------------------------------From: max_at_gte.netDate: Mon, 1 Apr 1996 19:06:48 -0600Subject: resistor in wiring harness to fix idleScott was wondering why a mechanic suggested a resistor in line with hiscoolant temperature sensor to improve idle on his E30. I can take a stab atthis as I had a Saab with an idling problem and the same solution worked for me.Why add a resistor?The temperature sensor for the fuel injection system is an NTC (I think thismeans negative temperature coefficient) sensor. The colder the engine thehigher the resistance. The engine computer reads this resistance and setsthe fuel mixture accordingly. In freezing temps, the resistance is high -thousands of ohms. At operating temperatures, the the temperature sensorresistance approaches zero. In some of the early LH Jetronic Saabs, idlespeed went up and down between about 500 and 1500 rpms when the car wasstarted cold in ambient temps of about 60 degrees F. The problem was thatthe engine was running too lean. Adding a resistor tricks the enginecomputer into thinking that the engine coolant is colder than it actually isand causes the computer to order more fuel. A revised ECU (about $1,000)was also supposed to work but a 29 cent resistor from Radio Shack did thejob for me.A word of caution. Determine the value of the resistor BMW added to the wiring harness. ForSaabs circa 1986-87 a 500 ohm resistor usually worked fine. A half wattresistor is ample as the circuit carries little current. Too muchresistance and the car will run rough and too rich. No flames about Saabs, please. They are fine cars, even though SatchCarlson and I might be the only BMW drivers to think so. And keep in mindthat it was the German Bosch system causing this trouble. Nothing that theSwedes built caused this little glitch, although they might have been theones responsible for the fuel mapping of the ECU. PaulDallas TX------------------------------