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My battery has been unexpectedly dying while left over night. I did the usual fuse tests, found a couple of non essential things, so removed the fuses, but the the problem persisted. I've just checked my alternator, it's giving out 13.5 volts, but doesn't hold this voltage under load. Changing the multi-meter to AC I see an AC voltage of 26 volts (ouch). So I guess this means one of the regulator diodes is broken and needs replacing, I think this could also explain the battery drain overnight. I'm currently disconnecting the negative lead for long periods of standing.
My question is, is it okay to drive the car like this for a couple of weeks, or will the AC out of the Alternator damage the electrics, battery and or other things?
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You sure you dont have a bad cell in the battery.
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That was my first thought, so I went out and treated the car to a nice sealed, maintenance free battery. Installed at the garage, started like strong, 5 minute drive home, 8 hours later over night, dead. A new battery shouldn't do that. Unless I was very unlucky to get a duff one, which I doubt.
There's still the 26AC volts to account for, what will that do, to the battery, and the electrical system in general. I need to run the car as it until the end of the month, which isn't far away, but, am I risking more (expensive) damage?
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"...am I risking more (expensive) damage?"
I know if my alternator/regulator were putting out 26v DC I wouldn't run the car. I've had a regulator failure in my 240 when the voltage spiked over 20 volts... The headlights went crazy bright and the wipers danced. Luckily it was brief and no damage done.
26v AC? Not sure what to make of that.
But if you see the headlights are brighter or the wipers moving fast I would not run the car. Don't want to fry the ECUs.
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I've had bad experiences with sealed maintenance free batteries in my cars made in the 90s including my current 940. They fare better with conventional batteries with the engine seems to exhibit less alternator load (i.e lighter rev-free engine). Seems to me that those sealed batteries need higher charging volts compared to conventional ones. I've had two alternators burned out within a few months of using sealed batteries. That was the time when I tried to be tech-savvy up-to-date with those new battery technologies. New cars that originally came with sealed batteries seem to have no such ill effect. I installed back the conventional ones and had been carefree all this while.
Regarding the AC my current 940 uses conventional battery with no any charging issue and still have AC reading when measured with voltmeter (about 15 volts) in AC mode. I don't think thats really AC. My cheap voltmeter could be reading pulsed DC and still treat that as an AC.
For your case I think your previous old battery died when it became aged along with your old worn alternator not providing enough charge to maintain it. Then after installing this new sealed battery your old alternator really knocked itself out in trying to maintain the charge. There could be an internal short within the alternator but what if the diode failed in open position?
Best to bring your car to the shop for thorough diagnosis. I think a car's charging system was designed to use components with same period technology.
Regards,
Amarin.
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are you sure that you have a good ground??
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Never had a bad alternator ground wire...especially an intermittent one...more than twice.
that said, overnight draining of the battery while sitting was never a symptom.
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Thanks for the reply. The meter I'm using the the cheapest possible meter you can buy, so it might not be reading the correctly. I might borrow a FLUKE from someone to test.
But the DC voltage isn't maintained under load either, dropping to 12.6 volts, I think the regulator should maintain it at a steady 14.2 volts.
So far the electrics are behaving normally.
I will see if I can test the regulator diodes.
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The voltage dropping under load is normal. I used to rebuild small aircraft alternators for a living, forty years ago, when they had separate regulators. I haven't had to mess with the one in my 960. I would follow the troubleshooting guidelines in a charging system manual. Sorry I can't be more helpful.
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Okay, so with the negative battery terminal disconnected, I put my meter into diode mode and placed the probes on the alternators B+ post and the casing, I got a reading of around 400mV coming back from the meter, I then reversed the probes and again I got around 400mV coming back from the meter. So does this mean my regulator needs replacing?
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I don't think thats a proper test. You're measuring the collective voltage drop across ALL diodes in the alternator. So you can't tell which one is defective/affected. On the same note I can't tell if your regulator needs replacing as it must be tested with the alternator working and engine ON (i.e testing its regulating function).
