Electrical issues 74' 450slc - Mercedes-Benz Forum
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#1 (permalink) Old 06-28-2013, 10:57 PM
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Electrical issues 74' 450slc

My 450 has been having all kind of issues lately the blinkers/hazards flash so fast they are almost solid. That has been an on going thing, but now all my front right lights went off one night as well as the instrument panel no longer lights up. So i cant see or tell how fast i am going at night. Which is always fun.

I doubt this is all fuse related as they all seem in place to me. but i know very little about how cars work. Any thoughts would be great thank you all.
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#2 (permalink) Old 06-29-2013, 03:56 AM
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1. Blinkers/Hazards Flash fast: Cycle your hazard button 20+ times. If it binds, it can be removed, opened up and cleaned out. Dielectric grease ages into dielectric crud which causes problems. If yours is shot, I have spares.

2. Front Right Lights: If Right Rear lights work, I agree that fuses are not the problem. Unlikely too that right front bulbs would fail at the same time. Look at the connector to the right front headlight fixture. Clean up the pin bushings (sockets) with a wire brush. Clean the pins on the headlight fixture in the same way. Check bulbs while your are there.

3. Instrument Cluster lighting dead: Cycle the instrument cluster rheostat (dimmer knob) 100+ times. The coil of the rheostat gets corroded and can interrupt the circuit.

4. Check the status of your fuse box - Replace aluminum over plastic fuses with copper over ceramic ones. Ceramic Bullet Fuse Kit Mercedes BMW Volvo Saab Porsche Audi VW | eBay Clean any corrosion on the fuse holders while you are there.

Try these and report back - continuity checks are next. Do you have a multi-meter?

Brad Cushman
73 450 SL
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#3 (permalink) Old 06-29-2013, 04:18 AM
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Electronics is a weak point on all old cars, fortunately your 74 SL does not have a lot of it and it is not too complex.

First the indicator flashing, most likely the flasher relay, not expensive and easy to replace.

The ligth circuit needs a bit of trobleshooting, could be several things:
- Bad grounding (usually corrosion)
- Blown/corroded fuses
- Blown bulbs
- Corroded sockets/wire lugs
- Broken wires
- Corroded lightswitch

You will find that a voltmeter is very handy when trouble shooting. It is also possible to troubleshoot with wires.

Grounding can be checked using a wire connect to the chassis at one end touching the ground connector (brown wire), if the bulb then works, you have found the problem.

If grounding is OK, you can connect a long cable to the battery +, connect this wire to the light socket positive, if the bulb lights up, the problem is the wiring, lightswitch or fuse.
You can then connect the cable to the lightswitch output, if the bulbs light up you have a wiring problem. Connect then to the lightswitch input. If the bulb lights up, the switch is the problem.

Be careful not to short the cable connected to battery+, it can easily melt and cause damage to surfaces it touches !

Electrical troubleshooting is not difficult, but requires patience. Make sure you get wiring diagram which shows the wiring colours.
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#4 (permalink) Old 06-30-2013, 05:02 PM
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Update:
I got the Dash light back on after moving around the dimmer til it clicked back.
I got the low beams back on but when i put the high beams its still out and turns the low off as well bulbs look good haven't had a chance to really get under the hood to pull apart.
The blinkers/hazards i cycled though the button and still nothing.
Thanks for the input so far.
ill keep trying and updating.
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#5 (permalink) Old 07-05-2013, 05:26 PM
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Update 2.0
I went away for the 4th Came back turned the car on and now...
The blinkers seem to be working fine now the speed as slowed down.
I guess it just took sometime for them to reset.
Just gotta figure out the highbeams.
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#6 (permalink) Old 07-05-2013, 06:17 PM
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Please let us know what you find out. I am really interested in knowing the cause of the problem.

Last edited by dgosh28; 07-05-2013 at 06:37 PM.
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#7 (permalink) Old 07-05-2013, 06:35 PM
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Quote:
Originally Posted by Bohimore View Post
Update 2.0
I went away for the 4th Came back turned the car on and now...
The blinkers seem to be working fine now the speed as slowed down.
I guess it just took sometime for them to reset.
Just gotta figure out the highbeams.
Wen my right side high beams did not work, I took the headlight assembly out of the car. To do this, first take the plastic around the headlights off. Be careful as the plastic is old and brittle. Next you need to take the headlight door off. The door is the metal piece under the headlights. Once you take that off (three screws) you can get to the headlight assembly. Remove the assembly and unplug the connectors. The plastic is going to be very brittle so be careful.
In my case, the brown ground wires were corroded. I cleaned them up and the lights came back on.

Before reassembling the thing, check to make sure everything is working again.

Good luck!!

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#8 (permalink) Old 07-06-2013, 10:02 AM
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Quote:
Update:
I got the Dash light back on after moving around the dimmer til it clicked back.
I got the low beams back on but when i put the high beams its still out and turns the low off as well bulbs look good haven't had a chance to really get under the hood to pull apart.
The blinkers/hazards i cycled though the button and still nothing.
Thanks for the input so far.
ill keep trying and updating.
Does this mean your high beam problem is no longer limited to the right side? If both high beams are misbehaving, the combination switch can be a problem.

I have several salvaged hazard switches and at least one flasher relay we could try to solve that part of the problem - PM if interested.

Brad Cushman
73 450 SL
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#9 (permalink) Old 07-20-2013, 09:48 PM
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Quote:
Originally Posted by cushjbc View Post
Does this mean your high beam problem is no longer limited to the right side? If both high beams are misbehaving, the combination switch can be a problem.

I have several salvaged hazard switches and at least one flasher relay we could try to solve that part of the problem - PM if interested.
No still just the right side.
ill try to take it apart one day soon.
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#10 (permalink) Old 07-21-2013, 08:00 AM
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I've found that most electrical problems are caused by corrosion on the contact surfaces. After 25-30 years the layer of crud builds up enough to act as a resistor or an insulator.

Tom H.
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