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Aternator/Exciter Wire

Dale Konkol

New member
Joined
Sep 16, 2016
Location
Madison, WI
Having an issue with alternator not charging.
Sorry if this has been covered elsewhere but could not find solution searching.
Issue might be two-fold. Not sure.
Dash battery light comes on when in ignition position number 2. It didn't at first when I started investigating problem, so I disconnected the wire at the D+ terminal at the alternator and jumped a connection to that exciter wire from the + battery. The dash battery indicator lights up when I did so. After that, the battery dash light will now come on after reconnecting the wire to D+ terminal. Alternator will still not charge after starting car. Dash battery indicator goes out after starting car. Battery is good and (more or less) fully charged.
So I decided to jumper directly from the battery + to the D+ terminal. Start car and the alternator does then work and is putting out 13.5 volts. But.....
It will only do so for about 10-12 seconds, then stops charging.
It will do this every time I try. It will charge for only that short period of time then stops.
This is will only happen if I jumper to the D+ terminal. No jumper, the car will not charge at all.
Anybody have any thoughts as to what I should be searching for?
Thanks for any input.
Dale
 
The D+ connection acts as a bootstrap to excite the alternator field winding on initial start up. Once the alternator starts working the voltage regulator takes over to provide the field current. Sometimes alternators have enough residual magnetic field in them to start up without the D+ connection; but, most of them won't.

Do you have a service manual? If so, check the procedure for testing the voltage regulator because the fact that you can get the alternator to start working and then it quits seems like you have a problem with the voltage regulator. Unfortunately, most internal voltage regulator tests consist of 'test by replacement'. Brushes that are worn out can also cause the alternator to fail to bootstrap; but, in that case you typically would not get the initial voltage rise and then drop off that you are experiencing.

Replacement voltage regulators (which include new brushes) run just slightly less than the cost of a rebuilt alternator from Rock Auto making for a less than obvious repair decision. You probably don't want to try the test by replacement on the regulator unless you are pretty confident that the regulator is the problem. Replacement brushes are fairly inexpensive, about $5; but, require some soldering skills. When you pull the voltage regulator / brush holder to examine it should be readily apparent whether the brushes are toast.
 
Thanks for reply.
I'm going to check what the voltage is at the exciter wire with it disconnected from D+ terminal with engine off/ignition position 2 and also with engine running.
I will do the same two measurements with exciter wire connected.
I'm just wondering why I can only get the alternator to start charging only if I jump from battery positive. Again, it will only charge for 10 seconds or so.
I just have the feeling I will have to replace the alt.
This problem has just come up. No issues in past.
 
Normally, the alternator field just requires a small amount of current through the D+ terminal to build a little field and then the alternator starts to generate a little voltage which it uses to supply current to the field winding and the alternator voltage builds quickly to the point where it can supply all of its field current internally via the internal voltage regulator. The little light bulb in the charge warning indicator actually acts as a current limiting resistance during the boot strap process. That's why if the light bulb burns out or you have a bad connection in the circuit, the alternator will not 'boot'.

The alternator internally supplies power to the voltage regulator through the D+ terminal. There are some internal isolation diodes in that circuit and if those isolation diodes are fried or there is a bad connection the alternator cannot pull its voltage up and nothing happens. However, when you apply +12 v to the D+ terminal you temporarily eliminate the need for the supply through those diodes and the alternator should be able to crank out whatever the regulator set point voltage is just fine as long as 12v is connected to D+. The fact that the voltage decays after the alternator starts up even with D+ connected to 12 v keeps pointing a finger at the voltage regulator.

My 1987 745 turbo had the miserable biodegradable Volvo wiring problem. This cause all kinds of weird and random electrical problems until I put in a replacement engine harness. Had Volvo fixed this problem by 1990? If not you might want to check the external wiring to the alternator - the D+ connection might not be doing what it is supposed to be doing. You said in your first post
Dash battery light comes on when in ignition position number 2. It didn't at first when I started investigating problem, so I disconnected the wire at the D+ terminal at the alternator and jumped a connection to that exciter wire from the + battery. The dash battery indicator lights up when I did so
.
If the ignition switch was on when you did that test the light should not come on because there is +12v on both sides of the light, no voltage difference hence no current flow. If the ignition switch was off then there was no connection to one side of the dash light and again it should not light. However, the dash light may light up in these circumstances if there is a short to ground in the circuit. A short to ground in this circuit could also have fried those internal isolation diodes and might have damaged the regulator
 
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