Weirdness in the gagues...

[Multiphonikks]

A long, on-going thing here...

After buying Kate's old gauges from Rebecca, I set about putting them in Hebe (Pretty easy as I got the wood panel she'd made too).

Wired 'em up, and everything worked from day one. The thing however, is that the voltmeter has always acted rather weirdly. On startup, Hebe's Voltmeter reports about 13.5 to 14 Volts, and then, as the engine gets hotter, it slowly goes down to 13 Volts. Now, even if the load doesn't change (ie I keep everything off) the voltage drops. If I drive for a few hours it'll drop down to 12.5 volts. It's weird, and doesn't change regardless of speed. Now, I've eliminated all the usual supsects:

The fanbelt seems to be tight enough
The Alternator is fine
The voltages seem to be fine on the battery itself - and it's showing a healthy charge.

Both leads on the voltmeter are cleana and connecting on clean metal. Admittedly, I've taken the earth from the mechanical temperature gague which is connected to the head of the engine. I wonder if the heat from the engine is affecting the resistance of the cable, and thus the reported voltage across the battery*

So am I just needing to be a good girl and taking the negative wire from right by the negative terminal on the battery, or what? - I really didn't think that there would be any difference...

So any physics type people... What's going on?!

*Or possibly the bi-metalic strip used in the voltmeter is a bit weird.... maybe when it heats up it's inccorectly reporting voltage. Thing is, I know it was fine on Kate's car...

[doobry]

I've taken the earth from the mechanical temperature gague which is connected to the head of the engine

the clue is in the question? - firstly try using a good earth ;-)
The dash is completely metal so it's not going to be hard to get one.

[les]

I'm sure the readings are quite normal, as the battery is further charged the less voltage it will take/need, I may be getting my amps and volts mixed up here, but I think a similar thing is being displayed on your gauges, as on a charger when a battery is being charged; the indicated amps drop. Anyway there's enough students on this board to put me right!

[rayofleamington]

I may be getting my amps and volts mixed up here

yes.

An alternator output will remain at a constant voltage (normally 14 volts give or take a bit). However the temperature sendor on the head is not an ideal earth...

[les]

I am thinking the battery voltage, as indicated by the gauge, can vary depending on the amount of charge in the battery. I didn't think the alternator output was the issue.

[Multiphonikks]

see - I was feeling rather silly posting it... but :0

[Cam]

That's not an earth!! it changes resistance accoring to temp. No wonder the gauge has been going funny!

Take it off and connect it to a proper earth (black wire on the speedo) or directly to the metal dash.

[Multiphonikks]

That's not an earth!! it changes resistance accoring to temp. No wonder the gauge has been going funny!

Take it off and connect it to a proper earth (black wire on the speedo) or directly to the metal dash.

Yessir... Thankyousir... *looks innocent and plays with pigtails*

Actually Cam, that's what I thought, but I talked to several "nameless people" who told me I was barmy...

I shall go and change it round

[Cam]

That's not an earth!! it changes resistance accoring to temp. No wonder the gauge has been going funny!

Take it off and connect it to a proper earth (black wire on the speedo) or directly to the metal dash.

Yessir... Thankyousir... *looks innocent and plays with pigtails*

Actually Cam, that's what I thought, but I talked to several "nameless people" who told me I was barmy...

I shall go and change it round

Hmm........ my comments do look a bit harsh there! They were written in humour but you can't tell that as I did not include any smilies Div

[Alec]

Hello all,
Battery condition indicator is a bit of a misnomer, it actually monitors the system voltage, which when running is the alternator output voltage. According to my Triumph workshop manual the 17ACR alternator is regulated at 14.7 volts, so that is about what you should see unless load is greater than alternator output.

Alec

[rayofleamington]

Actually Cam, that's what I thought

well it didn't seem to sink in when the first 2 people mentioned it so Well done to Cam for labouring the point ;-)

[Cam]

Well........ a few folks mentioned it not being an 'ideal' earth perhaps giving the impression that it was an earth.

It's not. It's a sensor for the temp gauge. I thought I'd just clarify things a bit. OK?

[rayofleamington]

It's not. It's a sensor for the temp gauge.

well yes, but as the temperature gauge earths through the sensor, it's all relative...
I guess we best not discuss the potential difference of earths at different points around a car as that's not axactly gonna help niKKi

[Cam]

Well..... it does not earth as such as I'm sure you know. An earth is a (theoretic) zero Ohm connection. The current passing through the gauge alters the reading (as it's basically an ammeter), and the current is determined by the resistance of the sender unit (varying with temperature). If we start saying that it 'earths' then transistors do too which is VERY messy! I know perhaps it IS labouring a point which was meant to avoid confusion But you can make quite a few mistakes if you are not aware of what's going on.

The fuel gauge is the same as well, but it helps (certainly to the average mechanic bod anyway) to think of them as senders which should not be used or thought of as earths. Otherwise you get into low and high resistance connections, and like you say problems with PD at different points. So we'll perhaps leave it there then! I think it was the usual case of terminology leading us astray!