my 240 is killing alternators!!

todds240 · Post by **todds240** » 23 May 2003, 09:22

ok, here's a question for someone who knows a lot about the charging/electrical system of a 1986 240.
What would be causing the charging system to damage the alternator? I believe it's burning out the diode/s. I'm on my third alternator, the first two I replaced myself and then I decided to try my local volvo mechanic, and after a week or so after installing a third one, it doesn't charge anymore. Actually, what happens is that when I cold start it, it's charging, but after several minutes of driving, it quits, for the rest of the day and then the next morning, it'll charge temporarily again. I'm confused and my mechanic doesn't believe me and just wants to put another alternator in, the fourth one!! I hope someone here can point me in the right direction. Thanks!
Todd

dpauto.com · Post by **dpauto.com** » 23 May 2003, 14:39

Make sure the ground at the alternator is good and tell your mechanic to do a voltage drop test (if he knows what that means) from the alternator to the battery.

Dave

240lover · Post by **240lover** » 23 May 2003, 16:17

could I do the voltage drop test myself? If so, how?

dpauto.com · Post by **dpauto.com** » 23 May 2003, 17:47

With a digital volt meter, you can first measure the charging voltage at the alternator. Then compare that voltage at the battery while the engine is running. Note the differences. If the differences is more than .5 of a volt, then you most likely have a voltage drop either on the positive or the negative side of the cables. That is when u need to do a voltage drop test.

To do a voltage drop test, you need to place the volt meter in series of the circuit, and that circuit has to be lived. For an example, if you want to find out how much voltage is being dropped from the alternator to the battery on the positve side, then you need to place the red lead of your volt meter at the alternator's charging post and the negative lead at the positive terminal at the battery. Whatever voltage that is being lost will show on your volt meter. You can do the same way on negative side. You can apply this method to any circuit at any time you think there is a voltage drop from a source to its component or components.

Dave

Guest · Post by **Guest** » 23 May 2003, 21:33

thanks, I'll give this a try on sunday and see what I find.

salty44 · Post by **salty44** » 13 Jun 2003, 10:43

I have a '85 240GL, which gave me fits. After 3 alternators, each of which charged well when installed, and stopped charging after driving awhile, then start working again for some unkown reason. It turned out to be the crumbling wires in the harness. I replaced the 4 wires going from the firewall plug to the (Alt, oil pressure, temp, and starter selenoid). The problem was fixed. It has been 3 months. All 3 alternators are good, and the charging system is working properly.