1999 Jeep Grand Cherokee • 162,500 miles

Hi I was wonder what could cause my Air Bag light to be constantly on? And the brake light on the dash keeps coming on? Also sometimes when the car is idling it will rev up to 1500 RPM's I didnt know if that was the throttle positioning sensor
September 19, 2013.

When the Air Bag light turns on it means the computer detected a problem, set a diagnostic fault code, turned the system off, and turned the light on to tell you. Your mechanic will read the code(s). Those will indicate the circuit or system that needs further diagnosis. The most common cause is a broken clock spring. That is a wound-up ribbon cable in a plastic housing under the steering wheel. As it continues to break, the horn and cruise control will also stop working. The fault code description for that is "Open squib" or "Open initiator circuit".

Is the brake light red or yellow?

The throttle position sensor has no affect on engine speed, and they have a very low failure rate. A vacuum leak will cause high idle speed, but if it's intermittent, the Engine Computer is usually commanding the higher speed in response to something. You need a scanner to view live data to see if the computer is requesting the higher speed or if it is trying to lower engine speed without success.

Sep 19, 2013.
The Brake light Is Red.

Sep 20, 2013.
There's up to three switches that can turn on the red warning light. First pull up on the parking brake pedal. If the light goes off, a cable is sticking or stretched and it's not pulling the pedal all the way to the released position.

Next is the low fluid switch. Check the level in the master cylinder reservoir. If it is low, just add a little. Do not fill it to the top. There is either a leak that must be addressed, or the front disc brake pads are worn and in need of replacement. As they wear, the self-adjusting feature causes the pistons to move out of the brake calipers, and brake fluid fills in behind them. Those pistons have to be pushed back in to make room for the new, thicker pads, and doing that pushes the brake fluid back up to the reservoir. If someone filled the reservoir previously, that fluid is going to spill out and make a mess. Brake fluid eats paint too. Not all vehicles have this low fluid switch. Also, be very careful to not get the slightest hint of any petroleum product like engine oil or power steering fluid mixed in with brake fluid. That will lead to a very expensive repair bill.

The third switch is the "pressure-differential" switch. It responds to a difference in pressure in the two hydraulic systems. That will be caused by a leak in one of those systems or a defective master cylinder. This one is probably the least likely to turn on the warning light. If you haven't found the cause by now, you can unplug each switch one at a time to see which one is responsible.

Sep 21, 2013.