MGB Battery Monitoring

Sometimes, when you're working on one thing, it is a really good idea to take your mind off of it by working a little bit on something else. Today's post covers my distraction from Hapy's electrical issues.... with the battery issues on Oliver (the 1978 MGB). LOL. For my local-to-the-Pacific-Northwest friends, I hope you have heat and power. I know some friends have been without both since Friday. Grateful we are finally above freezing.

Flat Battery

Our saga begins with Oliver's battery going flat from being parked for a few weeks. I first noticed it in early fall after parking him in the garage for the winter and forgot to hook up the float charger. When discovered, I put the battery on the float charger like when it was parked outside, but wanted to know the root cause. I started with swapping out the battery for the battery we had pulled from donor Zed. That was a good battery that still held 12.5V almost a year after pulling it out of the donor. It barely fits in the battery box, but it would run flat if left off the float charger when hooked up to Oliver's battery cables. I decided there was something pulling vampire amps off the battery.

Clock

original oil temp
and clock look

I am sure there are faster, more scientific ways to figure this out, but this is what I did: I pulled all the fuses, and put the battery cables back on the battery. I left it like that for a week and found that the battery did not run flat. This told me that the drain wasn't from some ShadeTree wiring that did not run through the fuse-box. Then, I checked the amp draw across the fuse pins, and found that the draw from the bottom-most fuse registered on the meter. That fuse provides power to the interior lights, the trunk light and the clock. These old analog clocks consume much more power than their digital counterparts, because they need to run little motors that turn gears rather than a tiny computer to keep the time. I decided I would pull the clock and put in a gauge that might be more useful: a volt-meter.

 

This removal is interesting. The VDO gauges with which I am most familiar have a threaded collar that goes all the way around the base of the gauge. You push the gauge through the hole in your dash and then thread on the collar from behind. These original MGB gauges are not like that. Instead, there is a cross bar that is shaped like a "U" with a hole through the very center of it. That hole aligns with a threaded stud exiting the center front (again, front is front) of the gauge. Once the stud passes through the hole, the ends of the "U" press against the dash, and a nut is threaded on to hold it in place. While more elegant, it is actually harder to manage with 2 hands. Otherwise, I think it is the better mount because you can wire your gauge while it is dangling through the hole in your dash, test it and then mount it without disturbing the wiring. With the VDO mount style, you need to either be optimistic and run the wires through the collar suspended behind the dash -OR- do that after you have tested.

Volt Meter

volt-meter installed

The gauges in the old MGB were made by a company called "Smith". So, if you want your gauges to look authentic, it is best to buy Smith gauges. Here in the US, that is quite difficult to do in-person or at least within the borders. Enter eBayUK. I found someone selling off a perfectly good voltmeter that was era-correct from a different British car. It cost about $100US delivered, and it arrived faster than some other things I have gotten from the east coast. The wiring is quite simple. there is a ground protruding from the front (front-is-front) corresponding to the far bottom end of the voltage scale. On the other side, opposite the top-end of the voltage scale is the signal input. In-between is a tube into which a bulb is set. This requires the bayonette-style bulb similar to the clock, and the other gauges in the MGB. I just reused the one from the clock.

Testing

Of course, I could not just pull the clock and slap the voltmeter in without some tests. First, I wanted to see how/if it worked. So, I ran a wire from the negative post on the old MGB battery (sitting on the garage floor on a float charger). To the positive post, I ran a fused wire (wire to a fuse to another wire) to the supply-side of the voltmeter. The needle slowly moved up and settled just below 13V. Perfect.

 

Next, I grabbed the clock I had removed. I confirmed that the clock would run using the same test wiring set up first. Then, between the fused wire and the supply side of the clock, I ran my meter to see what the amp draw is. On the second to most sensitive setting... or is it least sensitive.. setting, I got an amp draw: 30. Interesting. I did the same thing on the voltmeter and got 3. So, the clock draws 10x as much juice from the battery as the voltmeter does. I figured that the voltmeter, even if it was on all the time, would allow the battery to hold a start-able charge 10 times longer. So, I decided that running the voltmeter to a switched source was not necessary. I may revisit that one day, but for now, I figure that I can look into the window of the car and see if we can go for a drive. More importantly, I could simply re-use the wires that I had just removed from the clock.

Install

volt-meter test with 
running lights on

Installing the voltmeter was quite easy. I plugged the red/white wire into the light bulb I scavenged from the clock. I plugged the black ground wire into the ground and the purple wire that used to run the clock into the voltmeter sensor plug. I confirmed the wiring by putting the battery cable back on. Yes, I could have done something with less potential for failure, but I would have had to do something like this eventually, and it was only 3 wires. I wrapped the end of the purple wire with electrical tape so I didn't accidentally short the circuit (and pop that fuse). Then, I set the gauge into the hole, put on the "U" with the arms touching the dash such that they were not anywhere near the supply side (running approximately from 1:30 to 7:30 on a clock-face). A quick thread-on of the retaining nut and it's in.

Between the weather and the pile of non-operational cars in the driveway, I cannot take Oliver out to test the gauge. But, since I hooked it into the always-hot circuit for the interior lights, I can verify that it works simply by looking at it. It currently reads just under 13V. So, we're good. I intend to leave the battery hooked up without a float charger and I will be checking the battery condition every couple of days. If the voltage drops enough for me to notice (or below 12V), I will put the float charger back on. at that point, I may search additional vampires. At this point, I think I slayed the one that was draining the battery.

That's it for today. Thanks, as always, for following along--