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Rheostat dimmer switch

All instrument cluster and dashboard lighting is controlled by the rheostat or dimmer switch. This component is part of Electrical System and Dashboard Instruments

Definition

When the Pagoda series of cars were first produced, roads and motorways were generally unlit. A rheostat was inserted in the circuit in order to dim the instrument lights to prevent reflections of the instrument cluster in the windscreen.

Nowadays with lots more ambient light, many owners find their dashboard lights very dim, especially on the speedometer and the tachometer. Usually a very dim light is the result of corrosion or failure of the Rheostat. The Rheostat or Dimmer is a coiled resistor wire. The tab with a brass plate moves along the wire, and varies the level of output voltage, thereby reducing or increasing the brightness of the instrument lights.

Maintenance

  • Use a plastic-safe contact spray cleaner with a thin nozzle tubing and flush out any debris between the contact wiper and the rheostat wire.

To do this it is best to pull out the center instrument cluster far enough to gain access to the rheostat. The Caig Company sells several suitable contact sprays that are safe for plastics: http://www.caig.com/

Testing

The central instrument cluster receives its power and signals through a cable with a 12-pin terminal male connector that mates up with a 12-pin coupler under the dashboard.

Underdash Cable Connections


Unplug the cable with a male terminal connector going to the central instrument cluster (it may have a blue tape collar) from its "handshake" partner (see photos above) and test the resistance between the two pins furthest away from the cable entry on the left side of the connector when looking at the pins.

When the rheostat knob is turned completely counterclockwise (left turn) the Ohm meter should read 18 Ohm and when turned completely clockwise (right turn) the resistance should be 0 Ohm. It may be necessary to turn the knob a few times for and back and to press firmly against the stops on both ends of the turn to get a stable reading. (Note: the resistance reading seen in the photo above is 18 Ohm, not 0.018 Ohm because the meter is set at 1 kΩ).

An unstable resistance reading, or readings that do not change smoothly and linearly between 0 and 18 Ohm may indicate that either debris is stuck between the wire coils preventing good contact or the contact wiper is corroded.

Most often a through cleaning with a good contact spray will recondition the rheostat. To do this the rheostat needs to be removed from the central instrument.

If the Ohm meter indicates infinite resistance then most likely the rheostat wire is broken.

  • There are two common remedies. Shorting out the Rheostat is often done, because with the high ambient light reflections in the windscreen are no longer an issue.
  • If you intend to short out the Rheostat, but if you also drive rallies in darker areas, it may be sensible to wire a resistor in series with the shorted Rheostat (experiment with the values but start with low resistance (5 Ohms) and reasonable wattage 25W) with a switch in parallel with the resistor so that it can be bypassed. This will give you two brightness settings: high (when the resistor is bypassed), and medium (when the resistor is in place of the Rheostat).
  • Alternatively you can Rebuild the Rheostat

Rebuilding the Rheostat

All instrument cluster, dashboard lighting is controlled by the rheostat or dimmer switch. If it fails, you will have no lights on the dash. But wait! It can be fixed… The first step would be to remove the instrument cluster from the dashboard, and then remove the rheostat.

Once you have the rheostat out, remove the metal cover by prying the two retaining tabs just enough so the metal cover comes off. Careful, it will now fall into pieces. In your hands will be: the ceramic base; a small rotating brass plate; and two small springs that push the brass plate down onto the coiled wire that serves as the variable resistor.

The two problems most commonly found with the rheostat are:

  • debris build-up on the brass plates; and
  • failure of the coiled wire.

The coiled wire is connected at one end to a brass connector that provides electrical continuity and at the other end it is held in place by some sort of adhesive.

The entire coil sits in a channel within the ceramic base. If the coiled wire is broken – you may be able to install another one by using fuse wire. The rheostat spring wire measures 17-18 ohms of peak resistance. Use some JB Weld to hold new spring in place on both ends. The "spring" is actually the resistor used to vary the impedance and hence the amount of current flowing through the lights. The resistance is varied by shorting the spring with the rotating stator. Maximum intensity is a complete electrical short of the resistor; minimum intensity is when the resistor is partially shorted. By stretching the spring, you essentially short-out most of the resistance!

To address the debris build up on the brass plates you can use an ink eraser if the build-up is not significant, or you can soak all the pieces in some jewelry cleaner (sulphuric acid and urea) and let it soak until clean.

Reassembly is straightforward although there is one trick. When you put the springs back in place and put the rotating brass part on them you need to hold it down with a small knife blade when you re-attach the other half. Once you've got it on just put the metal cover back on and fold the tabs in and you're done.

Getting access to the Rheostat

Disconnect the Battery.

  1. Remove the tachometer. See: Rev Counter
  2. Disconnect the 12-pin connector at the end of the cable from the Instrument Cluster. Consult the photo showing the Underdash Cable Connections

The Instrument Cluster

  1. Go under the hood and disconnect the small black plastic cover tube to the oil filter fitting. Fish this tube loose so that it can be free to be pulled through the firewall.
  2. Find the other tube from the engine block for measuring the coolant temperature. My tube has enough slack that I didn't take it loose. I just carefully worked it so that it too could easily be pulled through the firewall.
  3. Back under the dash. Remove the thumbscrew and keeper that hold the main cluster.
  4. The steering column gets in the way of pulling the cluster straight out. You have to tip the top out first. WATCH THE TUBING. Mine gave a "pop" when the top edge cleared the dash.
  5. Watching the coolant line, rotate the cluster counterclockwise and lay it on the steering column.
  6. Remove the two screws and the plastic cover from the resistor. Solder a small piece of wire connecting the two posts on the resistor.
  7. Put the cover back on and test.

Shorting out the Rheostat

  1. Disconnect the Battery!
  2. Disconnect the “12 prong plug” on the metal bracket that the “Flasher Can” also hangs on.
  3. Remove the cover off the back of the male plug (3 screws on a 230 or snaps-off on 250 and 280's.
  4. Remove the two wires that go up to the rheostat. One is grey w/violet stripe the other grey w/blue stripe.
  5. Hold each terminal pin with a pair of needle nose pliers, heat with a soldering iron until the wire pulls out. While the solder is still hot tap it against the floor and the excess solder will come out and clear the holes.
  6. Make a short (insulated) jumper wire and solder it with the original wires in the terminals.
  7. Replace the terminal pins back in the plug where they came out.
  8. Reinstall the cover on the back of the plug.
  9. Reconnect the “12 prong plug”.
  10. Reconnect the Battery and check for operation of your dash lights.

The photos below were taken on the workbench, but the job could easily be done in the car with the steering wheel removed.


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