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I have an aftermarket cruise-control system which only somewhat works in a 2005 Ford Focus.

By this I mean that it controls the speed but is largely inaccurate (+/- at least 10mph). Going up/down hills causes the speed to slowly increase/decrease for a long while. So it works... somewhat, but, obviously something is broken/failing with the system.

What can I do to diagnose the problem?

It looks identical to this product. I do not know the exact model because it was installed prior to me purchasing the car.

Here are some pictures on console

I think this is the unit under the hood (I'm not sure though...).

what I think is the unit under the hood

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It's an aftermarket product, how do you know it just isn't designed to work well with your ford? – hillsons Dec 28 '12 at 20:47
@hillsons it used to work a lot better – enderland Dec 28 '12 at 20:52
Any idea what brand the unit is and have you changed your brake lights lately? – mikes Dec 29 '12 at 1:28
I need to know the type of system, does is use a vacuum servo, or electronic? The name and model number of the system would be a great help. – Move More Comments Link To Top Dec 29 '12 at 19:42
@mikes I added pictures and a link to the question. I have not changed brake lights recently (or ever). – enderland Dec 29 '12 at 19:56
up vote 4 down vote accepted

It appears to be a Rostra unit their site has an extensive support page. That is why I asked about the tail lights, they mentioned it. I would go to the site input you vehicle info and click the support tab. They will give you step by step instructions included are calibration steps, which may be your issue.

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oh wow. I had no idea they had that much information there, guess I'll be reading and hopefully be able to find the needle in that haystack... – enderland Dec 29 '12 at 20:18

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