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Discussion Starter · #1 ·
I posted awhile back about a dead speedometer and hard shifting. I replaced the VSS (or ABS if you like) which did not fix the problem. I then traced the wires out to the 4WABS (wires are good) and then traced the one orange/LB wire to the PSOM (wire also good). My question is this: If I run a jumper from the O/LtBlue wire to a voltmeter, then go driving, should I see a varied voltage as I accelerate? If I ran a jumper from the VSS and performed the same test, would I get the same results, or are there easier ways of testing for a faulty PSOM? I'm guessing they're expensive. This is a '94 Bronco by the way.
 

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Since no one has responded, I'll at least give you something...I can't answer your electrical jumping questions, but related to the PSOM, it's expense, fixes, etc, try Miesk5's site:

http://mywebpages.comcast.net/miesk5//

I was just out there reviewing a TSB for speedo needle jumping, not quite the same thing as you, but perhaps you'll find some useful info.
 

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A good used PSOM isn't that expensive - I have at least 1, but the mileage on it might not be close to yours.

Have you looked at the Haynes wiring diagrams? There's a test connector built into your wiring harness for the ABS sensor signal. To test the signal correctly, you need a true RMS multimeter ($50-200) that can measure frequency. I've posted the procedure & calculation before, but maybe not ont this BBS. It might have been on FordF150.net or FordTruckWorld.com, but their searches don't work as well as FSB's. If you can't find it here, I'll look it up again & repost it.

Does your cruise control work smoothly?
 

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Discussion Starter · #4 ·
Shadofax - I'll have to keep that website for future ref. - nothing applicable to my PSOM problem though, it seems.

Steve83 - I searched for your info, but couldn't find it. Is this next to the EEC connector? How do you extract codes from the ABS test connector?

Couldn't I just disconnect the PSOM connector and put a voltmeter in the VSS input just to see if there's anything at all? I don't really care if it's perfect, but at least this would tell me if I was good all the way to the PSOM, right?
 

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The VSS doesn't send out voltage, it sends out a pulse signal. Thats why Steve mentioned having a true RMS multimeter with FQ testing ability.
 

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Yeah, actually it will tell you if the thing is generating any kind of current... but I dont think that will give you an exact yes or no on whether or not the VSS is working properly.
 

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Discussion Starter · #9 ·
UPDATE:

1. In answer to Steve83's question, no the cruise control does not work.

2. I ran the test that involves holding down the reset button on the odometer, and it said something like P 08 06, can't remember exactly - and the speedometer went all the way to top speed then back down again. Does this mean the PSOM is fine??
 

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The needle sweep on the speedo is a indicator thats it is OK, but its usually used when you change the conversion constant of the PSOM.

here I just cut and pasted this...:

4. Release RESET button The Speedometer will prove out(sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display id the dealer mode lockout count. This count shows the number of allowable conversion constant changes remaining. When the count is zero (0), no additional changes can be made to this instrument cluster. If a revisionis required a service instrument cluster must be ordered.

Here is a link to the page.....I know its not exactly what your looking for but....???

http://www.mudnuts.org/modules/sections/index.php?op=viewarticle&artid=23
 

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The rear speed sensor signal goes to the 4WABS module where it is converted from 72,900-86,190 pulses/mile to 8000 pulses/mile.The signal goes from the 4WABS module to the PSOM,PCM,Speed Control and other speed signal users.If your ABS lite is on the inst cluster,repair the 4WABS first.Voltage at the cluster(across pin 4-orange/light blue and pin 5-light green/yellow)should be 0-3.5 volts on the A/C voltage setting on the digital VOM when accellerating from 0-30 mph.

To check the wiring from the rear speed sensor to the 4WABS module,first make sure the rear speed sensor resistance is 800-1400 ohms.With the rear sensor plugged in,disconnect the 4WABS module connector,check for 800-1400 ohms across pin 21-light green/black and pin 22 red/pink.If you have the 800-1400 ohms,then the wiring from the rear speed sensor to the 4WABS module is good.If you don't have the 800-1400 ohms at the 4WABS module connector,then you have an open circuit in the rear speed sensor wiring.Most of the open circuits that I've seen have been in the wiring from the rear speed sensor to the frame rail.

Hope this helps,bronco1
 

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damonwolf said:
I noticed an earlier post by Steve83...
You're taking that out-of-context; keep ALL the bold text together. This is the link to that post. You can't pick one step out of a test procedure & expect to get useable results.
bronco1 said:
...the 4WABS module ...converted from 72,900-86,190 pulses/mile to 8000 pulses/mile.
:twak
That's wrong. The PSOM makes the conversion.
 

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I thought the VSS always went to the PSOM first, because that way it allowed you to send a corrected tire size signal to the ABS/EEC?
 

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Sorry steve,you are correct,the signal does go to the 4WABS module first but continues through to the PSOM unconverted.From what i can see on the diag,if the ABS light is not on,then you are probably getting a signal from the rear speed sensor,if the light is on,then you may not be getting a signal.I believe the diag above that I posted is still correct.
 

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Discussion Starter · #15 ·
I put the multimeter on the connectors at the 4WABS with the VSS plugged in and it read 1.28, which I assume is 1280 ohms, so I believe the wiring is OK. I also did a continuity test which registered at 1 ohm.

So, it looks like we're good to the 4WABS. My multimeter has a freq. setting. I don't have the luxury of jackstands, so what if I pull the instrument cluster (again) and drive on down the road with the meter plugged into the VSS input to the PSOM? What should neg. be plugged into, ground? I only see one wire coming from the 4WABS to the PSOM in the schematics.
 

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bronco1 said:
Sorry steve,you are correct,the signal does go to the 4WABS module first but continues through to the PSOM unconverted.From what i can see on the diag,if the ABS light is not on,then you are probably getting a signal from the rear speed sensor,if the light is on,then you may not be getting a signal.I believe the diag above that I posted is still correct.

That still doesn't make sense though, because if you change tire sizes, your ABS module isn't getting a corrected vehicle speed. It just doesn't seem right that they would do it that way, because you would want the ABS module to get corrected speeds right?

Or does it not matter because there are multiple abs sensors, and its not using just the VSS?
 

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I would say that the 1.28 is 1.28K which is 1280 ohms on the one in the Bronco which is good.Now with both ends disconnected,test each wire to ground,you should get the same reading as with the tester disconnected.The set the meter on DC volts (approx 20 volt scale)and test the same wires to ground,the meter should read 0 volts.If all this checks out,then the sensor and wiring to the 4WABS module are ok.The next step is probably to hook the meter to the instrument connector-pin 4(orange/light blue)and pin 5(light green/yellow),make sure you have the sensor and the 4WABS module hooked up,have the meter set to AC volts, and road test to approx 30 MPH(or have the Bronco on jack stands and run the rear wheels to approx 30 MPH),the meter should read 0 when stopped and approx 3.5 volts at 30 MPH.You haven't said whether the ABS light stays on or goes out.
 

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Discussion Starter · #18 ·
I edited my original message I guess while you were typing yours, bronco1 -- you must have read my mind. I'll go do the road test now - have to take that damn panel out again. I'll post back in an hour or so (I'm slow and I dropped a couple of screws God knows where when I put the panel back on last time).
 
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