1984-1989 Cluster Photocell Dimmer Repair
Figure 1 - Example of Cluster with defective photocell
Dimmer Circuit Behavior
With the headlights off, the headlight dimmer control has no effect - the Photocell alone controls the brightness of the cluster. The bulbs should be at full brightness on a sunny day, medium brightness on a cloudy day, and low brightness at night or when the photocell is covered.
With the headlights on, both the headlight dimmer and the photocell control the brightness of the cluster. In general, the bulbs should be at full brightness on a sunny day, medium brightness on a cloudy day, and low brightness at night or when the photocell is covered - plus or minus any adjustment dialed in via the headlight dimmer. The headlights should dim when the headlight control is rotated to the Dim setting, and should get brighter when it is rotated to the Bright setting.
Dimmer Circuit Troubleshooting
Digital Cluster Wiring Diagrams Click Here
Start by checking the fuses which support the cluster. Those are marked "LCD", "CLSTR", "TAIL" and "INST".
At the wiring harness for the digital cluster, check the two wires which tell the cluster about the status of the headlights - see the wiring diagram.
- Terminal C6 of the cluster wiring harness should be battery voltage with the ignition turned on and headlight switch set to either Park or On. If it isn't, the TAIL fuse may be blown, or there may be a break in this wire.
- Terminal C9 of the cluster wiring harness should be battery voltage when the headlight switch is rotated to full brightness setting, and around +6V when the headlight switch is rotated to full dim setting. If it isn't, the TAIL or INST fuse may be blown, or there may be a problem with the headlight switch (a known point-of-failure).
If the above checks pass, the issue is likely inside the cluster.
Remove the cluster assembly (use the docs below).
Remove the back cover and top board from the cluster assembly (use the docs below).
At the board connector (the 12 really long pins sticking up from the bottom board), use an Ohmmeter and measure the resistance between pins 2 and 3 while the photocell opening in the cluster is exposed to daylight or lamp light. The reading should be < 80KOhms for the cluster bulbs to be at maximum brightness.
Measure the resistance between pins 2 and 3 when the photocell is completely dark. Wait a few seconds for the meter reading to stablize. The resistance should be > 1 Megaohm for the cluster to dim properly in dark conditions.
Measure the resistance of the photocell itself. If it measures infinite, the photocell needs to be replaced. If it measures correctly, the issue may be the board connector solder joints.
It's easy to replace the photocell sensor. Partial disassembly of the cluster will be necessary, and it will be necessary to solder two large solder connections, but the skill required is minimal.
What you'll need to service the Digital Cluster
Photocell Kit from batee.com - Click Here
Optional: Xenon bulb kit from batee.com. While the cluster is apart, it's a great time to replace the factory bulbs with cool-running Xenon bulbs! Click Here to buy
Small drill bit or Xacto knife to remove factory photo cell from its housing
A Soldering Iron (Radio Shack has these for $8.99 as of 2013)
A way to desolder - use a vacuum solder removal tool or desoldering braid
Optional: Static Wrist Strap
Removing the Digital Cluster
Follow the removal instructions located here.
Disassembling the Cluster
Disassemble the cluster using the documentation here.
Warning! Avoid removing the bottom board if at all possible! It's a huge amount of labor, and the potential for damage to the LCDs is fairly high! If you do, follow the directions here to disassemble, and here to reassemble.
Replacing the Photocell
Figure 2 - Photocell Location is shown in red. The dimming transistor is shown in blue.
Using desoldering braid or another desoldering technique, remove the factory solder from the two photocell solder joints circled in red in Figure 3.
Remove the two black plastic rivets and transparent photocell cover from the front side of the cluster.
Using needlenose pliers or foreceps, remove the photocell and its black housing from the front side of the cluster.
We'll need to reuse the black housing which contains the factory photocell. Make a small hole in the back of the housing using a 1/16" or smaller drill or Xacto knife.
Figure 3 - Photocell Rear
Place the housing on its side on a hard surface, and roll it forward while scraping the glue from the inside walls of the housing using an Xacto knife.
Using a paperclip or small punch, push the factory photocell out of the black housing.
Clean the residual glue out of the inside of the factory housing using an Xacto knife. Place the housing on its side on a hard surface, and roll it forward while scraping the glue from the inside walls of the housing using an Xacto knife.
Figure 4 - Photocell Front
Insert the new photocell in the black factory housing. Leave the leads long for now.
Using needlenose pliers or foreceps, replace the photocell. Make sure one lead from the photocell goes through each of the two holes in the circuit board. The photocell is just a resistor, so it doesn't matter which lead goes through which hole.
Warning! - the solder provided contains lead, so take reasonable measures to protect yourself: Don't hold the solder in your mouth, don't breathe the fumes the solder produces, and don't eat anything until you've thoroughly washed your hands after handling the solder.
Using the solder provided in the kit (or another Tin/Lead mix solder, but NOT acid core or lead-free!!!), resolder each of the two connections to the photocell.
Trim off the excess length of the photocell leads using wire cutters.
If the yellow 33uF capacitor near the photocell shows any signs of having leaked, replace it with the blue capacitor supplied in the Photocell kit. Cut the leads to the yellow cap very near the capacitor, and use the remaining lead soldered to the board to attach the new capacitor.
In most of the clusters we've seen, heat from the factory bulbs has caused the dimmer transistor to become detached from the bottom board. Using the Xacto knife, scrape the sides of the heatsink tab for the transistor marked in blue in Fig 3 until you see bare copper color. Then use the remaining solder to solder the sides of the transistor to the bottom board heatsink pad.
Clean both sides of the clear photocell cover before reinstalling it.
Reassembling the Cluster
Note: If you removed the bottom board, follow this process exactly to replace it before proceeding.
Reassemble the cluster using the documentation located here.