Tacho reading high

[maulonir]

I have a saas tacho fitted to an ea81 with the standard points ignition. For some reason it is reading about 1000 rpm high

[aussie_paul]

I have a saas tacho fitted to an ea81 with the standard points ignition. For some reason it is reading about 1000 rpm high

[chook]

Rob, in a previous life I serviced and calibrated automotive tuning equipment. Many were multi function units (tacho, dwell, volt etc) and had to be calibrated in a sequence and then for "average" best result. Your unit will no doubt have some form of internal adjustment which sounds as though it needs a "tweek". Some of the adjusting potentiometers are a bit fragile and yours may have received a "bump" in transit or may not have been locked.I recently had a problem with a gyro I am rebuilding putting out 16volts from the alternator. I was expecting problems with it as the previous owner had placed an external regulator in the cct which already had an internal regulator fitted to the alternator. I checked everything, pulled the alt down and eventually bought a second hand alt. Guess what - same problem. Only then did I pull the VDO voltmeter out and check it - reading high - bugger- caught again.

[maulonir]

G,day chookAre you saying my meter is out or do you mean the gauge is out of cal. I returned the tach and swapped it for a new one and the same thing, reading about 1000rpm high. Took it back again and got my money back.went to another shop and bought another, another saas tach which is all they had. Different model though. Plugged it in same thing reading high. My meter might not be 100% acurate but the lowest I can get the gauge down to is 1000rpm and with the idle screw wound out that much that engines is right at the point of konking out. When my meter is showing 900rpm the engine is purring along nicely but the tach is showing 1800 rpm. This is driving me nuts.

[chook]

Ron are you using an electronic point system?Some tachs don"t like the signal that they get off those electronic units.If you are using the standard points system (not electronic) then you have to take the connection from the lead going between the distributor and the coil. (i"ve seen guys get the pickup for the tach from the battery side of the coil which also causes problems)Are your earths to the tach correct? Try connecting your tach straight accross the battery. It sounds like a dirty signal there somewhere Ron.

[maulonir]

Chook,It is a standard points setup. I have connected the tach as you suggested, and signal wire directly to the negative side of the coil at the same point I have the multi meter connected and still the same result, as in the tach is about 1000rpm higher than the meter. Thanks for the suggestions but not sure what to try next.Rob

[chook]

Ron what type of coil are you using?Where are you located?Or contact me on my email - see my profile.I"ve been out of the loop for a while now and don"t recognize the brand but I still have the calibration jig. Regards......... Chook.

[mad max]

I have built several gyros and the VDO tachos all read high. I placed a small 1 amp diode in the signal line. That cured them all. Point or electronic ignition. I allso put a 24v 3w globe in the line and did some tests and found that if the tacho shorted out it did not stop the engine. The globe fits in a black universal fuse holder. max (retired auto. sparky and full time Gyronaught) However not flying at the moment, been kicked off black fellow airstrip for not paying extortion fees. At present pleading to higher authority Two below God.

[birdy]

Only then did I pull the VDO voltmeter out and check itNever ever known any VDO to be any where near accurate.Tho they are consistant.Wot ever the error is, its always the same.Even them [so called] rotax dial tacos aint accurate. The one in the wasa is readn bout 10% high.

[chook]

Ron I was discussing your problem with Adrian Stoffels on the weekend and he came up with the same answer as MM. The diode you are chasing is a 1N4004 (50volt)or 1N4007 (1000volt). You will have to experiment with the direction that they are wired in (a white band on one end signifies the cathode end). You will be able to obtain them at an electronics store (not Dick Smiths - they don"t sell components anymore - the younger generation can"t be bothered building stuff - they want instant).Birdy I was able to look inside the VDO voltmeter (took the light out) and spot a variable resistor. I then carefully drilled a small hole in the side of the case through which I can put a screwdriver and now the meter is calibrated

[maulonir]

G"day Chook and Max,thanks for that info, I will give it a go.As Birdy said the tachos I tried were consistant and smooth through the rev range just showing about 1000 rpm high.Chook, I am using a Bosch GT 40 sports coil, the small square resin filled one.RegardsRob

[maulonir]

Max,Does it matter which side of the diode the globe goes on?Rob

[GraemeMonro]

Max,Does it matter which side of the diode the globe goes on?RobThe Outside would most likely work best.Graeme.

[chook]

Rob, (sorry I picked up Ron somewhere) the globe will be in series so it does not matter. What is important is that the globe insertion point is close to the power source or the power feed. This means that any wiring after the globe coming into contact with earth (or shorting) will pass the current through the globe (harmlessly). Your engine will stop but at least you won"t have to deal with a fire on the way down as well. Regards....Chook.

[mad max]

I put the globe in serries with the signal wire down as close as possible to the coil neg. That protects the engine from stopping if there is a short anywhere from there up to the tachometer. The diode can go anywhere in that line. It simply breaks down the signal by half waving it. I even had to put a diode in my new EFI machine in the signal wire to the computor. A diode simply being a one way valve for electric current. Any small alternator exiter diode will do.