Fixed: Hesitation during acceleration and wrench light ... after master cylinder and booster replacement: Be sure to calibrate brake light switch

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Asob1963

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Not sure where to post this, but I hope it helps someone. Sorry if someone has already posted, but so happy I have to share. I have a 2013 Expedition, King Ranch. 170,000 miles. Brake Master cylinder failed and pumped fluid into booster. Had it repaired at a remote town in West Texas. Nice guys, but almost immediately after picking up the truck I experienced hesitation and a shift lag during acceleration. Mostly after getting to 30 to 50 mph. Returned to repair shop. They fiddled with it for 3 hours and told me they made some adjustment. Seemed to help, but about 6 hours and hundreds of miles later I same hesitation during acceleration problem. Brake pedal seemed to sag a little, I put my left foot under it and pulled it up. Ran fine that way. NO warning lights. Took it to my local mechanic a few days later after I returned home. Said, it needed new one new ignition coil and plugs. $750. Probably did need those repairs. But, 20 minutes after driving away, a Wrench" warning light for power train fault came on. Wrench Light would clear when vehicle turned off then back on. Returned to shop for 2 more days. Could not get a code. Eventually retrieved vehicle for a weekend. Drove it all day Saturday with left foot under brake pedal. No warning light and no hesitation in acceleration. Next day drove without foot under brake pedal. Got erratic wrench warning light and hesitation during acceleration at various times. Returned to shop this morning with wrench light on and had them scan for codes read before turning off vehicle. Two BRAKE LIGHT SWITCH codes. Made appointment for morning. I had replaced the brake light switch just before taking it to the local shop in my home town. Had watched tons of videos on how to do it. Some said no calibration was needed. I even tried to pull the plunger out by hand but felt it was too hard to do and did not use pliers. Tonight after watching random video on replacing brake light switch in a Ford Edge after a master cylinder change, I see that the brake light switch must be fully extended and then calibrated by inserting it while pedal is fully depressed. Took out brake light switch, gave plunger a tug with pliers. Ratchet sound and it came out and remained very stiff. Started motor and depressed pedal fully, inserted switch and then allowed pedal to return to its top position naturally. Heard a couple of clicks. Did not pull pedal up. Took for long test drive. Seems to have FIXED both the hesitation and wrench light. Acceleration is really responsive. Bottom line: there are tons of videos out there on how to replace a brake light switch, but most don't mention calibration. I think this is a must. I suspect the mechanic that did the master and booster did not calibrate the switch after putting it back in. I know I did not when I put in the new switch. I realize there are 20 other potential reasons for similar issues, but making sure you calibrate the brake light switch is a cheap and easy place to start.
 

Trainmaster

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Thank you for sharing this. You are right that the specifics are seldom discussed. New switches come with the plunger fully extended. When you install them (with the pedal down) the brake pedal arm sets them when released. There's a ratchet in the switch that sets it in to the proper depth. That's a one-time deal.

You only get one shot at this, so don't push the button in when you play with it out of the box.

Some of the cheap replacement ones don't even do this, so good luck with them.

Thanks for your post and welcome.
 
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