TechTips
TechTips is a monthly feature in AutoInc. Each month, specialists who field calls for autoline Telediagnosis®, a technical hot line service, share some of the most common vehicle problems and their solutions.
Motronic engine with flat spot in acceleration Vehicle: 1983-1987 BMW with Motronic engine management problem: hesitation or "flat spot" on acceleration.
The Motronic engine management system integrates both ignition and fuel control functions into a single control unit. It contains a multidimensional "map" of timing advance and injector duration values based upon engine speed, load, temperature and throttle position signals. If any of these inputs is inaccurate or missing, the effect on driveability can be severe.
A case in point is that of a 1986 BMW 325e with a very bad "flat spot" in mid- range acceleration. This car would accelerate normally to 3500 rpm in any gear, then simply refuse to go any faster. Routine testing of inputs from throttle switch, air flow meter, coolant temperature sensor and engine speed sensor showed no abnormalities. Fuel pressure and delivery volume were adequate. If the engine could be coaxed past the flat spot between approximately 3500 and 3800 rpm by feathering the throttle or by downshifting to a lower gear, the car would then accelerate normally to engine redline.
When I questioned the technician working on the car in order to double-check his test results, I found that he had checked engine speed sensor output by testing AC voltage at the disconnected sensor plug, and had approximately 1.5 VAC at a fast cranking speed. When I asked if he had a lab scope available to test the speed sensor, and when it was found that he had, I examined the output waveform he faxed to me. It showed a nearly normal sine wave at cranking speed, with an occasional "jag" in an otherwise smooth pattern. As speed was increased, the pattern remained essentially the same until 3500 rpm, when it dissolved into an almost unrecognizable "hash" of jagged peaks. Checking service records showed that the car had a starter replaced for poor engagement and grinding at about the same time that the driveability problem was first noted. Examination of the flywheel showed that the ends of three ring gear teeth were ground off by the poor starter engagement. Replacement of the flywheel completely cured the mid- range acceleration problem.
Another victory for lab scope testing!
By Dave Tidaback, autoline European import specialist (23 years experience)GM Cruise Control Puzzler
Have you ever gone "off the clock" to figure out a problem not for the customer but for yourself? You pull together all the information available. Soon you become an authority on the subject. Most of us enjoy this kind of challenge occasionally. After all, who in their right mind would open their tool box drawer every morning if they didn’t? It often begins with a customer who casually announces that they’ve been to two other garages with this problem and nobody can fix it. The conversation usually drops off as they stare you in the face waiting for you to break out in a cold sweat. Odds are that you can figure it out. But what if your voltmeter and dual-trace lab scope show every circuit is perfect, the part has been replaced twice and the car still refuses to operate?Let’s talk about the cruise control on a 1992 Caprice with a General Motors B body. It’s a self-contained module, calculating vehicle speed, and operating a stepper motor for throttle control. The circuit is simple, but can be confusing when it doesn’t work. One technician told me, "This is not rocket science. Everything tests fine, but the module won’t engage." He had completed a voltage drop check on the grounds and verified power inputs. The VSS input was the expected digital square wave and the inputs from the cruise switch were all in place. After two days of work, why wouldn’t the cruise module engage?
The answer lies within the module. In this case, the throttle control stepper motor is grounded through the center high-mounted brake light circuit! When the brake pedal is depressed, power is applied to that same circuit, which of course cancels its grounding characteristics and disengages the cruise control. Not a bad idea if you know about it. It indirectly alerts the driver that the stoplight circuit has a problem. If you were fortunate to figure it out, you were among a select few. A TSB now explains the problem and how to fix it. For Caprice wagons, it also explains how and when to solder a resistor into the circuit.
But don’t believe everything you read. In this case, the brake light switch was drawn with the contacts in a normally closed position. Wrong! If you follow the circuit on the wiring diagram, you will notice this switch operates the brake lights. Common sense tells you that it’s a normally open switch that closes when the pedal is depressed. Very often, as in this instance, you can catch mistakes in wiring diagrams by understanding what the circuit does. If you understand these "quirks," your frustrated customer will become a rolling advertisement for your business. By Dave Martin, autoline GM specialist (24 years experience)
©1996, Automotive Information Systems.
autoline Telediagnosis® with its 29 phone specialists handles over 12,000 phone calls per month from shops all over North America. As an ASA member, you’re already signed up to use the hotline service at a discounted rate. Call (800) 288-6210, Monday - Friday, 7 a.m. to 7 p.m., Central Standard Time.
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AutoInc. Magazine ®, Vol. XLIV No. 2, February 1996