questions about gauges!

[wibble_puppy]

ello

I've been reading through the old threads about gauges; what sort to have and where to site the dials.

Some questions occurred to me as I read through, mostly because I'm new to this gauge business.

1. What is the difference between an ammeter and a voltmeter? in terms of being able to judge the condition or performance of part of your car? What does each one measure, exactly? Why does an ammeter need such thick wires?

2. what is a "vacuum gauge"?

3. If I put in an oil pressure gauge, is there any reason why I shouldn't then use the oil pressure warning light on the speedo dial as an alert to something else, eg that my reversing lamp is on?

4. Those of you who have a rev counter, how much do you actually find yourself using it?

5. On one of the old threads, someone (Roni) commented that the gauge bracket under the dash vibrated and that although it was in keeping with the character of the car the dials were harder to see (compared with siting the dials in the driver's side glovebox area). Any comments?

6. What is a battery condition meter and how useful is it?

I'm sure I'll think of more questions out of my ignorant brain but those will do for the present

wibble xx

[bigginger]

1) One measures amps, the other volts. Ammeter a dodgy thing to have, IMO - wires carrying the car's full current, and I can't see much point in the volt meter either.
2) Measures the vacuum in the inlet manifold, meant to aid economy if you drive by it
3) Nope
4) Never had one on a Mog, but never used it in other cars
5) Can't help
6) Not a clue, and can't see a need for one either,

[wibble_puppy]

Thanks big man, such lightning-fast reflexes, like a panther

1) One measures amps, the other volts. Ammeter a dodgy thing to have, IMO - wires carrying the car's full current, and I can't see much point in the volt meter either.

But what is the point of having them - either one or t'other? What do they enable you to find out about the functioning of which part(s) of the van?

[alex_holden]

1. An ammeter measures how fast the battery is charging or discharging. You can also tell when the battery is full because the charging rate gradually drops off until it's at zero. I find it very handy to keep an eye on the charging system. A voltmeter tells you the battery voltage. Personally I rarely found that useful to know when I drove a car that had one. An ammeter needs thick wires because all the charge/discharge current to/from the battery passes through it (apart from the starter motor current).
2. A vacuum gauge measures the amount of vacuum in the inlet manifold. Higher vacuum = lower fuel consumption, apparently. Personally I wouldn't bother with one.
3. Best to keep the oil pressure warning light as well as the pressure gauge because you'll probably notice sooner if a warning light has come on than if a needle drops to zero.
4. A lot. I didn't think I would, but actually I find it helpful to decide when to change up, plus when I'm cruising on the motorway at a constant speed I tend to go by the rev counter instead of the speedo because it's in a better location (behind the steering wheel rather than in the middle of the dash).
5. Mine are in the driver's glovebox area.
6. Not sure but I'm guessing it's a voltmeter with markings that show what the voltage equates to in terms of the amount of charge in the battery.

[pyestudiocolour]

Hi. AFAIK, a voltmeter and battery condition meter are the same thing, monitoring the voltage of the car's electrical system - I had one fitted to my old Mini and it proved very useful as it indicated that my alternator wasn't actually charging the battery half the time (a quick tap with a hammer on the side of the alternator usually got it going) strangely, even though there was no battery charging happening and the wipers started slowing down at one point, the ignition warning light never lit up. I found the gauge so useful, I swapped it to my Minor. It also shows if the battery is being over charged, due to incorrect cut out settings etc. on the control box. They are very easy to wire in.

I wouldn't consider an ammeter as I consider the wiring to be a little dangerous - all the current being used by the electrical system needs to be passed through the gauge - the consequences of a wire coming off whilst driving don't bear thinking about!

I put a mechanical (mini) oil pressure gauge in mine with a T-piece so that the original oil pressure switch could be fitted at the same time - this works well but proved a major swine to fit. . .

My gauges are fitted in a bracket bolted through the parcel shelf, centrally beneath the heater. They are not so easy to see there as on the glovebox (where I used to have them) but they don't vibrate too much - only a little at tickover, and it leaves the glovebos free for other things as well as looking more more like a period mod!

[MoggyTech]

Battery voltmeter and condition meter are the same animal. This guage will show you alternator output when running, it can also show you V drop due to high battery load, such as extra lights etc.

I don't like ammeters due to the full current going through the wires to the gauge. Never use an ammeter with an alternator.

