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I recently purchased a 2003 V6 mustang with automatic transmission. It has 105,000 miles on it.. It ran fine for several weeks and then I started noticing that the speedometer would fluctuate wildly. It would stay stuck at zero then go up to 70 mph then back down into the 30 - 40 mph range, then finally settle on the correct speed. The engine then began to surge and jolt when attempting to accelerate. That issue got progressively worse and I took it to a transmission place. Following is the history on what has been done to the car.
1. One transmission place told me the diagnostic showed the transmission slipping in 1-2nd shift range and 2-3rd shift range. They recommended an RDI costing $585. They also detected a burnt odor to the fluid. They quoted me a price between $1100 - $3500 to repair.
2. 2nd place said no need to fix tranny it was in good shape. He said it was low on transmission fluid and he also replaced the filter and pan gasket. However he referred me to another place to check out the sensor.
3. 3rd place said there was a code P0721 - Output speed sensor malfunction. They replaced the sensor and reset the engine light.
The car improved considerably, however the speedometer fluctuating and slight surges continue. This seems to be more prominent when the engine is transitioning from cold to warm. When I was leaving the shop and driving home (approx. 8 miles away), the cruise control would not engage at 65MPH. I attempted several times and had no luck. I also noticed that the "O/D off" was blinking on and off. However the car was not in OD and operating fine. Not sure what that was all about.
Later that day my wife and I decided to take the car for a long cruise. I engaged the cruise control av=bout 90 minutes into our trip and it stayed right at 70MPH. However on the way home it would not engage at all.

A friend of ours on the east coast is a 2004 mustang owner and also a mechanic. He mentioned to have the overdrive lockup solenoid checked out.

As a newbie I do hope whoever is reading this will forgive me for being long winded. I just want my pony fixed. I do have an appointment to take it back in on Monday and they are renting a car for me while they tinker some more on the car.

If anyone has any other suggestions or ideas I would greatly appreciate them.

Thanks
 

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This is probably not your problem, but my speed was erratic and I later found that the transmission harness wires were damaged and sorted.

There's so many shady scammer transmission shops.... have you tried a reputable Ford dealer? They should be more knowledgeable and more honest.
 

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Go to a dealer. They hate doing transmission jobs so they will/should actually trouble shoot the problem. Tranny shops are shady, every single one of them. I thought I had finally found a good one here to send my work too and they have tried to burn me on the last 2 jobs I sent in. They have lower ethical standards than lawyers. They are never to be trusted.

Also, the cruise control systems in these cars are a joke. Mine has failed twice and I refuse to repair it again. These cars were built sooo poorly and with such crappy parts it amazes me sometimes.
 

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Discussion Starter #4
Thanks for the input guys. Not sure if name dropping is forbidden, but I plead newbie ignorance with this reply.
First place I went to did not catch the senor issue I mentioned. Rapid Transmission just told be they needed to do the RDI to determine how much to charge for rebuilding the tranny. My wife did a review on them and found that 99% of their reviews were negative. So yes I feel they were more into getting a large profit instead of fixing the problem.
The second place we went to (Discount Transmission) had nothing but positive reviews. He could have simply agreed with Rapid Transmission and undercut them a few hundred dollars. He said he would not rebuild a tranny if he felt it did not warrant it, I felt he was being very upfront with me on that.
The third place where we were referred to (Aiwa Auto repair) had awesome reviews as well. They even did an oil change on my wife's 2008 Nissan for free, He said he will also do the 35,000 mile check up for free. The manager said most of the work required are simple checks that the dealer will charge close to $300 to perform. They are also paying for a rental car while they have mine.
I feel confidant that the last two places are not out to gouge me, unlike the first place. But if they still cannot figure out the speedometer, cruise control, and surging/learching issue then I may have the issue diagnosed at a dealer and go from there.
 

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Have you checked the fuses? Especially F2.17, F2.35, and F2.33? Tons of always on circuits. Easy to blow a fuse if working without disconnecting the battery. A blown fuse might easily explain why the cruise control stopped working.

Before you ask were they are, prob simpler to just check them all.

Any evidence of rodent or water damage?

+1 on checking for wiring harness damage. Especially if an aftermarket exhaust has been installed.
 
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