Eletrcial problem

[Frederik]

Hi,

After having an issue with my alternator lamp is took apart the fan and checked all cables on the alternator, I put it back and started the car, the light was still present, increased when I raised the rpm's. Wiped all cables and put them back in place, suddenly the car will not start and the alternator light is one even though the key is out! I did change the alternator regulator after looking in the repair book, still the same!!

Any tips ?

Many thanks in advance

[FrayAdjacent911]

I don't have a suggestion, but I would comment, you may want to give a little detail about the car... what year and model, and maybe what engine.

Most people put it in their signature

[304065]

Open alternator diode. Have alternator rebuilt. Doesn't matter what year, car or even make, for that matter.

[Frederik]

Many Thanks!

I will update my profile, 911 Targa 2,7 Ltr, 1977

[seventyeight911]

You cant have an open diode; it either works or it is burned out.

[304065]

All right, since we have an EE here on the thread, you have one or more diodes in the alternator that have failed allowing current to flow as follows: From the battery positive terminal through the large black cable to the starter; through the fat red wire to the B+ terminal on the alternator, through the busted output diode into the stator, where it then flows into the "trio" diodes and out through the D+/61 terminal, through the engine wire harness to the 14-pin connector, through the electrical console to the blue wire, out of the blue wire into the bulb holder on the oil pressure gauge, through the bulb (lighting it) and then through the red wire with black stripe to the ignition switch, then to ground.

But no, you can't have an open diode. There's no such thing. You do need a rebuilt alternator, though.

[Frederik]

Thanks for all your help, however being a Hotel & Hospitality Man and not a great engineer, could you try to explain how I change the diodes?! How do they look?

I used a lubricant spray (5-56) to wipe all the connectors in the alternator can it have short circuit there some where ? My oil pressure sender is broken, just found a replacement, can that have something to do with it ? I have put it back temporarily and the gauge went to the top...a small cable (red) was sticking out...

But the car started fine after I put back the alternator, fan and the belt and then suddenly died....

I have a SEV Marchal Alternator......

Thanks for all help and I must say this is a great site with great members!!

Greetings from southern Sweden!

[seventyeight911]

I think the problem is somewhere in the starter switch; here is my explanation. In order for the alternator to start charging, it has to have some sort of out side electrical input or "excitation" if you will. You might say, this is what the excitation or "trio" diodes are for but they will not excite the alternator initially i.e. when the car is just starting and has been off. So, the way this happens is when the key is switched on, the current flows through the ignition switch, through the bulb, and to the DF on the rotor of the alternator. This "excites" the magnetic field in the rotor allowing it to start producing a small amount of power. Power is then taken off of the stator and routed through the exciter diodes or "trio” through the voltage regulator and to the DF side of the rotor. as the power increases in the alternator, it will be regulated through the regulator and limit the among of power to the rotor, controlling the magnetic field strength and the voltage output. now when the exciter diodes are able to pass power to the rotor, it will also send 12 volts back to the light bulb (through the same wire because they are connected) canceling the power and the light will go out. When the alternator is not producing 12 volts, say 8 or 6 volts, the light will be dim, because only 4 or 6 volts are being canceled. Now, the only reason the light can stay on is if the alternator is not producing any power, not enough power, or if there is a short in the ignition switch somewhere. One might thing that it could be grounded somewhere but it is grounded at the alternator and any other ground would be a short circuit. The first thing to do is to see if the alternator is charging by using a voltmeter on the battery when the car is on. it should be producing about 14.5-14.7 volts. If it is, then the alternator is fine, if it is not, then you probably need a new alternator. Because the car will not start, and the light is on even when the key is out would lead me to the ignition switch. Now that we can all trace wires on a wiring diagram and be sarcastic, lets clarify a few more things. An open diode is a burned out one, which will not conduct no matter which way you apply current. A shorted diode will conduct current both directions and a properly working one will conduct when properly forward biased and thank god we have an engineer on the thread.

[autobonrun]

I wouldn't spend the time trying to replace a diode myself. Not worth the effort. I'd take the alternator to a local electrical shop and have them diagnose the problem and at the same time service it; basically the same suggestion as John. Once you go to the effort of removing the alternator, you might as well replace the brushes and if need be the bearings as well. I paid $99 for this job at one time.

Alternately (play on words here), you can buy a rebuilt unit for a couple hundred dollars and be done with it. However, before you do, it's still worth taking it to an electrical shop to have them test it under load and confirm it's bad. It is feasible that it is something other than the alternator but your description of a bulb that changes in brightness does point toward the charging system.

The bulb you mention works on a differential between the alternator charging voltage and the battery voltage. If the alternator is not charging, there will be a differential and the bulb will light.

Thie link I've attached is from the Triumph board but provides one of the better explainations I've seen on how alternators work.

I've replaced mine 3 times in the 21 years I've owned my car and each time I also replaced the voltage regulator as well. I don't think it's mandatory but I do it just in case whatever damaged the alternator also affected the VR.

I won't try to diagnose an alternator problem over the internet as there are several things that can cause it not to charge. As mentioned, one is a bad diode. A second is a broken wire to the brushes; and a third is a broken hold down spring that keeps the brush in place. Ask me how I know.

One other thing, put your battery on a charger if you have one so it will be at full capacity when you reinstall your alternator. It may be worth a trip to a local Sears to have them test your battery and recharge it as well if you have any doubt whatsoever that your battery is good. I like to eliminate as many possibilities as I can when troubleshooting electrical issues.

http://www.vtr.org/maintain/alternator-overview.shtml

[Mark McClure]

seventyeight911.........fantastic.........it should be read without breaks to the tune of "old McDonald had a Farm"

[randywebb]

when a diode burns out, isn't it "open?"

[seventyeight911]

Originally posted by seventyeight911
You cant have an open diode; it either works or it is burned out.

i made a dumb comment there...sorry=]

[304065]

Frederik, there's nothing wrong with your ignition switch, one of the diodes is shorted (not "open," that was the wrong word to use.)

I found the technical reference I was looking for. . .

http://www.pelicanparts.com/914/914QA/914Q_electrical_alternator.htm

[randywebb]

"it either works or it is burned out."


Well, yeh - when it burns out it is 'open' -- true?

[Frederik]

I have now removed the alternator and will try to get it of the fan housing, will return when the electrical shop has looked at it, once again thanks for all the tips!!

I'll be back !!

[randywebb]

Be careful withe the vanes - they break easily.