According to Bosch alternator Repair instruction (this is downloadable from the web - search for "Bosch external fan alternator repair book") you have to dismantle the alternator and separate the diode assembly out. This includes soldering off the wire connections from stator (the body windings) from the diode assembly. Then you would have to test the diodes one by one on the assembly. If any one defect found the entire diode assembly must be replaced. Its a meticulous work. Much easier to just replace the alternator.
A simple test (my own test - this not from official repair instruction above so I can't vouch for its validity) is to do a resistance reading between B+ terminal and alternator body (with battery disconnected). It should read infinity. At least this could tell if your alternator is draining the battery slowly.
Regards,
Amarin.
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Amarin, you wrote "A simple test (my own test - this not from official repair instruction above so I can't vouch for its validity) is to do a resistance reading between B+ terminal and alternator body (with battery disconnected). It should read infinity. At least this could tell if your alternator is draining the battery slowly."
Is this a reading across the diode rectifier bridge? If so, the reading should be infinity or a high resistance reading one way, with the red lead of the ohmmeter on the B+ and the black lead of the ohmmeter on the alternator body and near zero ohms when the leads are reversed. But, I have had a cheap ohmmeter that had the leads reversed inside the ohmmeter. So, near infinity one way and near zero with the leads reversed?
--
Mine:5-940s running, 2-740, 1-940 parts, dtrs:4-940s
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Touch red meter probe on B+ terminal and black meter probe on alternator body. Same way how the battery ought to be connected. Because you're testing for any current drain from B+ to ground. If the meter leads are internally reversed then test first on the car's battery (or any other battery - e.g. flashlight's) to get a positive 12v reading then label the one connected to the positive battery terminal as positive probe/red probe.
Regards,
Amarin.
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Thanks. I will try this later. By the way, I was able to borrow a FLUKE meter and you were spot on, the AC reading from the FLUKE came back as 0.0198 volts, while my cheap meter was reading 26 volts. So I am now confident that no AC is going into the system.
I just did a stress voltage test, under full load, main lights on, cooling fan on, air conditioning at max, radio on, the voltage came out at 12.05 at 2000 rpm.
Is that bad? or should the regulator hold is roughly at a constant voltage between somewhere between 13.8 to 14.8?
PS note to self. After testing the amps, be sure to remember to swap back the probes to volts before reading volts to prevent a short. That's what I just forgot. I now need to replace the Fuse in the FLUKE. Doh!
Thanks.
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Thats good news on the AC!
I wish I have my own FLUKE meter if not for its exorbitant price. I did shop for one before but after seeing the price was about 20x to 30x more than a normal meter I froze. Worth for industrial use rather than DIYs.
Regarding the stress voltage test it MUST be done with the battery fully charged. Several choices here - borrow a fully charged battery, get a new battery or have a long nice drive (an hour would suffice I think). Otherwise it would be difficult to tell if that low voltage is due to alternator still charging the battery OR alternator's own fault (i.e faulty regulator/short carbon brushes/worn slip rings).
While you could straight away swap in a known good regulator with long brushes and inspect the slip rings at the same time, the readout voltage could still be low if battery is not fully charged. And testing should be done using conventional battery. I can't tell of any test result using sealed battery simply because I don't use it. The voltage should read between 13.2v to 13.8v at idle. Maybe 14.2v at high rpms.
Regards,
Amarin.
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Yeah, I wish I had my own FLUKE two, but my 5 dollar meter has been good enough up until now :-)
Good tip. I've put the battery on charge, will see how long to takes to reach full.
Will update with results later.
All the best.
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Once you have the battery fully charged check its voltage. Might be around 13V. Turn on the headlights for a few seconds and check the battery again (this eliminates the "surface charge") and a good battery should read about 12.6 to 12.8 volts.
Start the engine and at idle with no headlights or other heavy load, you should see approx 14V (not less than 13.7V) if the alternator and regulator are functioning properly.
This is the way my 1983 240 with untouched original 70-amp Bosch alternator behaves.
--
Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), 98 S90 (recently sold) and 2010 XC60. Also '77 MGB and four old motorcycles
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