A bad bulb blows out alternators

Last year in mid October my Datsun 100A's alternator broke down. It stopped charging the battery. After I was left on the road a couple of times I checked the battery, which naturally was almost dried out. I refilled it, charged it with a home charger and tried again. Finally I was forced to admit that there's something wrong with the charging system of my car, so I took it to the local mechanic for repairs.

The mechanic took out the alternator, measured the voltage and said there was something wrong with the charging field i.e. the alternator was broken unfixable. He then went to the parts shop and brought back a new alternator, which in fact was not an original 100A alternator. The new alternator had an internal voltage regulator opposed to the external on the original hookup. The mechanic fitted the new alternator and left the old voltage regulator intact! At first he even connected the lamp wire to the wrong lead, so that the charge indicator would flash whenever I used my turn signal! I went back to the shop to get it fixed, and for three months the whole system worked like new.

Then one afternoon I was just about to back up from our school parking lot when I noticed that the charge indicator lit while the motor was running. Wisely enough I opened the hood and checked the alternator. It was so loose I could wiggle it with my bare hands. I drove immediately to the repair shop and had the mechanic take a look at it. At first he suspected it had just loosened from shaking, but when he pulled the alternator out, he was pretty shocked. One of the alternator attachment wings had just broken! And if that's not weird enough, he then had the wing REWELDED back onto the alternator! The alternator had a 12 month guarantee, and he goes and rewelds it!

I noticed after the charge indicator had lit, that my turn signal wasn't working and my fuel meter was showing empty. The mechanic refitted the broken alternator back, put a new fuse and checked the system. It was charging 14.1 volts as it should, but when I backed up out of the shop the charge indicator lit again! The mechanic's response to this? "The voltage regulator is probably broken, but the guarantee doesn't cover for it. The alternator still charges, so drive off!" Can you believe this guy?

Well I couldn't so I drove up to the parts shop where the alternator had been bought from. The owner there immediately said that the alternator would be replaced due to the material defect. So I picked up another alternator and drove back home. The next day I and my neighbor started working on the alternator problem. We pulled the old one out and fitted the new one. At this point I noticed the same flasher fuse had blown again, so I fitted a new one. The alternator seems to be working fine now, it charges 14.1 volts and the turn signal works. My neighbor goes back home and I go for a test drive. I back up from our yard and BOOM! The fuse is out and the charge indicator is lit again! God was I pissed at that moment!

You might already know what is blowing the fuse, but I didn't realize it at first, so read on. The next day I call my neighbor (who used to own my 100A earlier) and say that it's not working and it blows the fuse. At this time I realized that the alternator won't charge if the fuse is blown, but what the hell is blowing the fuse? The next Monday my neighbor brings me the electric schematic of the 100A and I start checking it out. A-ha! The voltage regulator is in fact a simple relay which goes off when a certain voltage is reached! That must be it! I tried taking the old voltage regulator off, but then the alternator wouldn't charge. I put a piece of tape in the relay to prevent it from connecting. No cheese. The fuse would blow anyway.

Finally I remove the whole voltage regulator and rewire the connections. The fuse still blows. At this point I was just about ready to sell my car to anyone who would pay more than $100 for it. BUT THEN one afternoon when I was leaving school the fuse blew again but for some reason, god knows why, I suddenly realized just what the hell was wrong with it! Thinking back to what we had done to my car in the beginning of the week. The fuse blew on Friday, and on Tuesday we had FIXED THE BACK UP GEAR! The back up had been unfunctional for a couple of months due to problems in the gear shifting. Now that the back up was working, the back up light shorted out the fuse! This I discovered from the schematic. The same 12V goes to the alternator, the turn signal relay and the back up light switch! BINGO! And when the back up lights connect the 12V to the ground, the fuse blows and the alternator stops charging. Why? Because the remote power is supplied to the alternator via the charge indicator, which's other lead just happens to be connected to the fuse that is blowing!

While being in ecstasy (not the drug) from figuring it all out, there was the final phase. Finding out which part of the system is shorting out. The known fact is that whenever there is something wrong with your car and you start searching for the fault, it never occurs. This was the case this time, too. But in a flash of heavenly enlightenment, pure luck or whatever you want to call it I managed to isolate the problem unknowingly. We checked the back up bulbs and they looked horrible. I took the right hand bulb out for comparison. The bulb was rusted inside and had some shit on it. I left it lying on my desk for a few days. Needless to say, the problem disappeared after removing the bulb. Then I checked if the bulb was OK, so I hooked it up on my 2A voltage source. The bulb is 10W so it should consume 1.2A but when I try it, it shorts out my voltage source!

After measuring its resistance it seems to be 10 times too small! So instead of 1.2 amperes a total of 24A is taken when back up lights are lit! That is a BIT over the 15 amperes the fuse can stand. I replaced the bulbs and the problem was fixed.

Now you are asking why the alternator broke down the first time the fuse was blown? When the alternator is revolving at 1500 RPM and charging 14.1V and suddenly you remove the remote power it technically tries to spin itself off from the attachment. That combined with incorrect installation of the alternator can lead to the failure described. The thing where the alternator is attached to is about 4 inches wide and the alternator attachment wings are about 4.1 inches apart. If you don't apply washers between the wings and the holster, one of the wings is to bend and finally crack under pressure. Don't worry, I'm not going to use the same mechanic for repairs again.

So what is the moral to this story? NEVER ever look for the most logical answer to your car problems. The problem can lie just about anywhere as in this case. Murphy's law is in effect even in car electric systems.

Teemu Kolehmainen

Message me at BobHewitt@Misterfixit.com

Back to Brother Bob's Home Page

Back to the index page (top level)