speedometer again
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New User
Joined: May 2008
Posts: 21
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From: Colstrip Montana
speedometer again
alright guys im still confused about why my speedometer is not working. I replaced the vss sensor on the diferential and searched the forums and all I got was vss and psom. If i dont have to replace the psom then i dont want to because of the mileage would be diferent and i just dont want to screw with it. if you know where there would be a loose wire under the dash other than directly on the speedometer that would help. note that the speedometer is the only gauge not working but the dash lights are dead and the light behind the heater ***** is not working niether is the dome light. also the transmition is shifting harshly. please help, i would just take it to the local mechanic but he charges way more than I can afford as I am just in high school.
1993 f-150 302 automatic.
any help is more than welcome
thanks
1993 f-150 302 automatic.
any help is more than welcome
thanks
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
You can try unplugging the ABS module and see if it works OK then.
If not leave it unplugged and check the AC Voltage coming from the rear end while the back end is up on jack stands. The AC voltage should correspond as stated below.
Connect an AC voltmeter to Pin 4 (speed in +) and Pin 5 (speed in -).
The voltage should increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to 30 mph.
If you have the correct AC voltage for the MPH the wheels are turning under the hood or at the ABS plug if you are driving it vs the jack stands then it would be the PSOM being bad.
If you do not get the right AC voltage then you will need to pull the rear end apart and repair the tone ring, VSS sensor or the wiring from the rear end to the dash.
If not leave it unplugged and check the AC Voltage coming from the rear end while the back end is up on jack stands. The AC voltage should correspond as stated below.
Connect an AC voltmeter to Pin 4 (speed in +) and Pin 5 (speed in -).
The voltage should increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to 30 mph.
If you have the correct AC voltage for the MPH the wheels are turning under the hood or at the ABS plug if you are driving it vs the jack stands then it would be the PSOM being bad.
If you do not get the right AC voltage then you will need to pull the rear end apart and repair the tone ring, VSS sensor or the wiring from the rear end to the dash.
Post Fiend
Joined: Oct 2003
Posts: 6,214
Likes: 13
From: New Jersey
After you test using subford's method you can take it one step further if you have a good signal off the new VSS sending unit.
The 92-95 F-series have a lonely PSOM test connector behind the glove box in the wire loom that goes across the AC/Heater box assembly. On my 93 it's a green wire with a one pin connector on the end - the signal between the PSOM and the EEC is there. If you do not see a signal here, but do up to the PSOM, then the PSOM module is fried "for sure".
And yes, changing the PSOM messes up the mileage, but what can you do except document the mileage before and afterwards. Most states of a mileage waiver document just for this reason.
93 PSOM... you can use one from a 92-95 F-series truck (including Bronco), 92-94 E-series van, and 91-93 aerostar, though the aerostar units are not programmable for non-stock diameter tires whereas the F and E series PSOM modules are.
You can "add" this feature by unsoldering your pushbuttons off your old PSOM onto the aerostar PSOM. That's what I had to do because I couldn't find a local junked F-truck with a working cluster.
The 92-95 F-series have a lonely PSOM test connector behind the glove box in the wire loom that goes across the AC/Heater box assembly. On my 93 it's a green wire with a one pin connector on the end - the signal between the PSOM and the EEC is there. If you do not see a signal here, but do up to the PSOM, then the PSOM module is fried "for sure".
And yes, changing the PSOM messes up the mileage, but what can you do except document the mileage before and afterwards. Most states of a mileage waiver document just for this reason.
93 PSOM... you can use one from a 92-95 F-series truck (including Bronco), 92-94 E-series van, and 91-93 aerostar, though the aerostar units are not programmable for non-stock diameter tires whereas the F and E series PSOM modules are.
You can "add" this feature by unsoldering your pushbuttons off your old PSOM onto the aerostar PSOM. That's what I had to do because I couldn't find a local junked F-truck with a working cluster.
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