 | |||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||
|
Mercedes Diesel Service NotesPosted 10/7/1998By Craig Ahlman
One of the PCM's best features is telling you when it didn't do something right. For example, it might tell you that the mass air flow (MAF) sensor didn't get its burnoff cycle. This code is seen only on F and Y bodies (Firebird, Camaro and Corvette) and only those with eight plug wires and five MAF wires, which have linear sensors. Although it doesn't appear often, this code will never set when the fault is actually present. For this reason, it may be more difficult to diagnose. Because of their location, MAFs are in a good place for contamination. Like bugs on your teeth after a long motorcycle ride, residue builds up on the MAF wires and the PCM must remove it. The burnoff cycle will usually happen after the vehicle has been run in closed loop and then shut down. When it does happen, you will see the wires inside the MAF heating to about 1,000 degrees Fahrenheit for a few seconds after shutdown. At the same time, the PCM monitors the signal line to see if a feedback voltage was induced from the MAF to find out if the burnoff cycle happened. The PCM will remind you this went awry the next time you start the car, by turning the MIL light on after the fault is no longer present. At this point, your best bet is to energize the burnoff relay by grounding it at the PCM after unplugging the MAF. If you leave it plugged in for more than a few seconds, your MAF will be so clean it smokes! Check for power at the MAF. Both the power wire and the burnoff wire should have power. If one doesn't light, you have a relay problem, a wiring problem, or a meter problem. Check both the power relay and burnoff relay. If they light, tap the relays lightly with a small screwdriver. Did the light flicker or the meter jump? Check the connections and wiring - maybe an anemic hamster called it home. If this checks out OK, you have a few options. The relays should be updated but check the part number to make sure - if not, update them. There have been voltage drop problems through the relays that are only evident during the burnoff cycle. You can also try a road test, watching the voltage at the PCM on the burnoff wire. The voltage should drop briefly from 12 volts to 0 volts after the car is shut off. At the same time, watch the MAF wires - they should glow briefly. If this happens, the PCM did its job (this time anyway) and the problem may be a faulty PCM. One situation to watch for is crankcase pressure buildup. This happens occasionally. The breather plugs and the crankcase builds pressure while running. After the engine shuts off, the air bleeds off through the PCV valve back out through the throttle body. Remember, the PCM is watching a feedback signal from the MAF to tell it the wire got hot enough. This feedback voltage is proportional to the temperature of the wire. When the crankcase is bleeding off during the burnoff cycle, the air blowing past the MAF will cool the wire just enough to keep feedback voltage low enough to tell the PCM it didn't get hot enough. Result: Code 36. So be sure to check the breather and PCV for blockage. GM says the MAF shouldn't be considered as a fault, but the PCM is getting feedback on the signal wire to tell whether to set the code. The problem doesn't have to be in the burnoff circuit to set the code, but more than likely there will be other MAF codes as well if there is something wrong with the signal circuit. Another possibility is that only the burnoff circuit is going open in the MAF. But generally, problems are isolated to the relays or PCM trigger problems.
|
||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | |||||||||||||||||||||||||||||||||
This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.
