Transmission problem maybe?

Car Talk
I have a 2008 Mercury Sable that I bought used. It has been very dependable except for one problem: While driving the ‘tire sensor fault’ light comes on ( not the low tire pressure light ) then the ‘ABS’ light comes on, then the little ‘wrench’ light comes on then about 15 seconds later the speedometer (the tach still works), the odometer display quits and the cruise control quits working. I can continue driving with no problems I get to my destination shut the vehicle down. This is what happens next: I start the car all the ‘idiot’ lights are off, the odometer display is working, and the speedometer is working but the car starts off with clanging (like the transmission is trying to find a gear) and the car is actually in high gear as I gain speed it eventually downshifts then shifts normally. The car is normal until the next episode which is usually when I travel more than 20 miles in cruise mode, but not every time. Sometimes I’ve gone a several months before this has happened. I have had the car at a local high-end shop and they couldn’t find anything wrong. I’ve tried to duplicate driving conditions to make it happen, but it only occurs when it feels like it? I have noticed though when I am going up hills the downshifting is sluggish almost like it is trying to find the right gear. (but again not all the time) What do you think? And have you ever heard of this problem before?
Richard in Western PA

I’d replace the VSS (vehicle speed sensor).

Condition is screaming an issue with the High Speed CAN network. Vehicle Speed data is sent from the ABS module to the PCM and Cluster ober the HS CAN Bus. The Tire pressure data is sent from the GEM/SJB module to the cluster via the Medium speed network to the cluster. The Cluster acts not only for instrumentation but also a gateway between the two networks, sharing data between the two networks and all the modules.

Network dropouts usually result in U-code DTCs present in multiple modules which cannot be ignored. U codes for this vehicle will report either lack of communication with another module or invalid data received from another module. For example, the ABS module may report a U0401 DTC which stands for “Invalid data received from the PCM”.

If the network traffic is interrupted, modules will not have critical information they need to operate. Simple example, if the Cluster loses data, many, if not all the warning indicators will illuminate, gauges will likely drop out. Engine computers losing data from the ABS will have no idea what the true vehicle speed is and will get confused as to what gear ratio they need to be in…

Whoever is servicing this vehicle needs to record what U codes are present and what module the DTCs are located in. (A U0401 for example could be found in multiple modules) Quite often the technician can see a pattern in the DTCs that will lead to a failed module or wiring issue. I can say from experience that many CAN network faults are due to wiring issues (Within the CAN network wiring) such as intermittent shorts, connector corrosion or connector terminals that have become loose or damaged from previous vehicle repairs.

There are quite a few ways to diagnose network issues:

  • DTC driven diagnosis and looking at the u-code patterns. Remember the tech needs to know both code definition and Module LOCATION… Not just the definition.
  • DVOM OHM testing to see if the proper resistance is present between the HS CAN circuits to verify the network harness integrity (Should be 60 ohms) other tests for shorts and high voltage can also be done as well.
  • Ford Scan tool Network test which pings the module to see who responds during the fault. (This requires that the technician verify your concern however)
  • There is also a new LIVE Network Test available to Ford dealers that will allow the modules to be repeatedly pinged while the technician road tests the vehicle or wiggle testing the harness. Intermittent faults will be picked up by the tool and displayed… The tech would then look at the patterns and the wiring schematic to ID the origin of the problem. (I am not saying you have to go to the dealer… This LIVE NET test is only available there as far as I know)

A good electrical/electronics trained auto technician should be able to nail this down pretty easily with adequate equipment.