Electrical Gremlins Anyone? - Mercedes-Benz Forum

 
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post #1 of 8 (permalink) Old 04-12-2010, 07:33 PM Thread Starter
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Electrical Gremlins Anyone?

Hello all,

I've had some problems with the 1975 240D. When I got the car the blower fan didn't work . I also noticed after dark that the dash lights didn't work. It seems weird that they were all burnt out, but possible. The Chilton's manual is no help in these wiring diagrams, it's not detailed enough. I've checked the fuses under the hood and all are good. I've tried to take out the heater control, but I can't seem to get it out. I see two screws on the left and right behind the air vent and one in the middle. I've got the left and right ones out, but the middle one just turns, I'll get it out, but am I on the right track?. I'd like to get to the switch and take some measurements.

I started pulling gently on the speedometer cluster to try and get at the bulbs, but it only comes out so far. the right side seems to come out further that the left, but not far enough to see any bulbs. Anybody got any tips for this?

Thanks in advance.

Jason
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post #2 of 8 (permalink) Old 04-13-2010, 08:11 AM
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Vehicle: 1975 240d w115.117
Location: Galveston Texas
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I was in the middle of writing an extensive response, and the computer decided to reload the page, losing my paragraphs.
The bulbs can be taken out by reaching your hand in, if you have clearance, and pressing down on the bulbs tab and turning them. They should then pop right out. If you dont have enough clearance to do this, detach the rounded plug, speedometer cable, and oil pressure cable. Take the instrument cluster out and you can see the back of the cluster has round black prominences with tabs attached. These are your bulbs. If they happen to not be burnt out, then the dimmer control knob is your problem, located on the bottom left of the front of the instrument cluster. I am not sure how my car electrician fixed it, but he must have bypassed it somehow and soldiered a wire. These commonly get loose over time and cause a short. Mine used to flicker on and off. If you bypass the dimmer, your lights will be bright all the time, which is better than having no lights at all.
I am not farmiliar with the heater control.
Hope this helps you!
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post #3 of 8 (permalink) Old 04-13-2010, 06:33 PM Thread Starter
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Great, thanks for the response. I never thought of the dimmer control, but it makes perfect sense. The dimmer varies the resistance in the circuit, if the dimmer is bad and is an open circuit, the bulbs get no current. Soldering a wire over the dimmer pot will do as you say, you'll lose the ability to dim, but thay will will at least work.

Again, thank you for the response, hopefully somebody will have an idea on the blower motor.

I suppose my next step there is to check the motor itself, if it has a bad ground, that would act as an open circuit and keep it from working.
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post #4 of 8 (permalink) Old 04-14-2010, 10:17 PM
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i had the same problems on my 115. the dimmer wasn't to blame, but 9 times out of 10 they are so i'd start there. as for the blower motor, hopefully the motor itself isn't bad--those things are a bear on the 114/115. the whole dash has to come out. so for now, take your windshield wiper arms off and pop your hood, and remove the four (or six) pins that hold the cowl cover on. the center of the pins push through--you'll probably lose one or two of them but will be able to fish most of them out of the air inlet after it comes off. after pushing the pins through, the remaining portion of the fastener comes out (towards you), much like the glove box clips if you've ever had that out. once the cowl is off (careful--dont bend it) see if the fan spins. the bearings can be completely shot and the fan will still spin, mine's worked like that for quite a while. if the fan freewheels spinning by hand, folllow the wires to the plug on the firewall next to the heater selector valves, right above no 4 cylinder for a 240 or in my case no. 5. check all pins for voltage with the multimeter grounded to the battery with the defrost switch on I, II and III. also, check the blower resistor pack (big wirewound resistors under the cowl--cant miss it). i went through all this only to find that some kind of switch that shuts off the defrost blower when the AC blower comes on was stuck. it's by one of the control rods, under the dash, right above the fuel pedal. check this first since it's the easiest, and last, after everything else, since jiggling things under the cowl may solve half of the problem. let us know how it goes...there are a few other tricks to try. and pray to the god(s) of your choice that it's not the blower itself.

'83 300DT -- 284K (rust casualty-gone in 2009)
'76 W115 300D -- spun bearing at 400K+, RIP 10/31/12
'78 300CD -- 159K - sold or crushed by ex (cringe)
'83 240D -- 162K - totalled (not by me) (parts car)
'75 240D -- 91K - clean enough to eat off the frame
'85 500SEL Euro -- 196K - why not?

'85 300DT --
03/16/10 - 243K
05/30/14 - 381K (daily driver - it won't die!)
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post #5 of 8 (permalink) Old 04-15-2010, 11:15 AM
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This helps for me too! My car was underwater, so i'm pretty sure the blower motor is shot. Will be quite a project getting it fixed, but not immediate. I would rather handle the heat of a texas summer, at least for a little while lol.
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post #6 of 8 (permalink) Old 04-15-2010, 06:29 PM Thread Starter
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Wow, thank you for the detailed response. I took a few more pieces off under the dash, and found no motor. My next step was to take off the cowl, as that's the only place the motor could be. I've seen the plug on the firewall, I wish I would have known that was the plug I was looking for! That's the easiest thing to check! I can check resistance on the motor and voltage at the plug. The car does not have AC, so I'm good there.

Again, thanks for the response. It's shared knowledge like this that keeps these on the road!
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post #7 of 8 (permalink) Old 04-18-2010, 12:07 PM Thread Starter
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Well, I got the instrument cluster out yesterday. I had to temporarily move the heater hose to get at the speedometer cable, but I got it out. Sure enough, the dimmer control rheostat was open. I'll be pricing out another one at the dealer Monday. I was also missing part of a light bulb holder, the remaining parts were rattling around inside the cluster as I removed it. I jumpered the rheostat and plugged in the cluster, and I have lights!

As far as the fan, I got the cowl apart and turned the motor by hand. It's very stiff, so it will need replaced. That's a job I'm going to have to hold off on.

Again, thanks for the helpful responses.

Jason
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post #8 of 8 (permalink) Old 04-19-2010, 06:28 PM Thread Starter
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Another chapter in the sagaof the dash lights. I wne to the dealer today, and although up to this point I found most parts to be reasonable, I found that a new dimmer control is $88 USD! That's a little steep. I was missing a bulb holder and even that was $10. They even wanted $10 for the bulb! I said no thanks. Does anyone have a good source where I can get the rheostat used? Or does anyone have a cluster laying around with a good rheostat they would like to part with? Otherwise, I'm going to be adapting another rheostat. If someone can tell me the resistance of it, (from one that works), it would be appreciated.

Thanks,

Jason
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