Tech Tips
Dead Batteries And Solid State Electronics
Using The Old Test Procedures Will Make You Crazy
by Dave MartinThis is not an uncommon scenario: A customer complains of a dead battery in the morning. They have to jump start it three days in a row, and now they’ve brought it to you for repair. You run a charging system and battery check, and they both show normal readings. Something is draining the battery. It’s time to run some tests. Since checking voltage between the battery and cable is no longer an accurate test, you disconnect a battery cable and install your ammeter in series. Your meter reads 25 milliamps (mA), well within normal range. How can that be? "There has to be an excessive draw here that’s killing the battery," you tell yourself.
What’s Happening?
On-board computers and many other solid-state devices on today’s cars need full-time power to energize their "keep-alive" memories. When you break the circuit to install your ammeter, these systems shut down. Any defective component that had been drawing excessive current before you disconnected the cable, may work correctly when power is reconnected.The solution is to measure amperage flow without removing power to the circuit. This can be accomplished by two methods. Either run a jumper wire from the battery terminal to the cable while connecting your meter, or install a parasitic drain switch which may be opened after your meter leads are connected.
Important Concepts To Remember
1) If you install a parasitic drain switch, make sure you drive the vehicle afterward using all accessories. That will ensure that any defective component has been operated. Then, with your meter connected between the battery and the cable, either open the parasitic drain switch or remove the jumper wire and read the amperage draw. Any draw over 50 mA is excessive.
2) Some systems normally draw power for up to 15 minutes after the key is turned off. If your meter initially shows over 50 mA, leave it connected and check it again in 15 minutes. If the draw is still over 50 mA, there is a problem that needs to be located and repaired.Finding The Hungry Circuit
Leaving your meter in place, disconnect each fuse, one at a time. If the draw is still present after all fuses are removed, you will have to start disconnecting the fuse links in the same manner. Keep disconnecting fuses, fuse links and components until you find the one that makes current draw fall to an acceptable level. The problem is on that circuit. By the way, make sure you preauthorize diagnostic time with the customer. Make sure they understand that repairing the problem is usually easier than finding it.
Rebuilt Engine -- No Start
Engine Had Injector Pulse, But No Spark. What Gives? by Dave Martin There is nothing worse on your bottom line than putting in hours of work on an engine swap, only to find it doesn’t start.
While you may not run into this exact situation very often, the logic behind troubleshooting this car can be useful in other GM no-start, no-spark situations.
Background
The rebuilt 2.0 liter throttle body injection engine had just been installed in a 1989 Corsica. Tests showed consistent injector pulse during cranking, but there was no spark at any cylinder at any time!The tech’s first thought was that it was a bad ignition module. He knew that the ignition module was sending the five-volt digital signal to the PCM, because the PCM was firing the injectors. But for some unknown reason, the module was not grounding the primaries on the coil packs. Three new modules didn’t change a thing still no spark.
The tech then turned his attention to the crankshaft position sensor (CPS). Again, three new parts, and again no spark! Next came multiple coil replacements, the CPS harness, even a new PCM. Result? No spark.
The tech had done a thorough job of testing before he called us. He had checked powers and grounds, sensor reference voltage and primary voltage during cranking without the coils installed. He had power to the coil primaries, but still no ground side toggle. All connections were good and he was out of clues.
The Slots, The Gap, The Answer
The DIS system on many of these cars uses a variable reluctance sensor. The sensor, in combination with a slotted ring on the crankshaft, generates an AC voltage to the ignition module. The ring contains seven slots. Six are equally spaced in 60-degree increments around the ring. But the seventh is used as a synchronization signal to the ignition module. Once the module sees the seventh slot, it knows to start the firing order over again. How does it identify the seventh slot? Because the seventh slot is located only 10 degrees after the sixth slot.The variable reluctance sensor detects the slots due to their increased gap from the tip of the sensor. In this case, the CPS was too close to the slotted ring. The module could recognize the 60-degree slots, but not the synch slot. Without a synchronization signal, the module could not determine where the crankshaft was. So, it never provided any ground to the primaries on the coil packs. It did, however, pass on the 60-degree signals to the PCM, in the form of a digital five-volt pulse. That’s what told the PCM to fire the injectors.
The Fix?
We had the technician add a .020" washer to the CPS and the engine fired right up. The tip? Never assume the whole system works just because one part of it does.Dave Martin has 24 years experience diagnosing and repairing automotive problems. He works in the GM section at autoLine Telediagnosis fielding calls in all areas, including electrical, driveability and drivetrain. Dave is L-1 certified and also holds a bachelor of science degree in business management.
Each month, technicians from autoLine Telediagnosis share valuable TechTips with our readers. We print these TechTips on heavier paper, so you can three-hole punch them and save them for future reference. As an ASA member, you’re already signed up to use the autoLine service at a discounted rate.
ASA Main Page || AutoInc. Main Page
Directions || Stat Corner || News Briefs || Taking The Hill
More Info About AutoInc. || Tech To Tech || News Briefs Extra
A Look at Collision Repair In The North-Central Region || Tech Tips
Chairman's Message || Issues Along The Way || The Top 45 Tips For Success
Guest Editorial || The Changing Parts Market -- And How It Affects Shop Owners
Collision Estimating Times: Are They Shrinking? || Have You Heard About OBD III?
AutoInc. Magazine ®, Vol. XLIV No. 5, May 1996