My 1986 420 SEC had sat in the garage for seven years unused until November last when I completed some cosmetic and mechanical work before putting it back on the road. It is a catalytic converter car with lambda sensor.
Last saturday at the end of a forty mile round trip, on selecting 'park' the revs rose to about 1600 rpm at idle and refused to go back to normal tick over.
The drive had been over undulating A roads at around 50 to 60 mph and I used the "s" auto box mode for overtaking farm traffic twice but returned to "e" mode approaching home.
I do not have a Merc workshop manual, only a USA Bentley Publishers Bosch FI & EM book which gives generic but not model specific information so I am slightly in the dark for diagnostics.
With a cold engine I checked to ensure there is no throttle binding anywhere, and then connected my test meter to the two wires on the auxilliary air valve. With ignition on the the valve vibrates and voltage fluctuates in a range from zero to around 2 volts, much lower than the 12 volt battery voltage I expected, but that was just my guess.
The valve also works off the car with a 12 volt supply.
On starting from cold the engine ran at normal tick over speed, great thought I!!!!!!!! However before I could get the top off the can to celebrate the revs went back up to 1600 rpm. This was only about 1 minute after starting from cold, and the top hose from engine to radiator was a little warm but perhaps not even human body heat.
The low voltage at the aux. air valve prompted me to look for throttle body switches as I found these previously on another car with a Bosch motronic system controlling voltage to this valve, but I can see none and do not want to remove the whole FI system just to confirm it.
So I then thought about the warm up issue, and on the casting to which the top radiator hose connects there are two brass sensors in the cooling water flow. One has a single push on wire terminal(it has a hex. of 17 mm) and is stamped"120 C", the second is bigger(21 mm hex I think) with two separate sensor connections, one has a single push on wire terminal and the second has a double push on wire terminal. I cannot read the marking as access is difficult.
I have no idea what the electrical diagnostics for these sensors cold and hot should be?
My final thought is that the lambda sensor may be faulty, but so far I have not even looked in that direction.
I am up the creek at the moment and will welcome some sound advice.
Perhaps I need to get a workshop manual. Does anyone have knowledge of the CD versions sometimes advertised?
Terry
Last saturday at the end of a forty mile round trip, on selecting 'park' the revs rose to about 1600 rpm at idle and refused to go back to normal tick over.
The drive had been over undulating A roads at around 50 to 60 mph and I used the "s" auto box mode for overtaking farm traffic twice but returned to "e" mode approaching home.
I do not have a Merc workshop manual, only a USA Bentley Publishers Bosch FI & EM book which gives generic but not model specific information so I am slightly in the dark for diagnostics.
With a cold engine I checked to ensure there is no throttle binding anywhere, and then connected my test meter to the two wires on the auxilliary air valve. With ignition on the the valve vibrates and voltage fluctuates in a range from zero to around 2 volts, much lower than the 12 volt battery voltage I expected, but that was just my guess.
The valve also works off the car with a 12 volt supply.
On starting from cold the engine ran at normal tick over speed, great thought I!!!!!!!! However before I could get the top off the can to celebrate the revs went back up to 1600 rpm. This was only about 1 minute after starting from cold, and the top hose from engine to radiator was a little warm but perhaps not even human body heat.
The low voltage at the aux. air valve prompted me to look for throttle body switches as I found these previously on another car with a Bosch motronic system controlling voltage to this valve, but I can see none and do not want to remove the whole FI system just to confirm it.
So I then thought about the warm up issue, and on the casting to which the top radiator hose connects there are two brass sensors in the cooling water flow. One has a single push on wire terminal(it has a hex. of 17 mm) and is stamped"120 C", the second is bigger(21 mm hex I think) with two separate sensor connections, one has a single push on wire terminal and the second has a double push on wire terminal. I cannot read the marking as access is difficult.
I have no idea what the electrical diagnostics for these sensors cold and hot should be?
My final thought is that the lambda sensor may be faulty, but so far I have not even looked in that direction.
I am up the creek at the moment and will welcome some sound advice.
Perhaps I need to get a workshop manual. Does anyone have knowledge of the CD versions sometimes advertised?
Terry
