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Charging/Battery died + Tach & Idle questions

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Brian_C

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Sorry folks, this post is a bit of a beast.

The car: 1964 MG Midget

The battery died on the way to work this morning. It just up and gave out on me.

A few questions:

1. the date on the battery was 2/06 - does it make sense for it to stop taking a charge within roughly 1 1/2 years?

2. Is there a simple test to see if the generator is working correctly? Someone told me to start it and then disconnect the battery. If the car dies then the alternator/generator isn't charging. I'm skeptical, but would love it if it's that easy.

3. I had the lights on (for safety), fuel pump (obviously), and there's an aftermarket electric fan that was turned on. I don't think that's too much pull for the generator to keep up with. What do you guys think?


Other Questions:

1. The tach has never really worked. It usually moves when the car is cold started (reads about 500 RPMS, actual = ~1500) then it'll randomly bounce around during the first few miles. Eventually it gives up the ghost and sits on 0 and "catches" for a split second or two every so often.

After I put a new battery in the car this morning and started it up the tach picked up right away and was steady @ 900-1000 RPMs during idle. I didn't have any independent way to test the accuracy, but it sure felt right on. Bliped the throttle and the needle moved like it should. But, I got on the road and could tell that it was off. By my estimation I was @ ~4,000 RPMs, but the gauge was reading 3K.

Very long story short, what would cause such a thing? I was going to put off repairing the tach until the off-season, but I'm intrigued, now.


2. Last question, I promise (for this post anyway). This is an idle question.

Cold Start = car starts with no problems within the first crank or two.

Cold Idle (choke all the way out) = RPMS are @ ~1500 - 2K. Feels good.

Warming up Idle (120-160 degrees), no choke = feels great. Fairly smooth idle. no problems.

Operating temps Idle (185 - 210, mostly 190 degrees). Starts with a normal idle and then the RPMS start decreasing until it's barely turning over. I basically gotta keep bliping the throttle or give it a little gas to keep the revs up.

I originally thought it was rich and overloading, but now I'm feeling like it may be a lean condition. Thoughts?


Thank you,

Brian
 
I assume you have a volt meter to check the various components.
If I recall correctly a good battery will be putting out around 14.3V. Connect the voltmeter, start the car, the voltage will drop, but climb back up if the generator is working properly. If it is, start switching on the various components to put it under load (lights, wipers, stereo etc). If the battery continues to read 14.3 then it should be good.
If you still have the receipt for the battery it may be under warranty.
If it isn't, and your moral standards are a little below the norm, (kids don't try this at home) buy a new battery of the same type with warranty, wait a couple months, clean up the old one and take it back for a refund... of course I didn't just say that (nudge, nudge, wink, wink)
 
O...K! Lots of questions here.

First, yes, a battery can die in a year and a half. Kinda early, but it can happen. Especially if it isn't charged properly. But, be sure it's really dead, not just undercharged.

You might want to put the battery on an "intelligent" charger. These are designed to give the right current and voltage profile, over the charge period, and shut off when the battery is full. Sometimes this can even restore a battery if it has minor plate sulfation from long periods of undercharge.

Second, I wouldn't try running the car with the battery disconnected. The battery itself provides a certain amount of voltage regulation, and the whole system is intended to work with the battery in place. Not sure what would happen if you tried it, but that's mainly because I've never been courageous enough to try it. Usually it's not necessary, anyway; unless there is an internal short in the battery or some other highly dramatic failure, the charge system should keep even a bad battery up to at least 12V or so. Of course, if it is bad, it won't keep that charge, but the generator still should be able to get the voltage up to some reasonable value.

Third...you can check the generator by disconnecting it, attach the output to the field coil, start the car, and see if the output voltage comes up. It should rise quickly to 20V or so. Don't run the engine too fast; connected this way, the generator voltage can get quite a bit higher than the thing is designed for. This assumes you have a charged battery to run the car, however.

Fourth...people always seem to blame the generator for charging problems, but the regulator is often at fault. The shop manual describes some simple checks you can use to test it. These require a moving-coil voltmeter, however; if you try to use a digital one, the noise and spikes on the main DC lines will drive it nuts (pardon the heavy technical terminology), and you'll get nonsense readings.

