Can't dim dash lights

[Cubby]

I can't dim the dash lights on my 01 Forester. I have replaced both the control stalk and illunimation control module with ones I got from the wrecker (seperately - tried 2 of each so I assume one of them would have worked) but still doesn't work. The lights are on full brightness but it bugs me that I can't dim them. What else could it be? I have not done anything to the electrical other than wire in some driving lights but I never tapped into the wiring harness. I did remove the radio once to run wires behind it.

 

Any ideas? Thanks.

[Cougar]

It sounds like some honest troubleshooting is now needed instead of just module replacement on order to fix this problem. I would first check for a problem on the control wires between the adjustment control and the module. If you need some wiring info on this I may be able to get some for you.

[Commuter]

Don't most cars put the dimmer on the ground side? As I recall, the circuitry is configured so that a fault will cause them to go full bright, as opposed to go off. Could it be as simple as a grounding issue?

 

Sorry, can't help you beyond that.

 

Commuter

[ferret]

On your Forester, as previously stated, the +12v is applied to all the lights. Then the ground is varied by the Illumination Module which is controlled by the rheostat on the headlight stalk. I suspect you have one of those controlled ground wires 'pinched' or 'frayed' and touching the body metal somewhere effectively grounding the lights and keeping them at full intensity at all times.

 

One way to check, UNPLUG the illumination module. If the lights still stay on bright, then you definitely have a short in one of those wires. If the lights go out, the the module you got from the wrecker is defective.

 

Now to find it, you did state you had the radio out. Behind it are a lot of sharp edges which could easily pierce a wire and cause a ground short like this. That would be where I would start. If the short is not there, you have a lot of tracing wires to do.

[Cougar]

Very good advice Ferret.

[jon38iowa]

On your Forester, as previously stated, the +12v is applied to all the lights. Then the ground is varied by the Illumination Module which is controlled by the rheostat on the headlight stalk.

 

 

With respect, I think the rheostat is located behind the glove box and not on the stalk. At least, with my '99 Forester thats where it was when I had to replace it last year. I don't know if thats where your issue is, though.

[OB99W]

With respect, maybe I can help clarify this. In years past, before solid state (semiconductor) electronics was used in cars, dash lighting levels were directly controlled with an electro-mechanical variable resistor called a "rheostat". (Some "modern" cars apparently still use them.) The current for the dash lamps passes directly through this rheostat, so it is relatively large. See the "High Power Types" section of http://en.wikipedia.org/wiki/Potentiometer .

 

We now have a more efficient way to do the same thing; in the Subaru, there's a small variable resistance control in the stalk that is connected to a remotely located "Illumi Con Unit" (Illumination Control Unit). The device in the stalk only controls the Illumi Con, never "sees" the lamp current, and so can be much smaller than the rheostats previously used. The Illumi Con is what actually handles the lamp current.

 

Probably because some people associate the term "rheostat" with any control for lamp intensity in cars, it's become another name for the Illumi Con Unit.

[Cubby]

Thanks for the replies. When I unplug the illumination conttrol module, the lights stay on. So its a ground short. I'll take the radio out this weekend and see if I pinched something.

[Cubby]

Well I took the radio out this weekend and no pinched wires or anything out of the ordinary. I think the illunimation control wire is orange. I will try to track that one from beginning to end to see if there is a problem.

[OB99W]

The diagram I have for the '99 Forester shows orange/white as the color of the control line wire; might be the same on the '01. The diagram shows that wire going to a lot of places. Here's the list for the '99:

 

Illumination for:

Combination meter (dash)

Foglight switch

Hazard switch

A/T selector lever

Mode control panel

Rear defogger switch

Cruise control main switch

CD

Radio

 

The ashtray lamp apparently can't be dimmed.

 

Is the radio original, or an aftermarket unit? If aftermarket, perhaps someone wired it incorrectly.

[Cubby]

Thanks OB99W. Its the factory 6 CD/cassette player. I will check the wiring one rainy day when I have time. I may even pull out the glove compartment to see if the wiring is fine there.

[rverdoold]

Dont know about the US subaru's but if i press the bright button next to the clock i can not adjust any dash light they are all at max. But if i turn it off i can adjust the brightness. (in a 91 legacy).

In a 2005 OBW its the same by keeping the brightness button pushed on the satnav display.

I hope it is not as simple as this after all you have tried with measuring.

[Cubby]

Thanks rverdoold but my radio doesn't have a brightness button. I wish is was that simple!