Slip­ping clutch can make car sud­denly rev

Winnipeg Free Press - Section F - - AUTOS - By Jim Kerr

and feel like it is dis­con­nected from the wheels — like you have placed the trans­mis­sion in neu­tral.

There are a cou­ple prob­lems that can cause the clutch to slip. The first is a worn-clutch as­sem­bly. Ev­ery time you start the ve­hi­cle mov­ing, the clutch has to slip a lit­tle and this cre­ates a lot of heat. The slip­ping parts and heat do wear the clutch plate and even­tu­ally it wears to the point it can no longer hold. I have seen clutch plates wear out in a few hours be­cause the driver was slip­ping the clutch ex­ces­sively while try­ing to ac­cel­er­ate a high horse­power ve­hi­cle quickly, but typ­i­cally a clutch will last many years. Stop-and-go city driv­ing is much harder on a clutch, while high­way driv­ing places vir­tu­ally no wear on the parts.

A worn or dam­aged clutch will also usu­ally start to slip ex­ces­sively in the higher gears be­fore it be­gins slip­ping in all gears. This is be­cause the lower gear ra­tios pro­vide a me­chan­i­cal ad­van­tage to the clutch so it doesn’t have to pro­vide as much hold­ing power.

Re­plac­ing a clutch can be ex­pen­sive. The trans­mis­sion has to be re­moved, the pres­sure plate, clutch disc and re­lease bear­ing have to be re­placed, and of­ten the en­gine fly­wheel also needs to be re­placed be­cause it is part of the fric­tion sur­face the clutch disc ap­plies against. This can be four or five hours work plus parts on many ve­hi­cles.

An­other po­ten­tial prob­lem that can cause a clutch to slip is that the clutch isn’t be­ing al­lowed to en­gage com­pletely by the clutch pedal and link­age as­sem­bly. Some ve­hi­cles use a me­chan­i­cal link or ca­ble to op­er­ate the clutch but a hy­draulic clutch is now more com­mon and your ve­hi­cle has hy­draulics. If there is dirt or cor­ro­sion in the clutch mas­ter cylin­der, then the clutch ap­ply fluid may not be able to re­turn com­pletely to the mas­ter cylin­der reser­voir and it keeps pres­sure in­side the sys­tem. This pres­sure will hold the clutch only par­tially ap­plied, so it slips when power is de­manded from the en­gine. Be­fore go­ing to the ex­pense of in­stalling a new clutch as­sem­bly, have the tech­ni­cian check the hy­draulic sys­tem to en­sure it will al­low the clutch to ap­ply fully.

Q. My 1998 Volvo V70 some­times will only click when I go to start the ve­hi­cle. The lights on the dash come on, the head­lights seem to be bright enough, but the en­gine won’t crank over. The bat­tery is only three years old but if I give it a boost from an­other bat­tery, the car cranks over and starts right away. Do I need a new bat­tery al­ready?

A. Even a rel­a­tively new bat­tery can some­times de­velop a weak con­nec­tion in­ter­nally and this can cre­ate in­ter­mit­tent start­ing prob­lems, but if the head­lights are bright while try­ing to crank the ve­hi­cle, then the prob­lem is likely in the starter. The ar­ma­ture in the starter likely has some worn or burned con­tacts or the brushes are worn and by boost­ing with an­other bat­tery, you in­crease the volt­age just enough to en­able the starter mo­tor to turn over. If the ar­ma­ture is worn, once the starter has moved a lit­tle, it will usu­ally con­tinue crank­ing till the en­gine starts. Have the bat­tery tested, and if it is good, then look at the starter as the main prob­lem. Jim Kerr is a me­chanic, in­struc­tor of au­to­mo­tive tech­nol­ogy, free­lance jour­nal­ist and mem­ber of the Au­to­mo­bile Jour­nal­ists’ As­so­ci­a­tion of Canada.


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