Need Help Adding a Voltmeter

[Cold Beer]

I want to install a voltmeter on my Massey Ferguson 240, currently it only has the dummy light. Since I have just enough electrical background knowledge to burn up my tractor and any surrounding structures, I need some advice on how to properly install the gauge, which wires go where etc.

 

[Soundguy]

i'd check and see if the key switch has a run tab..if so, that would be my choice for a + pickup.

soundguy

 

[Transit]

FYI, not being all that familiar with volt meters, you may find some "Problems" that are not problems.
Digital meters are more sensitive than dial meters, in that they indicate down to xx.1, xx.01 volts. The low order digits may roc some and the voltage across the battery or were ever you make the connection to will also roc because the voltage regulators are not rock solid. Expect to see values in the range of 13.5 to 14.5 v and constantly changing as the regulator will hunt around an average value. You will soon learn what normal is.

 

[Soundguy]

can also install a large value electrolytic capacitor across the back of the gauge to compenstae it a bit. say something like 1000-2200 uf in the 15-25v range.. even up to 6800uf...

soundguy

 

[SPYDERLK]

Since a voltmeter draws almost no current you could hook it directly into the battery circuit before the switch. This gives you continuous reading and the advantage of being able to read your battery V under no load conditions vs the V under various load conditions that are seen when the Ig Sw is on. This can give you comparative values that one day youll find use for in diagnosing problems. Cap is a good idea if your going with a digital gage .. An analog type wouldnt need it, but vibrations are an issue with those.
larry

 

[Soundguy]

Since a voltmeter draws almost no current you could hook it directly into the battery circuit before the switch. This gives you continuous reading and the advantage of being able to read your battery V under no load conditions vs the V under various load conditions that are seen when the Ig Sw is on. This can give you comparative values that one day youll find use for in diagnosing problems. Cap is a good idea if your going with a digital gage .. An analog type wouldnt need it, but vibrations are an issue with those.
larry

I agree..the cheap non compensated gauges ( analog or digital ) are gonna be the less accurate .. digital are easy to compensate.. the analog with touch movements.. well.. not much to do for them.

I know some people put them on push buttons for momentary or on/off checking.. or automagic with key switch or by a sender like on oil pressure line.

lotsa possibilities

soundguy

 

[Transit]

Without going off the deep end into the electronics, i don't think adding a large cap will have any influence.

 

[Soundguy]

It will.. and yes.. you'd have to go off the deep end to know or explain why. Slew rate and 'averaging' will be part of the picture.. can get fancy and add an inductor.
looking at a PI or an L filter and thinking about emi/rfi filtering will be helpfull in working with non compenstaed gauges... ( won't do much for non dampened analogs though.. ) that are just plain jittery due to inertia..

soundguy

 

[SPYDERLK]

Without going off the deep end into the electronics, i don't think adding a large cap will have any influence.

Some cheap electronic/digital meters dont deal well with quickly changing voltages. A charging waveform is not a true, reasonably steady state DC, but instead its a series of hi frequency pulses that average out to DC. The digital meter is not updated real time - it samples at some rate not atuned to the charge pulses and its displayed value can jump around because of this. A cap across it would serve to average the pulses and steady the meter reading. ... or use an analog meter. The mass of its meter movement averages the pulses - no cap needed. Unfortunately, tractor vibes will shorten its life some.
larry

 

[Soundguy]

yeah.. what he said. same reason why I hate cheap-o digital auto rangeing meters with no set-scale function...

soundguy