Defroster switch not working

[hippie_elmo]

My defroster switch on my 1973 280 seems to be not working I pull the switch out and then it turns on but when I let go it shuts off. Is this a bad switch or a bad relay.

Thanks for your help,

Asssuming you are refering to rear window, sounds like the timed relay. If you don't have a subsitute to plug in , you will need to trouble shoot with wiring diagram. Window is supposed to heat for only about 30 minutes before cutting off. (high amperage draw)

[hippie_elmo]

Originally posted by Turbo240
Asssuming you are refering to rear window, sounds like the timed relay. If you don't have a subsitute to plug in , you will need to trouble shoot with wiring diagram. Window is supposed to heat for only about 30 minutes before cutting off. (high amperage draw)

I was thinking about buying a new relay. Could it also be the switch?

Thanks,

[hippie_elmo]

Were is the relay located?

Relay is located under left side of dash near turnsignal flasher unit. It is the standard dark brown color is plastic, and has 5 pin connections. If you have a voltmeter or test lamp and want to troubleshoot it, the values are as follows. With the window switch off, terminals number 1,2,4,and 5 should be 0 volt. Terminal number 3 should be 12 volts. With the window switched on, terminals 1,2, and 3 should read 12 volts, and terminals 4 and 5 will read 0 volts. This is a transistorized relay with printed circuit, and is probably your problem. I would check the above mentioned voltages before replacing it however. You should also check for continuity with an ohmmeter, the orange oxide lines that go across the rear window, sometimes they get scratched and lose continuity. They are tin oxide, and get warm when current is applied to them.

Hope this is of some help.

Turbo240

[hippie_elmo]

Could this problem be in the switch?

To check the switch, first check the power source, (fuse # 15,16 amp fuse.) Check switch with test light or voltmeter,#2 pin should have 12 volts at all times, switch in on position, #1 pin should have 12 volts applied. You can accomplish these voltage measurements by partially unplugging harness plug from switch and using your voltmeter or test light. Lighted knob for switch should also glow when system is on.

Turbo240

[hippie_elmo]

I checked the system today. Found that switch is bad. Will have to order a new one. It costs $92.00.

[hippie_elmo]

Originally posted by Turbo240
Relay is located under left side of dash near turnsignal flasher unit. It is the standard dark brown color is plastic, and has 5 pin connections. If you have a voltmeter or test lamp and want to troubleshoot it, the values are as follows. With the window switch off, terminals number 1,2,4,and 5 should be 0 volt. Terminal number 3 should be 12 volts. With the window switched on, terminals 1,2, and 3 should read 12 volts, and terminals 4 and 5 will read 0 volts. This is a transistorized relay with printed circuit, and is probably your problem. I would check the above mentioned voltages before replacing it however. You should also check for continuity with an ohmmeter, the orange oxide lines that go across the rear window, sometimes they get scratched and lose continuity. They are tin oxide, and get warm when current is applied to them.

Hope this is of some help.

Turbo240

How do you take the pins out that hold the under dash cover on?

Very carefully with a small screwdriver or other flat instrument to pry with. Insert under the small round head that you see holding the under dash panel in place and pry straight down. these clips are made of a spring type steel that expand after they are inserted. A "V" type instrument like on the head of a carpenters hammer is available or you can improvise with an old screwdriver if you want.

Turbo 240

[hippie_elmo]

I saw in the manual that when you pull the switch out it stays out for the 30min that it is on. Is this right?

Correct window heats for approx. 30 minutes then shuts its self off. this is a functio of the time delay relay under the dash. I believe switch only controls relay, switch does not stay out for 30 minutes. The knob on the switch has a bulb inside that stays lit and lights the white dot in the center of the knob when window is in the heat mode. Check the switch function with a voltmter or test light using the values in my previous post.

Rob

[hippie_elmo]

Took off the under dash cover to find the relay today. While I was unscrewing the screw that holds the relay on the metal stand the screw broke. I was able to get the screw out and the relay. I have found out that the relay is also the switch that turns it off after 30 min. New one costed $60.00!!!!

One of the joys of Mercedes ownership. $$$$$$$$$$$$$