Finally, I think you have quite a heavy electrical load. The Midget/Sprite generator is good for maybe 20 amps, and that's at some decent engine speed. This is not much above your lights-on load. At idle, it can't come close to providing the current needed for all the electrical equipment you have. It should handle the load OK at speed, though. With this load, unless you have some long periods of running, the battery never gets completely charged, so it slowly discharges. Lead-acid batteries don't like sitting partially charged for a long time, and they can deteriorate under these conditions. I always charge mine, with an external intelligent charger, between weekend romps.

It's possible that the tach problem comes from a low system voltage from an undercharged or defective battery. It's also possible that some of the electronic parts in the tach are getting soft--things like the little tantalum capacitors do have a limited lifetime.
 
Brian,
No harm comes from disconnectiong the battery when the motor is running. BTDT. It works to diagnose a "no-output" condition of the generator or alternator.
We used to jump start our autoX MGA and run it without a battery in the 70s, but if you stalled on the course, it was embarassing.

Glen Byrns
 
Glen, I don't know for sure if generators are significantly different enough to do that, but I do know disconnecting batteries on alternator equipped cars is a very bad idea. Since alternators (like generators) have residual magnetism and will generate voltage even if the regulator turns it "off", the battery needs to be there to load the output, or it can go significantly higher than 12 volts. Maybe since generators don't have lots of power to deliver you can get away with it, but still not a "good idea". There's a big warning label under the hood of my Midget specifically warning never to run with the battery disconnected, and I know a friend who burned out several light bulbs on his motorcycle when he tried the same thing. They got very bright and died once the engine started running.
 
I think it's entirely possible that you might be able to run a generator-equipped car without a battery, but not an alternator-equipped one. It might happen that some generator-equipped cars can do this; others, no. As I said, I've never tried this, and don't intend to. I'll leave this experiment for other people to perform. Dave's explanation seems entirely plausible to me.

It is true that the residual field magnetism in LBC generators is pretty weak, but still strong enough to get the generator running from a cold start. (That's why you have to "repolarize" the generator when you switch from positive to negative ground--to reverse the residual field.) In some kinds of generators it might be fairly high, though.
 
What they said about alternator disconnection is true. DO NOT do it with the engine running.

Generator, however can be unhooked, and it won't harm anything, other than a spark if you remove the output wire with the engine running. Difficulty in diagnosing this way. Is it's just a quick field check to see if the generator system is charging. Possible the regulator is set too low, or the component users are draining more amperage than the generator can produce.

Battery charging voltage needs to be minimum of 12.8 volts with a maximum of 14.6(on a generator vehicle). If you do energize the field for quick voltage check, make sure you watch the voltage scale to see a quick rise in voltage, and do NOT hold the connection for more than a couple of seconds, or when the voltage climbs above 16 volts.

With the fan and lights on, and if you have an electronic ignition, I suspect the amperage output of your generator is insufficient.
 
By the way, this is my first british car. I've had it for all of 3 weeks and I'm car dumb.

Update. Sort of.

Working on my wife's car sucked all my time away (bad car day yesterday). So, I didn't get a chance to test the car, yet.

But, there's more strange happenings since changing the battery.

- With the new battery, the car would not idle at all (even badly) unless the choke was pulled 1/2 to 2/3 of the way out.

There's a small stretch of 55 mph on my way home. After it got ran at this faster speed, the car felt like it had more power than it ever has and the idle was back to normal (still needs to be addressed, see above). I mean I was pulling away from traffic when I usually am reving it pretty good to keep up.

- Also, my high beam indicator started working for the first time since owning this car.

All this from a new battery?

B
 
I think you have a couple of problems here. The idle problem is almost certainly carburetor adjustment, or some other carb problem. Second, it seems likely that your charging system is not working right, and you need to go through the standard generator and regulator checks, and, of course, make sure the battery is good. Time to get a good shop manual, if you don't have one yet--all this is in the manual.
 
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