FORScan - 1995 F-250 7.3 Vehicle Speed / Overdrive Flashing Light
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Parts Nerd Extraordinaire
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FORScan - 1995 F-250 7.3 Vehicle Speed / Overdrive Flashing Light
Anyone definitively know if vehicle speed can be read live through FORScan? I have been trying to troubleshoot an Overdrive issue (light flashes) and replaced the vehicle speed sensor in the diff. Speedometer on the dash works, but FORScan reads speed of 0. Other live data like engine rpm and transmission position come through.
Last edited by bmoran4; May 26, 2026 at 08:22 PM.
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Parts Nerd Extraordinaire
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From: Western NY
That was one of the possibilities I was considering. I am seeking a way to certainly diagnose before firing more parts cannon. If the PSOM was suspect, wouldn't the dash speedometer also be wonky?
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From: Bahstun
I have seen the dash show fine and the PCM not see the signal on here many times from a bad PSOM.
VSS ------> ABS Computer ------> PSOM ------> PCM
It could be the dash it self and that is hard to trouble shoot over the internet.
VSS ------> ABS Computer ------> PSOM ------> PCM
It could be the dash it self and that is hard to trouble shoot over the internet.
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Parts Nerd Extraordinaire
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From: Western NY
VSS was replaced. This helped keep the dash speedometer from jumping erratically.
Is there a way to systematically test the speed signals at the ABS, PSOM, and PCM accordingly to narrow in?
Is there a way to systematically test the speed signals at the ABS, PSOM, and PCM accordingly to narrow in?
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Parts Nerd Extraordinaire
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So the PSOM is responsible for the analog dash speedometer? And the that means that the VSS, ABS, and PSOM units are not primary suspects; that would leave the PCM? I have an older analog oscilloscope - not sure what I would need to tap to look for the speed signal. (Forgive me as I am much more intimately familiar with the simpler first generation F series trucks!)
Last edited by bmoran4; Yesterday at 04:25 PM.
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Fleet Mechanic
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Here are wiring diagrams for the VSS signal. The VSS goes to the PSOM (red highlight). The PSOM sends it to the PCM (yellow highlight).
The speed control also gets the VSS. Maybe the speed control is affecting the signal. More likely there is a bad wire or connection.
The speed control also gets the VSS. Maybe the speed control is affecting the signal. More likely there is a bad wire or connection.
Last edited by Don Ridley; Yesterday at 05:12 PM.
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Hotshot
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From: Ashland City, TN
I don't personally remember a read code for the VSS in the OBS models. In the Super duty stuff, there is 100's more sensors you can pull from.
I have no idea where my read code chart is though. So I can't look and see if it's on there.
Maybe it was.
But ...
How the fk would the PCM not see it, but the dash see it??? that makes no sense.
I have no idea where my read code chart is though. So I can't look and see if it's on there.
Maybe it was.
But ...
How the fk would the PCM not see it, but the dash see it??? that makes no sense.
Fleet Mechanic
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The speed signal is created by the PSOM in the instrument cluster. Then the speed signal is sent to the PCM.
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