98 Speedometer intermittent bouncing, OD out, Cruise out no lights/codes

[HookerSplitter]

First post here, looking for advice on my issue. ABout 120k on the clock and have an intermittent problem with my speedometer. After driving 20-30 minutes and then restarting after a good amount of sit time my speedo often reads anywhere between 10 and current speed. It just bounces around randomly then a brief moment of actual speed if I'm lucky.
Overdrive kicks in and out rapidly when this happens. Cruise control kills when this happens. No codes stored at all, no lights just mayhem.
I've replaced the VSS on the rear diff and the connector. No dice.
What else should I look for? I've read that the vehicle speed is actually a power trio from front wheel speeds and rear diff VSS that's interpreted through the ABS module. Maybe it has a bum ground but am lost on finding it's ground point.
Any direction is appreciated and for incentive, I'll paypal beer money to the answer with a fix.

Cheers!

 

[Dave 1955]

If it was one of the front sensors you would think the abs light would come on.


I know if you pull the abs fuse the abs light will come on and the speedo will not work on my 2001..nor will the cruise work.
I'm chasing pulsing abs and I ended up for now just unplugging one lead from the abs sensor and all is fine but i do have tape on the light..lol.
I guess I have to switch the hub/bearings out even though I know the bearings are fine.

Same thing happened on my 95 years back and new hubs fixed that abs deal.


Your deal..I dunno...strange no light.

But I don't drink beer anymore anyways..now,Capt/Coke is a diff story!

 

[2000StreetRod]

4WABS output

If your vehicle is like my 2000:
rear axle sensor > 4WABS module > speed control, PCM, instrument cluster, radio, GEM, message center, ARC module and compass/OAT module.

Since you have replaced the rear axle sensor check all inputs to the 4WABS:
right front wheel sensor, left front wheel sensor, acceleration sensor. Also check the connectors at the 4WABS.

 

[dustinjorge]

First post here, looking for advice on my issue. ABout 120k on the clock and have an intermittent problem with my speedometer. After driving 20-30 minutes and then restarting after a good amount of sit time my speedo often reads anywhere between 10 and current speed. It just bounces around randomly then a brief moment of actual speed if I'm lucky.
Overdrive kicks in and out rapidly when this happens. Cruise control kills when this happens. No codes stored at all, no lights just mayhem.
I've replaced the VSS on the rear diff and the connector. No dice.
What else should I look for? I've read that the vehicle speed is actually a power trio from front wheel speeds and rear diff VSS that's interpreted through the ABS module. Maybe it has a bum ground but am lost on finding it's ground point.
Any direction is appreciated and for incentive, I'll paypal beer money to the answer with a fix.

Cheers!

You might take a look at the VSS harness and trace the signal wire back as far as you can to see if it's shorted somewhere. I had a situation where an installer cut through my VSS pulse wire with one of those stupid clamp/splice adapters when installing a nav unit... the result was similar to what you described, very random until the wire finally cut then it was in 'limp home mode' which meant hard shifting and a blinking overdrive light.

I was told by the dealer that O/D light blinking is indicative of 'limp home mode'.

Another situation I spliced in a Truespeed from procomp that's supposed to allow me to adjust the speed pulse instead of changing the VSS gear for speedo adjustments. The thing didn't work in my truck, and I ended up with strange speedo/trans issues as a result. Needless to say it went back.

Just some food for thought.

 

[HookerSplitter]

So I did some digging. I found the harness form the VSS on the rear axle was intact and doing fine until it joins the harness in the frame rail.

The only aftermarket work on the car is done by me, I'm an installer by day and only use solder to join wires. I can't stress how much damage T taps or various crimp connectors can do in the wrong hands.

Still no lights on my dash but that doesn't mean no codes stored. My old shop has a snap-on modis and a kegerator. I'll jam by this weekend and see what's up. My plan is to go for a drive, let sit, drive and attempt to log all my sensors to find an issue.

 

[HookerSplitter]

A little update I haven't scanned anything yet but did get my overdrive light to flash until I keyed off and on again. Overdrive was working during the, didn't test for cruise and my speedo was functioning correctly.

After about 50 miles of driving and letting sit, the heat soak must be screwing up something underhood. My speedo went on the fritz on the freeway and continued to flail around until I was home. I also felt a slight hesitation but the transmission wasn't shifting, rpm was not changing and I had good power. Maybe my ABS module was hitting the brake ?!
Weird stuff going on man.

 

[HookerSplitter]

Got a flashing O/D light the other day. The second I hit cruise control it kicked off, winked the O/D light at me for a few miles then calmed down.
I filled my tank and drove upwards of 70 miles without a single fault. Let the vehicle sit and a hot restart hour later had the same malfunction with speedometer.
Pulled code P0503 a few times, "P0503 Vehicle Speed Sensor Intermittent/Erratic/High"
Oh hey no ****.
After a few quick googles found a fellow with the same condition. He said the dealership diagnosed radio frequency interference in the VSS line from the rear axle. Turns out the fuel pump wiring parties alongside the VSS wires in the harness to rear. I don't have any issues with my fuel pump but if it is stressed from a clogged strainer sock or inline filter the pump could generate RFI (noise) in the lines and radio out - contaminating the VSS signal from sensor.

I'll try running two wires from axle to abs module inside the cab.
I only use this thing to commute to work really. Not often do I run errands with it so this thing isn't high priority.

 

[HookerSplitter]

Update time!
I ran new wires from the VSS harness at the rear axle to the ABS module. No dice it's still ####y.
Getting code p1500 Intermittent VSS now. Only happens when I drive more than an hour or so and do a hot restart up to 6 hours from last drive time. Speedo, cruise, everything works if I let it sit overnight. Heat soak has to be messing with it.
Does the ABS module attached to the pump fail often? Since I'm pulling codes from the PCM and not the ABS module itself I'm lead to believe the PCM is failing itself or the wiring between ABS module and PCM or through components between is kaput.

 

[SoNic67]

If you can get one of those cheap ELM327 clones, a laptop with Bluetooth and the free Forscan software, you can test the ABS module.

 

[HookerSplitter]

Tested every module KOEO and KOER, no codes everything checked out. I've put 2k miles on this since I cleared codes and have had quite a few miles (over 100) of them with no cruise control and speedometer bouncing less than actual speed. It never rests at 0 just flutters less than actual moving speed.
I've got access to a snap-on ethos at work. I'm going to try and monitor my wheel speed outputs through the abs module in real time while driving to determine if the ABS module is faulty. I don't care if codes aren't being thrown the car in question is a Ford here with an early OBDII hell going on.
Also I've noticed when I hot start and experience the problem my volt meter in dash is slightly lower than usual at idle and all around.
Currently tested 14.4v at idle all acc off, 12.8v at idle all acc all lights on. That normal for these things? Idle speed around 620.
I'm also starting to suspect faulty PCM at this point.

 

[HookerSplitter]

Drove around until I could get it to malfunction, while watching the abs module data live. All three wheel speed sensors showed appropriate speeds. Abs module checks out just fine.
Pulled the PCM, cleaned ground and inspected the connector. Everything looks fine.
This is turning into a ton of fun!

 

[HookerSplitter]

Anyone else have an idea?
When my speedo starts bouncing and I lose cruise control I've noticed a random misfire. No codes for it so maybe the PCM is failing when hot? I've ruled out wiring as being an issue. It must be a component at this point