Vaccum gauge, will tell you much about the health of your engine, as well as fuel ecconomy. You can diagnose a stack of engine faults with a vacuum gauge, including sticking/burnt valves (flickers at idle), inlet air leaks and more.

Oil pressure gauge far better than just a warning light, which only comes on when oil pressure drops to below 10 psi, by which time the damage is done.

Temperature Gauge vital IMHO

Rev counter, handy if speedo calibration is out a bit. Standard 1098 with 4.22 diff gives 15 MPH/1000 Revs in top gear.

The drivers glove box, just begs for gauges...

[Alec]

Hello Wibble,

I do like to know what is going on so I have a full range of gauges on my car.
A vacuum gauge is a little more than just an indication of how much throttle you are using, it also gives an indication of how well the engine is tuned or any malfunctions. They were a common test instrument in days before electronic test equipment, but the car mounted ones really are too small to be effective for that purpose.

Yes, a battery condition gauge is a voltmeter.

Alec

[Alec]

hello MT,

you beat me to it!

You say never use an ammeter with an alternator? Certainly a 30 amp meter is too small for most alternators but you do get higher range ammeters.

Alec

[Willie]

I think that the oil pressure is the most useful guage, but retain the warning light too. Water temperature is also useful, if only to balance
the look if you have the earlier type of dashboard where the two gauges
can be inserted either side of the speedo.

[alex_holden]

Never use an ammeter with an alternator.

Why not? They work fine together.

[Packedup]

The answers have already been pretty much covered, but here's my take:

ello

1. What is the difference between an ammeter and a voltmeter? in terms of being able to judge the condition or performance of part of your car? What does each one measure, exactly? Why does an ammeter need such thick wires?

An ammeter tells you how many amps are flowing, and in which direction. A voltmeter tells you the volts in the system.

I prefer a voltmeter if only because an ammeter needs the main live wires to pass through it, adding potential for major problems. A voltmeter can still be useful in letting you know what's going on - If it drops a marked amount when you put the lights, heater etc on then you know your charging system isn't up to the job. Also, if it always reads low, again your charging system isn't great. I wouldn't consider it a vital gauge though, just useful.

2. what is a "vacuum gauge"?

A gauge that measures the vacuum! It monitors the pressure in the inlet manifold, which can be a guide to getting more mpg if you use it to control your right foot. Handy for fault diagnosis, but then you can buy ones to use in the garage. VW had an economy meter built into the dash on some of their cars, and that was just a vacuum gauge. Pointless IMO, unles you really need a nanny telling you planting your boot makes the petrol disappear faster.

3. If I put in an oil pressure gauge, is there any reason why I shouldn't then use the oil pressure warning light on the speedo dial as an alert to something else, eg that my reversing lamp is on?

None at all, but I'd fit a T piece and have both. One of my Triumhps had a gauge fitted by a previous owner who didn't bother with the light, and also placed it by my right knee with no backlight. That was useful then. The Midget has only a gauge, and I find staring at the needle on a long run more distracting than simply having a shiny light to tell me my engine's kerplunked.

4. Those of you who have a rev counter, how much do you actually find yourself using it?

All the time. Both for when I'm taking it steady and want to remain nearer peak torque for efficiency, and when I'm putting my foot down and use it to let me know when the power curve is falling off and it's time to change gear. It's also helped me spot problems, such as the Midget having a massive problem above 5250rpm, and the Metro having cooling issues at a steady 3500+ rpm. It can also act as a backup speedo if the cable fails (or if I forget to put the cable back on after rummaging behind the dash..).

5. On one of the old threads, someone (Roni) commented that the gauge bracket under the dash vibrated and that although it was in keeping with the character of the car the dials were harder to see (compared with siting the dials in the driver's side glovebox area). Any comments?

Do you want to fit gauges to inform you of certain things? If so, do you want to be looking at your lap when hurtling along, or have your eyes somewhat nearer the road? The under dash panels are fine for flashy gauges, but personally if I wanted to fit gauges to tell me what the car was doing, I'd want them as near to my normal eyeline as possible!

The beast would be a heads up display I suppose... ;)

[MoggyTech]

Never use an ammeter with an alternator.

Why not? They work fine together.

Yes they do work but they have been known to cause problems with alternators that use Battery sensing rather than Machine sensing to start the charge cycle. Plus it's another pair of connections in the circuit, that if they go noisy or worse open cicuit, it's goodbye alternator The less connections in an alternator feed the better IMHO.

