Electrical Troubleshooting

Delco 50dn 220 to 300 Amp Alternator

Trouble Shooting

To test it to see if it is working you can bypass the regulator and all the switches. You will need a Volt Meter (digital would be best) and a Jumper Wire, preferably a larger gauge such as a 16, 14, or 12 would be safer (If you use a very light jumper such as a 18 gauge or smaller it might get hot in a short time and burn as you hold it to the terminals especially if it there is an internal short in the alternator). There are two major differences on the types of this alternator, some are Air Cooled, such as on most all the older Eagle buses, and a few were on some of the early GMs and the early MCIs. The housing on the opposite end of the belt drive or gear drive (depending on which vehicle you are working on) the housing will have the heavy NEG and POS posts located on the side of the round housing. The NEG will usually have a strap connected from the post to the case. The POS will be insulated and not grounded. On an Oil Cooled alternator, such as is on the MOST (but not all) of the old GM and MCI buses, the POS post is located on a step out hump on the back end of the alternator. (IF you have any doubts about which cable is which, use a volt meter to verify which one is the POS. Carefully put a jumper wire direct FROM the large 12v (or 24v) POS (big as your thumb cable) TO the FIELD terminal momentarily with the engine running. MAKE SURE that you do not jump to the GND or the RELAY terminals. What you should see is a "Soft Spark" and at the same time you will hear the engine slow down from the "LOAD" you have just put on the alternator.

CAUTION: This is only a momentary test!!
If you leave it on too long it can burn up the alternator by heating up the bearings, diodes, stator, and the field coil inside the alternator and overcharge the batteries possibly causing a the batteries to explode. What you are doing is making the alternator put out about 18 or more volts(on a 12 volt system or 27 to 32 volts on a 24 volt system). It SHOULD not harm it for a very SHORT momentary test.
SOFT SPARK with HIGH Voltage
IF you hear the engine slow down under the load, and have a soft spark (Blue or light yellow but NOT a hot loud crack) and you get a voltage reading above 16 volts (on a 12 volt system or 27 volts on a 24 volt system) the problem is more than likely the Voltage Regulator, a Relay, a faulty ground, a Circuit Breaker or loss of continuity in a wire loom. To isolate the Voltage Regulator from the possibility of being the problem, it has 3 wires going to it. The center post is the FIELD. Of the outer 2 post, one will be the GND, the opposite one will be the POSITIVE. Depending on how you are viewing the regulator, IF it is orientated in the vertical position (Tall) and the Hole with a HEX HEAD cover cap for the ADJUSTING SCREW is above the terminals, then the left outside terminal will be the GND, and the opposing outside terminal on the right will be the POS. If there is any doubt, test with your meter to insure which one is POS. You can jump across the FIELD (center) to the POS. If the system starts charging, the problem is NOT the REGULATOR but one of the 3 circuits has a fault.
SOFT SPARK with LOW Voltage
IF you hear the engine slow down under the load, and have a soft spark (Blue or light yellow but NOT a hot loud crack) and you get a voltage reading below 16 volts (on a 12 volt system or 27 volts on a 24 volt system) BUT you do get a noticeable increase in volts, the problem is more than likely 1 (one) or more of the 6 Diodes are blown.
SOFT SPARK with NO Voltage deviation
IF you do not hear the engine slow down under a load, and you do not get an indication of higher output on the volt meter, then the problem could be a bad Rotor.
HOT SPARK:
This could be caused by a direct short from a internal jumper wire shorted due to faulty insulation or it could a blown or shorted diode or a shorted out Stator.
NO SPARK:
IF no spark then a lack of continuity, a bad Field Coil, a bad Stator, burned out BUSS BAR, several Diodes blown or some other problem.

A faulty alternator should be repaired only by a repair facility that has experience in working on the 50dn series. IF THEY DO NOT HAVE THE Manual or have not worked on one they can do serious damage that could break some parts of the housings, bus bars, insulators that are not readily available with out incurring some overnight air freight charges causing an expensive delay in repairs. Case in point, a customer dropped on his Neihoff alternator to a repair facility NOT familiar with the unit to replace the bearings. When the customer went back to get his alternator, the repair person made the following statement: There is something wrong inside this alternator that can no longer be repaired. After bringing the unit to us, when the unit was inspected, the entire inside of the housing had been destroyed by the repair facility that was not familiar with the unit when they had attempted to press out the bearings with out totally disassembling all that was necessary prior to pressing on the bearings. The particular alternator common to the later Eagles cost approximately $3000.00. And as sad as it is, the repairman was correct in his statement.

Preventive Maintenance

To test the bearings you can use a long (24 inch more or less) stick, iron rod, broom handle, long screw driver or what have you. With the engine running at idle, wrap your fingers around one end of the stick with about 1 inch of the stick exposed on the end where your thumb is and cover the end with your thumb with the knuckle bent as tight as you can comfortably holding pressure against the stick with the thumb. (You can purchase a stethoscope for automotive diagnosis or a electronic listing device with ear phones to perform this test also) CAREFULLY place the opposite end of the stick on the end of the housing close to the pulley and place your ear on the knuckle of the thumb. You should hear a whine or a fine smooth noise as the alternator is spinning. You can go to the middle of the housing and perform the same test. It will be difficult to distinguish between the two bearings, but it may be possible. If the noise that you hear sounds ROUGH, or has a clicking with a odd rhythm, you should have the alternator checked out by a trusted repair facility to confirm your diagnosis.
This would also be a good time to check the bearings in the Pillow Block under the Alternator. It has two bearings in it also.
Do not allow the stick to get into any rotating shafts, blades, belts.
You can do a further test by shutting off the engine, loosen and remove the fan belts and carefully rotating (KEEP your fingers out of the cooling blades on the pulley) and listen to the bearing again with the sounding rod. It should be easier to determine if it is in the alternator with the engine not running while you make this test. While the belts are loose, check them for gaps on the inside surface of the belt. A gap of the rubber on a belt could be a source of an unusual noise while listening on the stick during the engine on portion of the above test and it also should be replaced prior to reinstallation.

HINTS

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