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Discussion Starter · #1 ·
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.
 

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Discussion Starter · #2 ·
Well, it does indeed work.

Finished board:



and bottom:



I need to pick up some spray varnish to waterproof it tomorrow and then it can go back in the car.

Jeff.
 

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Coolant warning light

Hello Jeff, my coolant warning light is burning all the time (ignition on). The level is fine and cooling well. What can you recommend me to check first, second, .... please?
Silver Shadow 1975

Thanks, Hemmo





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coolant level

First thing i would check is the probe in header tank as these can get gunged up and cause light to come on . If you disconnect wires 2 small nuts 6 ba (if memory serves me right) taking great care not to drop, the light should come on. if you touch wire together light should go out. probe works by using the coolant to make circuit . if you do above it will show you whether this part of circuit is ok.
 

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Discussion Starter · #5 ·
Hemmo,

The coolant probe should connect the sensor transistor to ground, causing the light to go out. In order of easiest things to check:

Pull back the rubber boot over the coolant probe in the centre of the header tank and see if the resistance is low between the two terminals (it should be on the order of ohms or perhaps Kohms, but not Mohms.

One of the terminals should go back to ground, so next check that you have a good connection between one of the terminals and ground. (The other terminal should be wired to the transistor in the amplifier.)

Assuming all that is good, next check the signal at the right-hand toeboard sockets. (It's socket H, pin 6 from inside the car for a Series II.)

Next remove the toproll and look at the circuit board behind the clock. You should see two of the little off-white boxes: the left-hand one is the ice warning amp and the right-hand one is the coolant level amp. (Again, on a Series II. I think the Series I had the board turned 90 degees.)

Pull the coolant amp out (rock it back and forth a bit). There are three rows of contacts on the bottom: one at 90 degrees to the rest (that's ground on a Series II), one in the middle by itself (that's the feed to the lamp), and two together (one is 12V and the other is the signal from the probe). So of the two together, you should get 12V on one, and pretty close to ground on the other.

Cheers,
Jeff.
 
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