It really needs to be a -60 0 +60 Ammeter as well to handle higher ouptut alternators, such as the Sierra flat back that can pump out 47 Amps.

[Axolotl]

It really needs to be a -60 0 +60 Ammeter as well to handle higher ouptut alternators, such as the Sierra flat back that can pump out 47 Amps.

It doesn't make any difference what alternator you have.

What you say is conventional wisdom for cars fitted with alternators, but in reality, whatever maximum rating the alternator has, it will never achieve it if it is connected to a single, starter type battery.

The alternator has a high rating because it has to power everything on the car, as well as charging the battery. It "sees" the battery as just another load to provide power to, along with the lights, heater etc.

The ammeter is measuring what is going into or out of the battery, not what's coming out of the alternator. That is a different thing.

On the charge side, maximum charge current for a standard battery will be limited by the battery chemistry and constuction to about 20% of the battery rated capacity. If you have a big battery, say 60 Ah (standard would be 35 Ah), you'd never see it charge at more than 12-15 A, even when flat. i.e. whatever the alternator's maximum output, you won't be able to "pump" it through a single battery. The battery "takes" what it can, no more.

On the discharge side of the battery, side lights, main beams, a couple of front and rear fog lights, rear screen heater, a radio, the heater motor and wipers would take about 500 Watts.

That's about 40 A with 12.5v, i.e. everything on with the engine not running and a nicely charged battery. but heavy loads like the heated rear screen should be wired so they can only be run with the engine running, and not many people turn everything on before they start the engine anyway.

The ammeter is measuring what's going into, or out of the battery, so as long as the engine is running and the alternator charging, you have to offset what it is producing to charge the battery against the load coming out to get the reading that would be displayed on the ammeter.

In practice, I have a 30 - 30 ammeter fitted on my Minor, and I can't get more than about - 15 A with everything on and the engine stopped. (The horn would take more, but that isn't wired through it).

Similarly, maximum charge rate is about 8 A with a flat battery.

The reason I fitted a 30 - 30 meter is that it has a bigger needle deflection for a given current, so it easier to see what is going on in a normal set up.

I used to have a 60 - 60 meter, and the needle barely moved.

Maybe that's why people don't think ammeters are particularly useful.

Oh, and for trivia fans, an ammeter is, in fact, just a voltmeter with a big shunt ( a very low resistance circuit) to take the actual current. It measures the voltage drop (or rise) across the known resistance of the shunt.

[wibble_puppy]

thanks very much indeed everyone for your posts - loads of useful stuff.

Axolotl I've read your post several times and still can't understand it. I shall try again. Just thought I'd mention it in case you thought I had ignored your taking the trouble to type out so much info for me!

Still pondering all this, so any more advice, tips etc will be really welcome

wibble xx

[alex_holden]

Axolotl is just saying that a -30 to +30 ammeter will be fine even if you have an alternator (and I agree with him).

[wibble_puppy]

oh i see thanks, Alex

and would it be a really useful thing to have? and what about these concerns people have expressed about the wiring issue?

[alex_holden]

I find it useful and I'm not overly worried about the wiring. The fact that not all the circuits on the car are fused concerns me more. I have considered inserting a large fuse as close to the battery as possible.

[wibble_puppy]

Cheers

The fact that not all the circuits on the car are fused concerns me more.

Er, yes

[MoggyTech]

If you exceed the shunt coil current in an ammeter it will overheat and burn out. If this happens with an alternator it is toast. With heated rear and front screens now avaliable for the Minor, a battery condition meter is a much safer gauge for use with an alternator. It simply connects between earth, and an ignition switched feed, and as the gauge coil is high impedance, it draws very little current.

Further, as a BCM only covers 9 to 15 volts, it is a 'short' scaled meter, and even small voltage changes can be detected, such as brake lights, indicators etc.

Ammeters were designed for dynamo charging circuits not alternator charge circuits. Running a cable capable of handling 50 amps through the bulkhead into the car, is not my idea, of a good idea. So if you opt for an ammeter, a fire extinguisher would be another good option.

[wibble_puppy]

Thank you for that info, MoggyTech

if you opt for an ammeter, a fire extinguisher would be another good option

I've already got a fire extinguisher

The battery condition meter sounds useful