[federicosl]

Hi all,

I usually post something when I run into issues and after I spend a good couple of weekends playing with my 350SL.

I recently removed the center console to fix a few cracks and repainted it. After re-installing it, the dashboard lights, the clock light and the heating control lights won't switch on... All the fuses are ok and it does not seem to be a grounding issue as if I apply the 12V directly from battery to the clock light, all the lights switch on.

This makes sense since according to the wiring diagram all these lights seems to be on the same circuit.

Most probably there is a loose contact between the light switch and the lead that goes to the dashboard, which is odd because I didn't touch that area.

Now to make this post a bit more interesting I can also tell you that a couple of days ago I hit a hole on the road and magically the lights went on until I stop parked and restarted the car...

Do you think I will have to remove the instrument panel and the light switch to fix this issue?

Would make sense to run a cable from the light of the shift case (this works as expected and turns on upon when turning the lights on) to the lights of the heater control (I know this might be a kind of a hack, but I am not sure how to remove the light switch).

Any idea, comments, insults and suggestions are really welcome.

Thank you

 

[Ianwright40]

Just behind the heater controls on my 450 is a distribution block which most of the accessories attach too. I could see that it would be possible for a loose connection there. Interestingly they are fixed with a screw rather than some push kind of connection.

Previous topic

The area outlined in red is where I would check first.

 

[MBGraham]

I think I understand what you are saying.

On the 1973 diagram, it shows a lighting circuit that goes to clock light (via instrument cluster lighting rheostat), then on to the heater control lights. Clock has it's own ground, and there is another for the heater lighting. If you applied 12v to the clock lamp and all the lights came on, then that would indicate that power is not getting to the clock lamp.

You have a 72 350SL same as I do. I have a Euro wiring diagram, but no clock shown. If lights came on when you hit a bump, then there must be a loose connection somewhere. Have you double checked connection at the clock light? Reseat the fuse? Redo you test and make sure that it is repeatable. Double check your wiring and grounds at the heater panel.

Hopefully problem somewhere you can get at rather than at light switch.

 

[federicosl]

Thank you all for your feedback.

I did all the checks suggested and indeed I don't get 12V at the lighting of the heater control, clock and dashboard.

By shorting any of those (apart the dashboard which I don't have access to) to battery i am able to switch all the lights. This suggest me that there is continuity and that the circuit is properly grounded and that the problem must be between the light switch and the dashboard light.

According to the wiring diagram, the instrument lights (4-h) are fed directly by the light switch (Pin K), the clock light is fed by the instrument cluster light and the heater control lights are fed by a the clock light and none of those lights is actually fused...

Initially I thought that fuse 13 was the one to check, but to fuse 13 only tail lamp, light switch and headlight unit are connected. In fact, I don't even have continuity between fuse 13 and the heater control lights...

Unfortunately it seems that I am not left with many options...

I think I will connect a lead between the +12 ignition radio and the heater control unit. In this way my dashboard will be turned on every time I'm driving, but better than never...

Thank you again for your help and I'll let you know if I catch on fire in the next couple of weeks ;-)

 

[rowdie]

If you mean instrument cluster it is easy to remove. 27MM nut to remove steering wheel. Cluster is a friction fit. Two 10MM wrenches to remove oil gauge line,. Just don't start engine with oil line removed.

 

[Ianwright40]

One last thought. The Dimmer control. I found that giving this a few rapid turns does bring the lights back no doubt due to crud on the rheostat. may not be your problem but symptoms are similar.

 

[MBGraham]

According to the wiring diagram, the instrument lights (4-h) are fed directly by the light switch (Pin K), the clock light is fed by the instrument cluster light and the heater control lights are fed by a the clock light and none of those lights is actually fused...

I had another look at the wiring diagram (was wrong this morning and corrected my post). My diagram is kind of hard to read, but it looked to me like the line to the clock comes off the cluster dimmer switch 4i that dims all instrument lighting. The cluster lighting wiring is on a printed circuit board that the dimmer switch is mounted on.

Are the main cluster illumination lights lit up when the heater and clock are not?

Common problem if there are no instrument lights, is faulty dimmer switch (sometimes easily fixed).

But if just clock & heater panel affected, could be that one of the traces on the cluster circuit board has burned out? That lighting circuit must come off a fuse earlier on, but it serves several users. I had cluster cct board traces blow on my 300D when I had dash apart without first disconnecting battery. Something shorted and that blew the trace instead of the fuse

 

[federicosl]

Thank you all,

As mentioned in the prior post, earlier on I ended up connecting a lead between the +12 ignition radio to the heater control unit and all the lights come up.

In this way all my lights turn on when I switch the ignition key (but this would exclude the issue mentioned by MBGraham), unless the trace blew between the dashboard light and the light switch.

I haven't tried the dimmer as Ianwright40 has recommended, but that could well be the root couse.

I should have done what Rowdie has recommended and pull off the cluster, but I don't have the right tools for it with me, and the cluster vinyl is very brittle (and every time I do so I add another crack)...

However, soon I will have to repair a crack in the dashboard and the instrument cluster as well (odometer stop running properly)...

On such occasion I will remove the cluster, try to trace the wiring and most probably break something else ;-)

Thank you again to all of you for your answers, help and kindness.

Federico

 

[MBGraham]

Thank you all,

As mentioned in the prior post, earlier on I ended up connecting a lead between the +12 ignition radio to the heater control unit and all the lights come up.

In this way all my lights turn on when I switch the ignition key (but this would exclude the issue mentioned by MBGraham), unless the trace blew between the dashboard light and the light switch.

I haven't tried the dimmer as Ianwright40 has recommended, but that could well be the root couse.

I should have done what Rowdie has recommended and pull off the cluster

Federico

When you say "all the lights come up", does 'all' mean heater, clock AND the backlights within the main instrument cluster?

If so, no tools needed to turn dimmer back and forth 50 or a 100 times to clean it's contacts. That is the usual fix.

But if cluster lights don't come on and the heater/clock lights do , then likely a problem on the cluster cct board or it's connection to clock.

 

[federicosl]

When you say "all the lights come up", does 'all' mean heater, clock AND the backlights within the main instrument cluster?

If so, no tools needed to turn dimmer back and forth 50 or a 100 times to clean it's contacts. That is the usual fix.

But if cluster lights don't come on and the heater/clock lights do , then likely a problem on the cluster cct board or it's connection to clock.

MBGraham,

thanks a lot for your message. The dimmer was the problem and as you suggested after turning it back and forth a few times all the lights (heater, clock and backlights in the instrument cluster) came up.

After spending two weekends with my head against in the fuse box, the problem was the dimmer and to be honest with you, without your feedback I would have hardly immagine the root cause...

Thanks again and thank you all on this forum.

Regards

 

[Ianwright40]

Good job. If you ever take out the cluster give it a squirt with switch cleaner. Mine still goes out from time to time. it's just lack of use.