Karl Seyfert Engineers place diodes in electrical circuits to con
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1. peared immediately afterward Do you have any idea how replacing an alternator could have damaged the a c compressor clutch diode Marvin Tow Chula Vista CA Well Marvin to state the obvious it appears that someone did something he wasn t supposed to do If you look in any automotive service manual nearly every R amp R procedure begins with the words Disconnect the negative battery cable It would seem that your customer and his friend skipped right over this step and possibly ground ed one or more of the alternators wires Other wires may also have been handled improperly and it s anyone s guess what position the ignition key might have been in at that time The damaged diode you found in the a c clutch circuit is a spike suppression diode at tached to the compressor clutch terminals to eliminate the potentially damaging voltage spikes that occur each time power is removed from the compressor clutch and the clutch opens If there were no diode in the circuit these voltage spikes could travel through the circuit possibly damaging other components and introducing noise into the vehicle s elec trical system at the very least continued on page 8 Trouble Shooter A diode is the electrical equivalent of a one way check valve It allows the flow of electrons in one direction but not the other A certain amount of electricity usually less than 7 volt must be flowing in the proper for ward direct2. Trouble Shooter Karl Seyfert Engineers place diodes in electrical circuits to con trol the orderly flow of electrons They didn t plan for the misguided efforts of a pair of DIYers however DIY Mop Up The son of a long time customer called to tell us the floor shifter on his 1996 Pontiac Grand Am was stuck in Park He also said the ERLS fuse blew every time he started the car And by the way a replacement alternator was on the back seat of the car The car was towed in and we began what we thought would be a routine electrical diagnosis and repair We confirmed that the ERLS fuse located in the engine compartment fuse center near the battery did indeed blow each time the car was started We hooked up an ammeter and noticed a substantial current spike on startup We continued by isolating each circuit routed through the ERLS fuse This included the back up lamp switch the electronic brake control re lay and module the neutral switch the transaxle 2 3 control solenoid the evaporative emissions canister vent and purge solenoids the digital EGR assem bly the powertrain control module and the alc clutch relay Isolating circuits protected by the ERLS fuse caused no change until we got to the a c clutch relay With no power to the relay the fuse would hold while the engine was cranked The a c relay checked Most digital multimeters DMMs have a Diode Check function that ups the vol
3. ion before the diode will open and allow the circuit to operate Electrical flow in the wrong reverse direction is blocked by the diode The diode terminals are called the anode and cathode The ring marking at one end of the diode body identifies the cathode Under normal conditions cur rent is allowed to flow through the diode only when the voltage at the anode is more positive than at the cathode A diode can become damaged in a couple of different ways If the forward current exceeds the diode s design speci fications the diode s internal parts may crack or melt and the contacts may sepa rate If it melts the diode may suddenly allow electrical flow in both directions The diode is a one way check valve but it s designed to hold back only a cer tain amount of electron flow So if an ex cessive amount of electron flow occurs in the reverse direction the diode will also fail and conduct in the wrong direction You didn t mention whether you had electrically tested the diode before re placing it Since the ERLS fuse blew whenever the a c clutch relay circuit was powered up it would appear that the a c compressor clutch spike sup pression diode was providing a short circuit bypassing the compressor clutch windings Although it appeared blown apart it would seem that your cus tomer s electrical shenanigans damaged the diode in such a way that it allowed current flow at all times So whenever the a c clutch re
4. lay circuit was powered up the diode provided a direct connec tion between ground and the a c relay supply voltage With no load in the cir cuit the ERLS fuse had no choice but to blow If it hadn t the rest of the cir cuit would have melted down too M Tough Problem Write to Trouble Shooter Motor Maga zine 645 Stewart Ave Garden City NY 11530 Or fax to 216 651 3016 Include your work number and email address interna The International Automotive Technicians Network is a group of over 30 000 professional automotive technicians These professionals use the network to exchange technical knowledge and information with other members from more than 100 countries around the globe Motor Magazine supports iATN and its Mission of Excellence To promote the continued growth success and image of the professional automotive technician by providing a forum for the exchange of knowledge and the promotion of education professionalism and integrity www iAIN net 8 MOTOR October 2002 MOTOR Editor amp Publisher John Lypen AF SAE 248 828 0000 Managing Editor Paul M Eckstein Art Director Harold A Perry Detroit Editor Tom Nash AF SAE STS 248 828 0000 Technical Editor at Large Karl Seyfert CMAT L1 Senior Writers Bob Gerullo AF SAE Mike Dale SAE Dan Marinucci AF SAE L1 Thomas Marcy Mark Warren AF SAE CMAT L1 Contributing Writers Ken Layne Bob 0 Co
5. nnor Paul Weissler Contributing Artist Russell von Sauers Production Manager Jennifer Herrmann 248 312 2700 Research Director Catherine Smith Circulation Manager Dale Boehle 516 227 1401 Classified Advertising Manager Donna Butera 800 858 3137 Director of Brand Marketing Lori Aemisegger 248 828 0000 Production Assistant Ann Ehlert 248 312 2700 PUBLISHED BY HEARST BUSINESS PUBLISHING INC A UNIT OF THE HEARST CORPORATION Victor F Ganzi President amp Chief Executive Officer George R Hearst Jr Chairman Frank A Bennack Jr Vice Chairman Richard P Malloch President amp Group Head Hearst Business Media William M Wright Executive Vice President amp Deputy Group Head Hearst Business Media William K Baker Vice President amp General Manager Hearst Business Media Robert D Wilbanks Group Controller Hearst Business Media Kevin F Carr President Motor Information Systems 5600 Crooks Road Suite 200 Troy MI 48098
6. tage flow through the test leads to force the diode to open The DMM then displays the voltage drop necessary to open the diode in the forward direction When the test leads are reversed the DMM should indi cate OL or out of limits If the diode is shorted the DMM indicates 0 in both di rections If the diode is open the DMM indi cates OL in both directions Photo Karl Seyfert out fine leaving only the a c compressor clutch we thought But when we discon nected the compressor clutch wiring the short was still present After reviewing the diagram we identified an alc clutch diode in the clutch circuit We 6 MOTOR October 2002 spent several hours unraveling and inspecting various looms before locating the suspect diode packaged in an oblong cream colored plastic enclosure that was hard wired within the har ness When we removed and opened the con tainer we found the diode had blown apart and was split along its entire length We installed a new diode which solved the problem When our customer arrived to pick up his car he confessed that he and his mechanic friend had diagnosed a bad alternator when his battery went dead and the car wouldn t start Their first attempt at replacing the alternator was unsuccessful but they got it right the sec ond time and the new alternator charged nor mally However they traded one problem for another as the stuck in Park problem ap
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