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I finally dug into my dashboard to figure out what was wrong with my coolant level warning light. (I had previously tested the bulb itself, which is fine.)
Turns out the whole warning light test circuit was missing its 12V feed. It still had the starter-engaged feed, which actually powers the warning circuit relays, so it looked like much of it worked -- but the coolant level amplifier and ice warning amplifiers weren't getting any power.
I hooked up a new 12V feed and immediately blew a fuse. I'm guessing a previous owner cut the feed somewhere because it was blowing fuses. I traced down the faulty component to the coolant level amplifier, which was shorting to ground. (The flyback diode in it was toasted.)
On advice from some helpful folks on the Aus RR forum, I'm going to replace the transistors on the board while I'm at it. There's a document by Stefan Knappe detailing his rebuild of a Series I amplifier (mine's a Series II) in the Aus RR Technical Documents section, but I'm using slightly different parts:
1) I'm replacing the flyback diode with a 1N4007.
2) I'm replacing the probe transistor (#1 in the Stefan's document) with a BC107 (same as the original part, which means I won't have to replace the 1Mohm resistor like Stefan did)
3) I'm replacing the two amplifier transistors (#2 and #3 in Stefan's document) with BC327's
Postage was more than the parts costs, so I got enough to do 3 ampliers. I'll probably do the ice warning one too if I'm successful with the coolant level.
One warning: my electronics skills are very stale (I switched to the digital side in the early '80s and then to software in the middle '80s). Before anyone follows my steps you should wait for me to post and say that it actually works.
-- Jeff.
Turns out the whole warning light test circuit was missing its 12V feed. It still had the starter-engaged feed, which actually powers the warning circuit relays, so it looked like much of it worked -- but the coolant level amplifier and ice warning amplifiers weren't getting any power.
I hooked up a new 12V feed and immediately blew a fuse. I'm guessing a previous owner cut the feed somewhere because it was blowing fuses. I traced down the faulty component to the coolant level amplifier, which was shorting to ground. (The flyback diode in it was toasted.)
On advice from some helpful folks on the Aus RR forum, I'm going to replace the transistors on the board while I'm at it. There's a document by Stefan Knappe detailing his rebuild of a Series I amplifier (mine's a Series II) in the Aus RR Technical Documents section, but I'm using slightly different parts:
1) I'm replacing the flyback diode with a 1N4007.
2) I'm replacing the probe transistor (#1 in the Stefan's document) with a BC107 (same as the original part, which means I won't have to replace the 1Mohm resistor like Stefan did)
3) I'm replacing the two amplifier transistors (#2 and #3 in Stefan's document) with BC327's
Postage was more than the parts costs, so I got enough to do 3 ampliers. I'll probably do the ice warning one too if I'm successful with the coolant level.
One warning: my electronics skills are very stale (I switched to the digital side in the early '80s and then to software in the middle '80s). Before anyone follows my steps you should wait for me to post and say that it actually works.
-- Jeff.