-9A. The needle bounced all over before eventually settling on zero. Then as I started my decent, it mysteriously started working again. After bouncing all over it now reads accurately with no fluctuation. I would have rather had it stayed failed so I could diagnose it better. Any advice how to diagnose this? The wiring is intact and all terminals tight. Thanks.

[rvbuilder2002]

Disassemble the coupler for the tach generator and look at square end (end that goes into teh tach gen.) of the short cable used to couple the generator to the drive on the engine.

On some engines the cable penetrates the engine side far enough that only a small portion of the cable penetrates the generator end it it becomes worn and slips.

[chuckwn]

You may want to consider improving the electrical connections by wicking solder into all of the crimped terminals and butt splices in the wiring to your tach. I have done this with my Van's oil temp and pressure instruments and am now getting reliable readings.

Also, we manufacture the T-30 Tachometer which reads directly from your p-leads or electronic ignition. No firewall penetrations are required for installation. $169

http://www.fdatasystems.com/T_30.htm

[pierre smith]

...where it goes into the sending unit. The cable had a raised 'spine' originally and we had the same left, right ,left needle fluctuation, then nothing.

During removal of the cable and sending unit, we saw the smooth end and just bought another cable.

Best,

[Doug Rohrer]

Thanks guys. I will check out the condition of the inner drive cable this weekend. It seems strange that it failed and then started working again. I will also try to spin the transducer with a short piece of speedo cable chucked in a drill.

[GLPalinkas]

I am getting the same erratic rpm readings. Checked the connections and all is fine. I pulled the short drive shaft from the engine/transducer and there is no binding or worn drive tips.

Just trying to figure out if I have to replace the entire setup or if I can replace the gauge or transducer separately.

Same setup as the OP Doug.

Any help troubleshooting is appreciated. How do I start?

Tach Gauge
Part Number = IE VTACH3500
Transducer for systems without vacuum pumps
Part Number = IE VTACHGEN 2

Or
Replacement drive key for VTACHGEN 2
Part Number = IE 104152X2.625

[Geeman]

I figured by the time I took it apart to troubleshoot the problem, I could have replaced it with an electric one. So I just ordered a 3" MGL Tach/Manifold Pressure. For less that $300 I got a new Tach and a MP gauge.

[scard]

I use the vans tach transducer. I simply wore mine out and replaced it. Others have done similar.

[GLPalinkas]

Bump

See my previous post.

Just looking for troubleshooting suggestions.

Thanks all