Bypass Rheostat - Update

[bpossel]

Recently fuse #7 was blowing when I turned on my lights.

I traced it back to the rheostat. It is toast! Cant be fixed!

There are 3 wires connected to the rheostat:

1. Grey/Blue wire & brown ground wire (connected to one side of the rheo)
2. Grey/Violet wire (connected to the other side of the rheo)

What I did was to completely remove the rheostat, by un-soldering the 3 wires from the back of the rheostat.  After removal of the rheostat, I soldered the 3 wires together.  Used some heat shrink tubing over the soldered ends and re-installed the center cluster.

Now have full bright lights on my dash.  No more worries about the rheostat...

Bob




bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

[Naj ✝︎]

quote:Originally posted by bpossel

Recently fuse #7 was blowing when I turned on my lights.

I traced it back to the rheostat. It is toast! Cant be fixed!

There are 3 wires connected to the rheostat:

1. Grey/Blue wire & brown ground wire (connected to one side of the rheo)
2. Grey/Violet wire (connected to the other side of the rheo)

What I did was to completely remove the rheostat, by un-soldering the 3 wires from the back of the rheostat.  After removal of the rheostat, I soldered the 3 wires together.  Used some heat shrink tubing over the soldered ends and re-installed the center cluster.

Now have full bright lights on my dash.  No more worries about the rheostat...

Bob




bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

So, where does the brown 'ground' wire go to?

This is a bit worrying !!!

naj

68 280SL

[bpossel]

Hello Naj,

I am hoping the electrical experts can jump in on this one!

I agree, I was also concerned with the ground and connecting all 3 wires together...

But...  from my understanding of how a rheostat works is that when you rotate the knob to achieve maximum intensity (full power to the lights) this is in essence, a complete electrical short of the resistor.  So, removing the rheostat and connecting the 3 wires together (brown wire (ground) included), you get full lights on.

Bob

bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

[al_lieffring]

quote:Originally posted by bpossel

Hello Naj,

I am hoping the electrical experts can jump in on this one!

I agree, I was also concerned with the ground and connecting all 3 wires together...

But...  from my understanding of how a rheostat works is that when you rotate the knob to achieve maximum intensity (full power to the lights) this is in essence, a complete electrical short of the resistor.  So, removing the rheostat and connecting the 3 wires together (brown wire (ground) included), you get full lights on.

Bob

bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

Bob, Naj

A red flag went up when I read that too, I went out and looked at the the back of my spare instrument cluster and saw that there were three wires, one was a (faded) reddish color, not a brown ground wire, A look at a wiring diagram shows that one wire is from fuse 7, the other goes to the panel lamps inside the cluster, the third goes out to all of the other pannel lamps in the dash.

No need to repost the photo of B9 the robot, Bob should be good to go.



Al Lieffring
66 230Sl
New blue top just arrived
new gas tank on order

[jeffc280sl]

Bob,

Here is a pic of one side of the rheostat.  I can see the three wires on the left side.  Can you take a picture of the other side?

I would think for this to work that fuse #7 would be connected to the 1 o'clock position on the "spring".  The brown ground would be connected to the othetr end at 3 o'clock.  The variable output to the lamps would be connected to the center as would an arm which sweeps across the spring.  As the arm sweeps in a clockwise direction from 3 o'clock the lamps get brighter as the lamp feed gets closer to the power source at 1 o'clock.  At 1 o'clock the lamps are at full bright.

The spring acts like a variable resistor.  When the arm is near the 3 o'clock position most of the energy from the power to ground connection is dissapated as heat by the spring and the lamps are dim.  As the arm moves closer to the power source less and less energy is dissapated as heat, because of the spring length and the lamps glow brighter.

I do not think the ground wire should be included in the connection you have made to power the lamps at full brightness.  Somewhere in this circuit there is an short that caused the rheostat to fail.  Let's see if we can find it.

Download Attachment: Rheostat.JPG
43.2 KB

Jeff C.
1970 280SL 4-speed

[al_lieffring]

Jeff

That is not how the rheostat works in this circuit.
The grounded coil circuit you are describing is used in transistorized control circuits, But in this case there are no transistors involved.
the coil is just as you said a variable resistor, at the far end high resistance (dim) at the other, low resistance (bright).

The components made by sublet manufacturers (VDO) don't necessarily use the MBZ color code on their internal wiring, In this case the brown wire is NOT a ground but the wire that goes from the rheostat to inside the case for the internal lighting of the instrument cluster.

Al Lieffring
66 230Sl
New blue top just arrived
new gas tank on order

[jeffc280sl]

Thanks Al,

If I understand this correctly the brown wire provides power for the center gauge cluster illumination. The brown wire and grey/blue wire are connected together on the sweeper arm at the rheostat.  The grey/blue wire provides power to the remainder of the dash board lamps. That leaves the grey/violet wire to provide power to the circuit from fuse #7.  As the arm sweeps clockwise illumination to all lamps is increased because the arm is getting closer to the power input point on the spring.

Jeff C.
1970 280SL 4-speed

[Naj ✝︎]

quote:Originally posted by al_lieffring

quote:Originally posted by bpossel

Hello Naj,

I am hoping the electrical experts can jump in on this one!

I agree, I was also concerned with the ground and connecting all 3 wires together...

But...  from my understanding of how a rheostat works is that when you rotate the knob to achieve maximum intensity (full power to the lights) this is in essence, a complete electrical short of the resistor.  So, removing the rheostat and connecting the 3 wires together (brown wire (ground) included), you get full lights on.

Bob

bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

Bob, Naj

A red flag went up when I read that too, I went out and looked at the the back of my spare instrument cluster and saw that there were three wires, one was a (faded) reddish color, not a brown ground wire, A look at a wiring diagram shows that one wire is from fuse 7, the other goes to the panel lamps inside the cluster, the third goes out to all of the other pannel lamps in the dash.

No need to repost the photo of B9 the robot, Bob should be good to go.



Al Lieffring
66 230Sl
New blue top just arrived
new gas tank on order

Thanks, Al,

I can go back to sleep now.  :|

naj

68 280SL

[bpossel]

Thanks Al, Jeff, Naj!
Bob

bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

[bpossel]

Al, Jeff, Naj,

Last night I drove my car and the dash lights are nice and bright!

Can we conclude that removal of the rheostat and connecting the 3 small wires that are connected to the back of the rheostat in the back of the center cluster is an option for dash lighting issues?

Mine seems to work great with the above procedure!  I wonder if this is what the vendors that rebuild these units do since the rheostats are no longer made?  Obviously if the rheostat is simply dirting, cleaning the spring and rotary plate is an option...  but if reo is not fixable, remove and connect the 3 wires...

Comments thoughts?

Bob

bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

[jeffc280sl]

Sure, I think its a good option.  I think rebuilding the rheostat or finding a substitute is the best.  Maybe someone else has done this already and can share the fix.

Jeff C.
1970 280SL 4-speed