Binman
May 21st, 2003, 20:26
Volvo 460 1.8Si 18BU – 1995, 70,000m Idle speed motor run away.
Potentially a dangerous FAIL to UNSAFE fault if the motor drives the throttle when driving on the road.
After the engine is switched OFF the idle speed motor can be heard, and seen to run. The plunger of the motor pushes the throttle open. This means that when restarted the engine revs wildly to over 4000rpm+ and the throttle idle motor will not return on its own.
Here is the question(s)
Why does the throttle idle speed motor receive power when the key is turned to the OFF position?
Is there a relay somewhere in the fuel/idle control system that is faulty? Is it a chip?
What can I do with the diagnostic box that is on the car, I see it has an LED and a short wire with a terminal plug that looks like it goes into the sockets on the diagnostic sockets.
The description of the fault.
The throttle idle speed motor moves one way with +12v and the other with –12v using the same 2 pins, so its supply is switched somewhere remote from the motor.
The fault started gradually by being intermittent and did not drive the plunger as far out as it does now. Sometimes the throttle would return to normal operation after a few miles. Now it has become so bad I have disconnected the idle speed motor and set it to a position that allows a reasonable idle speed when the engine is warm. This was done by applying +/-12V to the top two terminals of the motor to power the plunger in/out to set/fix the idle speed.
I have checked the motor by removing and disassembling it, it works fine. The switch which is the button on the end of the plunger works okay as well, it is connected to the lower two pins of the four pin connector on the idle speed motor. When the motor moves the plunger out the button on the end of the plunger makes the switch contact the end of the throttle arm.
I also checked the throttle switch on the other side of the throttle body, this isn’t a conventional switch, it’s a potentiometer with 4 carbon tracks and two pairs of wipers connected to pins 1, 2 and 4 that give a variable resistance between 1.8 Ohms and 7.2 Ohms depending on throttle position. There is no throttle closed switch, or WOT switch on this model, therefore no zero ohms and no infinity resistance.
I’ve checked the engine coolant temperature sensor and it checks out within range, as does the air temp sensor in the throttle body. I haven’t checked the O2 sensor because I don’t think it’s the culprit.
Please help any advice might be worthwhile.
Can you minimise your waste?
Potentially a dangerous FAIL to UNSAFE fault if the motor drives the throttle when driving on the road.
After the engine is switched OFF the idle speed motor can be heard, and seen to run. The plunger of the motor pushes the throttle open. This means that when restarted the engine revs wildly to over 4000rpm+ and the throttle idle motor will not return on its own.
Here is the question(s)
Why does the throttle idle speed motor receive power when the key is turned to the OFF position?
Is there a relay somewhere in the fuel/idle control system that is faulty? Is it a chip?
What can I do with the diagnostic box that is on the car, I see it has an LED and a short wire with a terminal plug that looks like it goes into the sockets on the diagnostic sockets.
The description of the fault.
The throttle idle speed motor moves one way with +12v and the other with –12v using the same 2 pins, so its supply is switched somewhere remote from the motor.
The fault started gradually by being intermittent and did not drive the plunger as far out as it does now. Sometimes the throttle would return to normal operation after a few miles. Now it has become so bad I have disconnected the idle speed motor and set it to a position that allows a reasonable idle speed when the engine is warm. This was done by applying +/-12V to the top two terminals of the motor to power the plunger in/out to set/fix the idle speed.
I have checked the motor by removing and disassembling it, it works fine. The switch which is the button on the end of the plunger works okay as well, it is connected to the lower two pins of the four pin connector on the idle speed motor. When the motor moves the plunger out the button on the end of the plunger makes the switch contact the end of the throttle arm.
I also checked the throttle switch on the other side of the throttle body, this isn’t a conventional switch, it’s a potentiometer with 4 carbon tracks and two pairs of wipers connected to pins 1, 2 and 4 that give a variable resistance between 1.8 Ohms and 7.2 Ohms depending on throttle position. There is no throttle closed switch, or WOT switch on this model, therefore no zero ohms and no infinity resistance.
I’ve checked the engine coolant temperature sensor and it checks out within range, as does the air temp sensor in the throttle body. I haven’t checked the O2 sensor because I don’t think it’s the culprit.
Please help any advice might be worthwhile.
Can you minimise your waste?