Old lurker with an old electrical question...

[Mental]

I expected a search for MG Electrical System would lock up my computer. But it came up with nothing.

Jokes aside, I have had a 72 Midget for about 10 months now.

I had the car for about 2 weeks when the alt light started coming on dimly and then the car started loosing charge. I replaced the alternator and the light went off. But the car was only good for three days of use, then dead battery.

I have changed the battery twice. The same problem. No charging light, but three days the car won;t start. Put it on a trickle charger overnight and I get the same use out of it. If it sits for 2 weeks, it will start, but three days of use and it is no good.

After the last battery swap I got about 5 days out of it, but it did the same thing. I can bump it when it does this, and that will get me about 20 more miles before it quits completely.

I cleaned up the ground off the battery and the main ground on the passenger fender (headlights weren't working)

Anyway, 72 Midget, US Spec, no idea how many miles, bad respray sometime ago, some creative wiring in there, but obviously I am missing something. I am a competent parts replacer and sometimes get lucky on diagnostics, but I am just not that good. This is a negative ground car.

I am looking for some tips and some directions to run this little gremlin down. I am sure I am leaving out info you smarter folks need, so ask me and I will do what I can to answer. I only ask you be patient with my lack of knowledge.

I get that is a 38 year old car with odd electrics, I just want to be able to drive it.

I thank all of you for your time.

 

[14dna]

Have you changed the alt?
I have had diodes go bad in the alt and drain a battery.
Results from an AVR test might be helpful.
Just curious, have you checked the charge bulb to see if it is OK?

Hope this helps

Dave

 

[Mental]

I did replace the alternator, but I should get it tested again. Thanks.

I have to ask what an AVR test is.

The light does work, at least the bulb does, it comes on when I start the car and before this last battery replacement it would sometimes come on dimly

 

[JPSmit]

Do you have a multimeter? With the car idling at about 1200 rpm, you have about 14.4 volts at the battery - it it is registering 12 volts or less, the battery isn't being charged and the problem is the alternator.

If you are getting 14.4 volts, then you likely have a short somewhere and the changing of the alternator is coincidental. This is my guess BTW since it sounds like the bulb is now going out.

BTW check your fan belt before you do anything else - a loose belt could easily cause what you are describing.

 

[Bob_s]

this is where a competent shop and only a few bucks will set you in the right direction to repair it yourself..bring the car with a fully charged battery and have the charge system and battery tested, they can check amp draw, cca in batt, alt diodes,etc in a matter of minutes, give you a printout and then you can fix the real problem instead of a lot of guessing.

 

[JodyFKerr]

For many tracing electrical faults and gremlins can seem to be never ending!

Diagnostics, though, are relatively simple, if painstaking. Your best friend for this is a good multimeter, and I don't mean one of the cheapo harbor freight deals.

You've got what's commonly referred to as a vampiric drain. Based on the time period (~3 days) I'd feel safe to guess that it's not an alternator solenoid, but right now all bets are off.

Step one is to figure out what the Amperage draw off the battery is when the car is turned off. Big draw (2+ amps) is likely to be somewhere in the starting circuit. Smaller draw can be anywhere in the wiring harness.

Work your way forward on the harness away from the battery. Every time a line splits off from the primary power supply, test it for amperage draw. You'll wind up figuring which circuits it's not and finish with what circuit it is (unless you're lucky and find it easily).

What I like to do is to disconnect the major components and connections as I go through the harness, clean the connections and reconnect, covering with vaseline or dielectric grease, your choice. If you find any dodgy splices, etc, take them apart and fix them properly. Also, clean up your grounding straps. They are often overlooked as issue sources when diagnosing electrical problems.

The wiring harness on the midget is simple enough that you should be able to do this in an afternoon on a freshly charged battery (tip, take it to Autozone and they'll charge it in an hour for free).

Also, any time I check a wiring harness the first thing I look for is electrical tape. 9 times out of 10 it's a bad PO bodge that is the source of your problems.

Jody

 

[JodyFKerr]

Forgot to add Stupid Electrical Diagnostic Trick #42:

Read the amperage draw while someone pulls the fuses one by one. If your Amperage draw dissapears when a fuse is pulled then you know the issue is in the circuit after said fuse.

Jody

 

[DrEntropy]

You can see it with a 12V test light seriesed between the battery post and ground. Pull fuses and when the light goes out, you have the feed the draw is tied into. Then a matter of replacing that fuse and pulling leads from each device, one at a time.

 

[Deleted member 8987]

Reading all the posts in this thread, if you canb get it started (or bumped) and it runs for a while then dies, I don't think it's a drain.
If it was a drain and the new alternator was charging, you'd run all day and into next week.

Even though the light is out, my suspicion is a bad alternator.
Just yank it and have it tested.

Diode, yes, more than likely, as that will drain it while sitting, too.

First Rule of automotive repair states:

"Just because it's a new part doesn't mean it's a good part"

 

[Mental]

For many tracing electrical faults and gremlins can seem to be never ending!

Diagnostics, though, are relatively simple, if painstaking. Your best friend for this is a good multimeter, and I don't mean one of the cheapo harbor freight deals.

You've got what's commonly referred to as a vampiric drain. Based on the time period (~3 days) I'd feel safe to guess that it's not an alternator solenoid, but right now all bets are off.

Step one is to figure out what the Amperage draw off the battery is when the car is turned off. Big draw (2+ amps) is likely to be somewhere in the starting circuit. Smaller draw can be anywhere in the wiring harness.

Work your way forward on the harness away from the battery. Every time a line splits off from the primary power supply, test it for amperage draw. You'll wind up figuring which circuits it's not and finish with what circuit it is (unless you're lucky and find it easily).

What I like to do is to disconnect the major components and connections as I go through the harness, clean the connections and reconnect, covering with vaseline or dielectric grease, your choice. If you find any dodgy splices, etc, take them apart and fix them properly. Also, clean up your grounding straps. They are often overlooked as issue sources when diagnosing electrical problems.

The wiring harness on the midget is simple enough that you should be able to do this in an afternoon on a freshly charged battery (tip, take it to Autozone and they'll charge it in an hour for free).

Also, any time I check a wiring harness the first thing I look for is electrical tape. 9 times out of 10 it's a bad PO bodge that is the source of your problems.

Jody

We had done a half effort on this which led to a cleaning of all the grounds and probably solved other problems as well

I did get a good multi-meter and a buddy who actually knows how to uses it (2 of them really)

Do you have a multimeter? With the car idling at about 1200 rpm, you have about 14.4 volts at the battery - it it is registering 12 volts or less, the battery isn't being charged and the problem is the alternator.

If you are getting 14.4 volts, then you likely have a short somewhere and the changing of the alternator is coincidental. This is my guess BTW since it sounds like the bulb is now going out.

BTW check your fan belt before you do anything else - a loose belt could easily cause what you are describing.

We did this. I am embarrased to say I think this is the culprit. At idle now I get 12.3 is at the battery but when the engine id revved a little is in high 13s. Previously the belt was slipping a bit and I didn't think much of it, at freeway speeds I couldn't hear it with all the other noise. But the alt light would come on during extended high rpm 4th gear driving.

The weather here has not co-oprated enough for me to test this past the 3 day situation, but if that doesn't work I will probably come back here.

Thanks for all the advice, I know I could take it to a shop, but that seems counter to the spirit of these cars. Not to say I won't if it drives me crazy enough.

 

[JPSmit]

Thanks for all the advice, I know I could take it to a shop, but that seems counter to the spirit of these cars. Not to say I won't if it drives me crazy enough.

it is counter to the spirit and there is a Ton O wisdom around here - glad it is working out.