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Old 04-10-2021, 08:09 PM   #8
fdryer
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Join Date: Jan 2006
Location: NYC
Posts: 44,891
 

2003 L-Series 3.0L Sedan
Default Re: Communications link error, service vehicle and brake warning

I'm not familiar with Ion master cylinder low brake fluid switches but other similar low brake fluid switches do not use resistors. A review of my service manual for my L300 shows a simple on/off switch in the wiring drawing. When brake fluid drops below the low level mark, the normally open switch closes, connecting ground to the bcm. The bcm interprets this as low fluid level and turns on the red brake light. Filling the mc with fluid with the float rising opens the switch removing the ground connection. The bcm not sensing a ground turns off the brake light. I presume this circuit applies to virtually every master cylinder low level switch circuit. Please correct me if I'm wrong.

If I'm not mistaken, the switch should measure zero resistance/open when float is high, closed/short when float is down. Disconnecting the wire to the mc should result in the brake light off. This presumes the parking brake switch is also closed/parking lever released in down position or wire to this switch also disconnected.
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