Headlights = NO cruise control?

I have a 2002 Saab 9-5 with a 3.0L turbo with 125k miles. The cruise control works fantastic UNLESS the headlights are on… huh??

Does anyone have any ideas on this? I can replicate 100% of the time.

It is getting dark earlier now - and my wife will not drive it anymore to work (35 miles) without the cruise control working.

I see the problem. Replacing a wife can be costly.

My usual suspect is a bad ground connection or broken ground wire. A wife will start acting strangely if she isn’t grounded properly. It isn’t only men who can be conductors after all. There is a wire that runs from some metal on the engine to the body (of the car). The body end can become rusty. Disconnect and clean. When that doesn’t work, there are some grounds under the dashboard that attach to the floor. Then there is the loose or corroded battery cable. When those are proven to be good, there are bad fuses to wonder about and it gets more expensive after that. Worn out alternator brushes, bad alternators… At least rings aren’t usually involved.

Your cruise control will kick off when you apply the brakes…yes? I think when you turn on your headlights the cruise thinks that the brakes are on… Have you checked to see that your brake lights work? OR are your brake lights on all the time/constant?

Not sure how or why turning the lights on may trigger the “brake lights on” wire…unless there was a short in one of the rear tail lights… Hmmm

There is also the possibility that the cruise control relay is having issues and is affected by the headlights being turned on… check the relays for headlights and cruise…maybe switch one around to see if the issue changes…

Since this is repeatable it will be rather easy to diagnose actually… I am VERY curious to know what the status of your brake lights are…that’s important The cruise wont engage if it thinks the brake is on…so keep that in mind…


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As pleasedodgevan2 mentioned, a bad ground circuit can be the problem. Honda Blackbird pointed to a brake circuit short. Both, I think, are your solution.

A bad ground at the taillights will cause the electricity to use the dual circuits for the tail and stop light to back-feed through both circuits looking for ground. This means the front running lights will glow when the brakes are pressed, and the brake light circuit will activate, cancelling the cruise control, whenever the running lights and headlights are on.

If this is the case, restoring the ground circuit at the tail lights will most likely fix the problem.

If the brake lights and tail lights are combined in one dual filament bulb, a sagging filament for the tail light may be touching the brake light filament. This would have the same effect as touching the brake. Since it won’t cost much, I think I would replace the rear tail light/brake light bulbs and see what happens.

A bad chassis ground could cause the trouble. A simple thing you could fo to prove that is connect a temporary jumper lead from the negative battery terminal and to a good ground point on the car body or chassis. Then see if the trouble clears up. If so you need to clean the ground connections.

Sorry about the late reply to your (great) replies! I had to spend part of the time in the doghouse because my wife had to use the car several days - AND - I was wrong: she travels 65 miles each way… anyway:

Tested a few things:

  1. created a good ground from the car battery - no luck
  2. pulled each side-lught bulb - no luck
  3. pulled each tail light bulb - nu luck
  4. applying brakes DOES turn the cruise off (when the headlights are off) but it RESUME’s just fine afterward
  5. all brake lights work as they are designed to

A few more interesting ponts:

  1. My backup lights do not work - never do
  2. When I turn ONLY the running lights on (not supposed to be headlights) the headlights come on too (and the cruise turns off)
  3. I took out the fuse that runs daytime lights (not required by law here in Mississippi) which is when I noticed the cruise working again - but NOT when the headlights are turned on manually

Still searching… not for a wife, but an answer! Help me please as I have just started a new job that requires travel several days a week on the road and my foot is getting sooooo tired. There are no Saab (ex)dealers around.

I think #2 is a big clue.
Find where the headlights are tied to the running lights and that might solve your problem.
Where is the running/head light switch located?
Might be a bad switch or a problem with the clockspring if the switch is a stalk on the steering wheel.

Agreed, cicuitsmith. There is a short somewhere. The headlight circuit is most likely energizing the brake lights, causing the cruise control to turn off. Since the running lights also activates the headlights, this switch is highly suspect, as well as the wiring between the relays and the switch.

From our friend Honda Blackbird: “…unless there was a short in one of the rear tail lights… Hmmm” Yea.

Did you test with all brake light bulbs removed from their sockets?

The problem with the light switch may be part of the issue here.

After looking at some data for your vehicle I would like to see if the trouble continues if you pull out fuses 10,11, 12, and 13 in the fuse and relay panel under the hood. This will kill power to the headlights but other parts of the circuit should still be alive. The wiring for the lights is fairly complicated. Let us know what the results of this easy test is.

I have the exact same problem with my 2005 saab 9-5. Every time I turn on the lights I lose the cruise control. Plus my navigation goes wacky and it shows me driving backwards on the map. Did you have any luck finding out the problems, SaabStories? I took the car to a Saab dealship and all they want to do is replace a whole bunch of parts instead of dealing with the wiring issue. Help, please!

I tried all ideas - but it was this:
3 of the wires going into the trunk lid were severed. This is also why the cruise control would not work when using the headlights, …why my backup lights did not work, …why the rear fog light did not work, …why the trunk lid would not open, …and why when putting the car in reverse the headlights would turn on when applying the brakes. Unbelievable. Cost of all repairs: $-0-
Spliced the three wires and all works perfectly now - including the cruise control when the headlights are on.

@SaabStories, glad you found the problem. Thank you very much for posting a follow-up. It is rarely done, but much appreciated.

Curious, tho. Any idea how the wires got cut?

Glad to hear that.

@Honda_Blackbird, thank you so much. Once I read your comment, I knew what I did wrong. :+1: