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Old 04-05-2005, 13:01   #1 (permalink)
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Join Date: May 2004
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confused EEC-V VSS readings way off - speedo OK

My Ford dealer can't locate the error, which shows like this:

From a full stop, the car is accelerated to 60 km/h (normal steady acceleration).

With the data logger connected to the OBD plug under the steering colum, the ECU speed readout fluctuate like crazy. It is worst just when the car starts moving (peaks up to 200 km/h), then gradually gets more smooth with higher speed.

The speedometer is "calm" and just shows increasing speed.

The Ford dealer tells me that this fluctuation may cause the very strange shifts we see in the automatic transmission. I find this plausible. He wants to replace the ECU - I find this very unlikely to be a cure.

But ... why is the speed fluctuating? One can rev up the engine in neutral without any ECU speed readout spikes. I've looked at the VSS signal on an oscilloscope with the engine off, ignition on while turning a wheel with the car lifted on a jack - it is a fine square 0-12 volts signal. The VSS has been replaced two times with the same observed behaviour.

The car is a feb/1997 Ford Mondeo V6 2,5L with automatic transmission, the car has done 260.000 km. The ECU is marked 97BB-12A650-RE

The ECU has no obvious defects (like full-of-water, corrosion or that sort of trouble)

Thanks in advance,
Niels Kristian Jensen
Denmark
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Old 04-07-2005, 05:17   #2 (permalink)
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Re: EEC-V VSS readings way off - speedo OK

I have a similar issue - same symptoms, but only minor fluctuations (not up to 200km/h !!) It makes the trip meter spike up also (L/km) but speedo is ok. new VSS didnt fix, so I am going to try and install a ground wire from VSS to body, check all plugs/connectors and wiring. Whe the car is still, very occassionaly the speedo will fluctuate up to 20 or 60 kmh, but moving forward just a few cm's stops it, as does changing into another gear in the auto. btw - this is an Australian falcon, but still EECV. i will send email with a doc on OBDC strategys, with a good section on automatic transmission malfunction codes/sensors.
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