Anyone ever recalibrate their CC thermostat, and have issues with voltage to the transducer staying high? Trying to figure out why my vacuum to the servo won’t drop off when cooling, and troubleshooting with Tim’s book doesn’t go into details about what to do if you can’t obtain 6 - 6.5 volts to the transducer...that I can find.
Give Tim a call or send him an email.
Thanks Russ! Did he ever get a webpage back up?
I always just email him.
The voltage to the transducer is regulated by the "sensor string", which are the ambient sensor near the blower, in-car sensor in the dash, and duct sensor in the air duct plenum. In the 1964 shop manual, you'll find a chart (fig 13-27) which shows the resistance value for each of these sensors at any given temperature. Start by checking those values with an ohmmeter against the temperature of each sensor location. You didn't post the actual voltage reading you're seeing, but if it's working and close to the right voltage, then it's likely one of the three sensors. If all three are accurate, then it's time to check grounds, connections, and the resistance of the circuit, then the amplifier board.
I used the steps in Tim’s book to calibrate the temperature dial. His process requires disconnecting the duct sensor. When I read the voltage going to the transducer, I was reading 10-11 volts. I also checked the ground going to the servo, and it was good. I do not know where all of the points are on the rest of the car for the CC system grounds, I will check resistance on the two other sensors.
I don't know Tim's book's procedures, since I don't have a copy, but if the voltage is high, then one of the resistors in circuit is likely shorted (low resistance). I forgot to list the temp dial itself as a possible culprit, that could be just dirty, and the dirt could be allowing current to cross the teeth of the rheostat. I don't have a value chart for that, but the shop manual says that there should be 1500 ohms from the connection to ground with the dial at 75 degrees (unplug the green wire and measure from connector to body of amplifier, i.e., ground)
Individual sensors have proper resistance, and I performed a string test on the three sensors. All results were where they should be.
Like I said, I don't have tim's book, so i'm not familiar with the test, but if you're performing it correctly, it seems like either their is an open circuit inside the transducer, the amplifier ground is bad, or the amplifier transistors are shorted internally. is the transducer unplugged during this test? Is it supposed to be?
My response was in answer to your input on the “sensor string” and sensors. I am unable to test per the shop manual since I do not have the tester it specifies, and therefore must work with other means. Since my initial question referenced Tim’s book, perhaps someone knowledgeable can help.
so let's try this: with the control panel and the transducer both plugged in, test for voltage drop. set your multimeter to dc volts and touch the leads on the back of the circuit board (the one that you replaced in your earlier thread). Get a volt reading between the yellow and light green/ blue wires, then again between yellow and black and white, and finally between light green/ blue and black/ white.
Then, to test the ground, connect the positive lead of the voltmeter to any of the screws that hold the amplifier circuit board and the negative lead to a known good ground. If this reading is anything other than zero you have a bad ground to the dash control panel
Thank you Mark. That was very helpful, and resulted in 10V across all three wires. I will try to find the grounds for the CC system, and try to figure out which one is the culprit.
the ground that controls the transducer voltage is thru the 1/4 screws that hold the circuit board to the comfort control panel , and through the dial. the dial is a variable resistor going to ground, if this is open, then that's a possibility.
The yellow wire should have battery voltage, it comes directly from the fuse. The light green/ blue comes from battery voltage through the sensor string. It should vary slightly with changes in the dial. When the system is plugged in, there should be a couple of volts difference between the yellow and the light green/blue, because of the resistance of the sensors. If there isn't a difference, and you know your sensors are showing the right resistance, then there are only two options, really: the dial to control switch is not grounded or defective, so that there is no load on the light green/ blue wire, or the amplifier circuit board is bad.
The black and white wire goes to the transducer from the output of the large transistor on the board. If everything works, you should see your 6.5 volts here.
I remember that you got a board from Russ to replace the burnt one, do you have an extra dial and board to try? You can test the dial without the testing unit, by unplugging it and checking it for ohms between the connection and the body of the control switch. If its around 1500 ohms at a 75 deg setting, it's fine and the problem is in the amplifier or the ground to the amplifier. If there's no continuity, then try taking it off and cleaning where it mounts. Could be just bad contact, these things are old.
I'm curious to know what Tim says, if you ever do reach him.