OBD - Throttle Position Sensor
TPS out of Synch
Recently, despite having a very
expensively reconditioned autobox, my Cosworth started to thump slightly into
gear. The engine, normally sweet as silk at idle, also started to sulk, idling
strongly and causing a lurch when selecting D from rest. This was not
good; I had a buyer for it and I had boasted about how well the car ran.
I ran a diagnostic using the OBD lead, and sure enough, I found a DTC on the
circuit low input
Wow, bummer. I looked at the DATA page and selected the TPS
sensor. I know from experience that it will show 16.1% when not depressed; other
vehicles, BMWs for example, show 0%, but all our Scorpios have a resting rate of
about 16%. The DATA page showed me the answer;
Ooops; only 4.7%. For the Scorpio, this
is far too low for the rest position.
There was the answer; 4.7% is simply wrong for the Scorpio - the
EECV expects a value of about 16%. What had happened? The EECV had gone out of
'synch' with the TPS. This happens from time to time; perhaps the throttle had
inadvertently been pressed when turning on the ignition, or perhaps there was
radio interference. Whatever the reason, this was the cause of both the poor
idle and the thumping gears. The TPS is an input for gear changes and fuelling,
so both functions would have been effected by the faulty initial position
recorded by the EECV. Somehow, both had to be re-synchronised to the correct
Luckily, I knew how. The TPS - and all the other sensors
of variable output - can be reset to base by the simple method of disconnecting
the battery for half-a-minute. Be prepared for awful running, perhaps stalling
for a few moments afterward because all of the fuelling parameters have to be
relearned - but once that has happened within a few minutes the idle will smooth
out and the auto gearchanges would be like silk once more. Make sure you have
your radio codes, though. If you haven't, someone on the Forum or the List will
be able to get them for you.
I disconnected the battery, checked
the TPS connector for both bright connections and the presence of the special
grease - both okay, then I reconnected the battery. Then I popped the OBD lead
back into the plug and checked the TPS - yes; 16.1%, the normal for my Scorpio.
A quick test drive confirmed that normal idle and perfect gear changes were
restored. This is a very worthwhile check to carry out if you experience a poor
idle and bad gearchanges.