Need help with cruise control problem

[tjthompson]

I have an 08 mazda 3 2.3L s Touring. I'm having some issues with the cruise control that I'm hoping to get some help with.

Symptoms are:

• Cruise control buttons all do strange things. Some times all the buttons will act like pressing the on/off button but they also sometimes just turn the cruise 'on' led on amber briefly while they're pressed and then it goes back off.
• This started happening once in a while and then gradually got to where it is now where it rarely works
• I have had some unintended acceleration where the vehicle will suddenly start accelerating for no reason. Tapping the brake stops the acceleration. I feel like I've only seen this happen when the cruise control is in the 'on' but not 'set' state.

What I've tried:
I initially took it to the dealer. They said they had found some codes related to the cruise control switches but they can't get the cruise control switch part any more and didn't do anything further.

So I went home and pulled the cruise control switches and the clock spring assembly.

I took apart the cruise control switch assembly and examined the switch mechanism itself. Everything seemed in good shape and I saw no signs of wear. It looks very well built and I doubt there's issues with anything on the cruise control switch PCB.

After poking around with a multimeter for a bit I located the ground and the cruise data line. I tested first from the cruise control switch harness and then from the downstream side of the clock assembly while pressing the various cruise control buttons. Both cases resulted in a solid resistance ladder as follows:

• No button press --> Open circuit
• On --> 0 ohms
• Set --> 680 ohms
• Resume --> 2200 ohms
• Cancel --> 120 ohms

These signals seemed stable while moving the clock spring around and wiggling it trying to induce odd values. I've come to the conclusion I don't think there's anything wrong with the switches or the clock spring. I haven't seen anything documenting what resistances I should see, but given the stability of the values I think it's unlikely something's broken here.

I think the next step is to check the resistances at the PCM but I'm a bit of a noob at car repair and haven't been able to find a good picture of where the pcm is (I think it's behind the battery on the driver's side at this point). More importantly I haven't been able to find a diagram for the pcm wire harness coming in so I know what wires to probe. I found a diagram that might be useful but I'm not sure if it's the right one.

So, questions:

• First off, does it sound like I'm on the right path here?
• Are there other issues I'm not checking that might cause a similar problem?
• Assuming my plan is solid, does someone know where to find a pinout for the PCM so I can locate the appropriate cruise control wires?

Any other suggestions are also welcome of course

 

[theblooms]

It absolutely sounds like you're on the right path! The PCM is located directly on the side of the battery box. There's a cover that you pop off, and then you'll see the wires and the PCM itself. Once you know what to look for, you can't miss it.

You may want to invest in a one time subscription to All Data to get the wiring diagrams and flow-chart to help with this. Getting an ELM327 adapter (a real one, not a clone!) and using ForScan may also help you out here. ForScan is a free nearly dealership level scan tool that you use on your laptop.

 

[negusm]

What was the code?

Whenever they say "we found a code" DEMAND a printout. Cruise is a funny thing, there are like a half dozen things that participate in it and it could be something like a faulty brake switch which might throw its own code.

Did you try and pull it with a code reader? If you do not have a reader, then I'd get one. Not sure if the cheap ones will pull this code or not as it's not directly engine related.

Also, you need a Factory Service Manual. You literally can't work on the electrical systems of cars past 2000 without one. I buy mine on ebay for like $15 for a downloadable pdf.

Here are a couple of sheets from the FSM I got off from ebay for the one cruise code I could find:

 

[negusm]

Some more:

 

[negusm]

Your switch readings are spot on. So you need to verify wiring and the PCM it seems.

Also those diagrams are from an FSM for a 2007 but they should be the same for 2008.

 

[tjthompson]

Your switch readings are spot on. So you need to verify wiring and the PCM it seems.

Also those diagrams are from an FSM for a 2007 but they should be the same for 2008.

Thanks for the super useful info. I have a scanner on the way so I can check the codes myself but I didn't get them from the dealer. I'll ask next time. I'll look over all the pics you provided in the meantime. I'll be busy this weekend so I probably won't get a chance to dig into things further until Monday or so. Glad to have the pinout on the pcm connector!