Wife was driving car and she said it felt like the car lost power and the speedometer quit working. The car never died and then it regained power and she was able to drive car home. The 97 Thunderbird with a 4.6 starts fine but the speedometer does not work, the overdrive light blinks on and off and the service engine soon light is lit.

lost speedometer, what does that mean 0mph at 50? ??????
YES, CHECK all fuses first. all cars, for any electric issues.

A./c lost, (ok there are 6 things that cut A/c some are normal)
one is HVAC dash blower fan must blow or AC brain cuts A.C
this prevent ICE up. (normal)
please:
why guess? please stop guessing and throwing parts and cash.
what does lost all power, driving mean.???
lost engine power (cant drive)
or lost all electric power (a prime reason for EFI to die , for lack of 12vdc electric power)
does lights die on the car, at the moment of problems?
Dash cluster , head lights, any lights.
does engine STALL? (0 RPM is stall) 300 RPM is bad idle.
800 RPM is normal hot idle, never less.

if this is just and engine power issue, (cant drive at all ,or only slow or
slow up hills?)O
my car will not move fast,? in any gear? and transmission don't slip
and no DTC scan codes (checked and smart to do first)

I'd be all over fuel pressure. this is a $20 gauge , that must be connected to the fuel rail to read pressure.
28psi is minimum for all cars.
if i knew what engine was in your car, id look yours up
alldata.com has this info.

the ecu can not report wrong fuel pressure, it cant
so is always and issue, with total engine power loss.
so is skipping 60,000 mile FULL TUNEUP's

if you need those charts they are easy to get. for any car.

tell more get more.... regards

SPEEDOMETER TROUBLESHOOTING

Can be loose speedo wiring connectors under instrument panel or its issue with speed sensor on transmission.

I suggest you to go through help links mentioned below with appropriate solutions to confirm the problem :------

How to trouble shoot speedometer?

http://schematicsdiagram.blogspot.com/2011/06/speedometer-not-working.html

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1996 fiat bravo speedometer not working?

http://technoanswers.blogspot.com/2012/01/1996-fiat-bravo-speedometer-not-working.html
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Speedometer works but odometer not working?

http://technoanswers.blogspot.in/2012/01/speedometer-works-but-odometer-not.html

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Speedometer runs backward?

http://technoanswers.blogspot.com/2012/01/speedometer-runs-backward.html

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Warning lights on and speedometer died?

http://howtobyme.blogspot.com/2011/08/warning-lights-on-and-speedometer-died.ht

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Speedometer not working on 1993 Ford F350?

http://repairhelpcenter.blogspot.in/2012/02/speedometer-not-working-on-1993-ford.html

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ABS & Brake lights warning indicator steady on dash and speedometer not working for Dodge Ram?

http://howtobyme.blogspot.com/2011/11/abs-brake-lights-warning-indicator.html

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Vehicles Cluster panel Not reading Miles?

http://schematicsdiagram.blogspot.com/2011/12/vehicles-cluster-panel-not-reading.html

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Odometer works properly but speedometer works intermittently?

http://technoanswers.blogspot.com/2012/01/odometer-works-properly-but-speedometer.html

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These will help.

Thanks.

You are throwing away money by replacing g parts that are likely just fine. Check for an ecu relay. My wife's 06 has an IPDM/ecu relay issue that will cause the vehicle to just die. It gets corrosion inside the contacts and cuts power to the computer. I had already purchased a spare for a couple dollars. Wife was close to home and car died, started back and was acting up. Swapped the relay in about 2 minutes and no issues since (several months now). Sometimes a small part can cause big problems. You can't just throw money at everything hoping to fix it. You need to do some research and troubleshooting.

Wife's 02 buick did that when the alternator quit working! Replaced it and everything went back to normal!

fouled spark plug.

Too early I think to have the automatic collision shutdown system. These can be troublesome Had a car that wouldn't start after the lady slammed the boot shut.
Are you using the original ignition key supplied by Ford and not a cut copy ?

I installed and after market stereo and my speedometer quit working 1999 chevy s10 blazer

Speedometer quit working-consider vehicle speed sensor.

timing belt slipped,bebt the valves need a valve job. these are interference engines the piston and the valves can not be in the same place at the same time,pistons bend valves

tranny gone. it might be repairable but the transmission must come off.... likely either the main input shaft from engine or one of the primary gears in ther tranny.,