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MG MGB Technical - volt meter

Not saying I want to do this, but where would be the most likely places to connect a volt gauge. Could one access the fuse block and any ground or would this be too easy. thanks,
jim mindy

Because a volt meter draws so little current, you can connect it to just about any ignition controlled source.

That should be OK - just make sure it is a good ground.

MJ Jones

MJ is right, in theory, but I think that I would attach it to the fuse block. With the frequency of corroded or dirty connections that occur in our cars, a wire under the dash could experience a small voltage drop that wouldb't noticeably affect the operation of, say the dash lights or the radio, but might cause you to think that you have an alternator problem, when you don't.

Get as close to the source as you can.

As MJ cautions, make sure that you have a good, clean ground.
Paul Noble

Paul caught me there. I meant to simply say connect it as close to the source as possible - the worst case I've seen is GM connecting their dash voltmeters to the same circuit as the A/C blower and/or headlight switch.
MJ Jones

As close to the source as possible would be the white at the ignition switch, but that is unfused so would need its own in-line. You could connect it to the green (which is already fused) in the cabin, but it would then be affected by any volt-drops in that circuit. But since those voltdrops are affecting the speed of the turn flashers, washers and heater fan and the brightness of reversing and brake lights then surely it is no bad thing if it encourages you to clean up those circuits. And the slow-acting thermal-type voltmeters do draw a significant current, much more so than the moving coil type of instrument found in an analogue test-meter, which is why they need good connections including the ground.
Paul Hunt

Why not plug in a volt meter to the cigar lighter ?
Mine works well there.
robert snider

It's permanently powered on most models and so would drain the battery over time.
Paul Hunt

On my 74 roadster I have a voltmeter. On the fuse block, on the side towards the radiator, I'm using the white wire that is second from the bottom. I don't know what circuit that is, but it's switched. Last year I had trouble with 1 of the 6 volt batteries, and when I would turn the switch to the on position and it did register about 9 volts. After I got a new battery then it started showing the 13 volts when running. One time it showed only 10 or 11 volts and it turned out the fan belt was loose. It's always had a rythmic jerk to the needle, and I finally realized it must be from the fuel pump. Each time the fuel pump clicks, the needle shows a jerk.
Overall, that voltmeter gives you another tool to keep track of your MGB's electrical system. I couldn't drive without it!
Ken T
Kenneth Thompson

Hook to any brown wire and you'll get an acurate reading of voltage. If you let your B sit long enough for a volt meter to pull the battery down it's time to get ride of your car and let someone who enjoys it have it! Bob
Bob Thompson

The white is switched as you say but unfused. You really should have an in-line fuse if picking up the white, which if the gauge is in the cabin is easier done at the steering column. If you can see the needle flick when the pump ticks then depending on how much it could imply bad connections back through the ignition switch to the solenoid. Which just goes to show.
Paul Hunt

Amen to that Bob-besides I long ago added a battery cutoff switch beside my seat and developed the habit of turnng it off whenever I get out of the car-if I were to forget the Volt meter sure tells
me-and it is a lot more imformative than a little red lite.
robert snider

I've just wired in a voltmeter temporarily to my 70GT after suffering a flat battery from a slow, dark, wet drive home the other week. Car is RHD so I cable tied it to the cross beam under the dash and poked the wired through the bulkhead to the fuseblock.

I think that the voltage can drop below the point at which the battery is being charged but still not have the little light come on.

What is the voltage at which the battery no longer charges but instead discharges? 13.8 volts?

With everything on, radio, GPS, lights, wipers and blower, I am sure my battery is not being charged at idle despite the little light not being on.

Simon
Simon Jansen

Simon

There's no magic number voltage-wise, but anything above what your battery sits at will charge.
Take the key out or disconnect the battery and check the voltage. Then start the car, wait for the electric choke to turn off -if so equipped- put as much load on the electrics as you dare and check the voltage at the battery.

Anything higher than what you starded with = charging.

Mike
Mike Phillips

Anything over 13v should be charging, it has got to be below 12v before it will be discharging significantly, but the voltage on *your* battery terminals after being left to stand for several hours gives the cross-over point for your system. Check the voltage at idle with minimal electrical load, it should be 14-14.5v. Now start adding electrical load and see what happens to the voltage. There are alternator faults that can give adequate charging on a light load but not a heavy load. When you add electrical load beyond what the alternator can supply that is when the battery starts taking the extra, discharging it and reducing the system voltage. This will *not* cause the red light to come on no matter how much load you add, that only happens when the alternator stops giving an output altogether e.g. if the revs are dropped below about 600 or there is a definite fault in the unit.
Paul Hunt

Paul, thanks for the advice--I'll put a fuse in the line to the cabin.
An interesting coincidence--on Saturday I drove with the british club up over Trail Ridge Road through Rocky Mountain National Park at elevations up to 12,000 ft. I noticed on the long downhill stretches where I used compression braking, the vacuum gauge stayed up around 20 and the jerk in the needle of the voltmeter slowed down considerably. The engine obviously didn't require much fuel and the clicking of the fuel pump had slowed down at the same rate.
Overall my MGB did great! it was a 247 mile round trip through Estes Park, down to Winter Park and then Denver, including Berthoud Pass, ranging from 4500 ft to 12,000 ft. I was running 91 octane and it pulled quite well on the steep slopes in 3rd gear. Since it has such good cornering capability, I was able to use the downslopes to build up momentum, take the switchbacks without braking, and use that momentum to get up the next slope. I actually passed a diesel pickup, which should have had more torque than my MGB. A guy in a WRX Subaru passed me, and there was no way I could keep up with him, but I'm still happy that I was able to do better than that diesel pickup!
Ken T
Kenneth Thompson

Confirmation, if any were needed, that an engine needs less fuel going downhill than up. Interestingly, going downhill using engine braking uses more fuel than idling, when in fact it doesn't need any fuel at all. Something the electronic injection systems deal with by shutting off the fuel supply all together if the engine is turning at more than 1000rpm or so with the throttle closed. In the 70s with rapidly rising fuel prices, and again more recently, gizmos (a bit like the gulp valve) were produced that open a valve in the inlet manifold at high depressions (i.e. engine braking) to dump air into the manifold, reduce vacuum, and reduce wasted fuel.
Paul Hunt

This thread was discussed between 20/05/2003 and 28/05/2003

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