2000 7.3L alternator replacement = speedometer problem??? - Ford Powerstroke Diesel Forum
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Old 06-21-2012, 06:32 PM
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Thumbs down 2000 7.3L alternator replacement = speedometer problem???

hi All,

my wife was driving home the other day when the truck died on her. she had previously gotten a jump start to get going. ended up having it towed home on a flatbed towtruck. it's an f350, LWB, SRW, 7.3L, crew cab, XL model.

i took the alternator off, had it tested (it was bad), and put a NAPA rebuild on it.

she also said when it happened, all of the gauges pegged, including the spedometer needle (pegged all the way clockwise).

truck has since started up and run fine, except for the spedometer needle has remained pegged.

i checked all the fuses in both boxes...one each was blown: radio fuse, and the laptop port fuse.

any ideas on what else could be wrong? i've seen other posts describing the speed sensor, but this just seems odd.

thanks in advance for the help!!!
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Old 06-21-2012, 10:13 PM
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I had that happen on a OBS when I was replacing the PSOM, the needle had rotated around while the cluster was out. Try taking the cluster out and flipping around till the needle is pointing the right direction then plug it back in and see if it works.
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Old 06-21-2012, 11:14 PM
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Is there a Check Engine Light ? Flashing OD OFF ? Do you have a way to monitor Vehicle Speed ? A CTS, AE or other capable code reader.

If there are no trouble codes and the Vehicle Speed is reporting appropriate values then you may just have a mechanical issue with the gauge as stated in the previous post.

If Vehicle Speed read through the scanner does not fluctuate appropriately with actual speed then you could have an ABS Module failure.

The ABS Module, or more correctly the Electronic Hydraulic Control Unit provides Vehicle Speed (VSS) to the PCM. This is a digital Volts AC signal. If the alternator failed it could have played havoc with the AC Rectifier and roasted the speed circuit in the EHCU.

Check for codes, verify Vehicle Speed and go from there.

If it is proven that the EHCU is bad, it can be rebuilt for around $130 by modulemaster.com vs the $600 for an OEM replacement.

Hope this helps.

Jim
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Old 06-21-2012, 11:42 PM
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BTW, for reference, the rear axle speed sensor is located in top of the rear differential and the wires run along the drivers side frame rail. It should read between 800 and 2500 when ohm'd. This circuit is often called the "VSS" - "Vehicle Speed Sensor" but in reality the VSS is the speed signal from the EHCU the PCM. If Cruise Control works and there's no flashing OD OFF then it's highly unlikely the rear diff speed sensor is bad.

Hope this helps.

Jim

Last edited by azc9a7; 06-21-2012 at 11:52 PM. Reason: because
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