| Re: Infamous Nichrome Alternator Wire [message #369715 is a reply to message #365955] |
Thu, 19 May 2022 21:33   |
 |
RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma:
|
Senior Member |
|
|
The GEN lamp circuit will only provide about 0.1 Amp of excitation current, whereas the 10.0 ohm nichrome wire will provide about 1 Amp of excitation current, so the alternator will come to life quickly.
Normally the GEN light will come ON if the alternator output is either a few volts above or below the battery current. With the Isolator in circuit on the motorhome, along with the voltage drop on the alternator output wire under heavy current flow, GM had complaints about the GEN light dimly glowing. So in later coaches they added a diode in series with the GEN light. This compensated for the voltage drop across the isolator and fixed the dim glow GEN light issue. The problem I see with this fix is if the alternator output voltage climbed above the battery voltage as would be the case when the Isolator failed OPEN, this diode would block the current back through the GEN light and the driver would be blissfully unaware of any problem. Meanwhile the high voltage from the alternator could be damaging electrical devices in the coach. They should have added a second diode in the opposite direction in order to give an indication of high voltage fault.
The same thing happens when you install a APC cable. The diode in the APC allows current to flow to the alternator when the alternator voltage is low. But it blocks current flow when the alternator voltage fails high so you are unaware of a problem. An external lamp in place of the nichrome wire will provide the desired higher excitation current and also provide a visual indication of a problem. I added two diodes (in parallel but wired back to back) in series with the lamp to compensate for the Isolator voltage drop (no dim glow). This way I have indication on this lamp of an alternator failure in both high and low voltage. If the alternator voltage goes too high, the lamp will be extremely bright and may burn out, but I should be able to see this bigger, brighter lamp during a failure.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
