ISC device

In a carburetor what does an ISC device do?

In a carburetor what does an ISC device do?

well ISC is SAE for 'idle speed control', I dunno if I have ever seen any IN a carb, but they have been used ON carbs....

for example, GM and Ford used a simple 1 or 2 step solenoid to vary throttle plate opening - step 0 or 'off' is what the plate goes to key off - prevents as much air as possible from entering the motor so you get no run on. Step 1 is normal idle and step 2 (optional) was used on A/C equipped cars to boost rpm a titch when the compressor would kick on.

then in the 80's GM got smart on some olds engines and used a gimmicky little thing called the ILC (idle level control) that used a reverse vacuum solenoid to control engine idle. reverse because if no vacuum would apply it would spring open. this default position was what allowed the E4ME to get the 2000rpm for the initial fast idle

as the car ran, a solenoid would allow vacuum to hit the ILC motor and pull the rpms down, if ac was needed then less vacuum would hit. when you shut the car off, another 'anti dieselling solenoid' would route the ILC motor vacuum from the ILC solenoid to a vacuum ball which would really suck hard on the ILC motor and keep it from dieselling.

as ugly as this sounds and looked, it actually ran just as well as the IAC equipped EFI cars of today - those olds 307s had a very stable idle.

Of course no one could understand (or bothered to learn) the operation so at first burp hoses started coming off...then everyone yanked the ILC motor, then got sick of the high rpms, so turned down the initial setting, then got sick of the run-on - such are the problems when dealing with retards, er I mean shadetree mechanics....

now if by your question you meant 'throttle body' instead of carb.....

the ISC aka IAC has 2 basic types - one that ford likes is the ISC-BPA - it lets air around the throttle plate without ever touching the throttle plate by using a solenoid and pintle. more duty cycle = more air and since the ECM compensates for mixture, more fuel is added hence more rpms. The other type is a sort of stepper motor that very quickly fluctuates the throttle plate more open or more closed to acheive the same thing.

Thanks for the great explanation. Just taking a course that has some trick questions. You the man.