Speedometer reads 20 mph in park. Transmission output speed sensor (vehicle speed sensor) codes. Transmission will not shift (2nd gear limp mode) from a bad alternator diode.
Clean, strong alternator output is critical for vehicle electrical performance. Without it, you can run into all kind of strange symptoms that you may not necessarily associate with an alternator problem, and this case study is a perfect example. I also supply some general guidelines for ripple testing that can be applied to any make or model out there.
Engine Performance Diagnostics chapter 1 page 27
- speedometer moves off of zero with the vehicle stopped and in gear
- check engine light (EGR fault code, unrelated)
- transmission shifting problems
- replace the alternator (failed rectifier diode)
- how to check for AC ripple using a lab scope with AC coupling
- identifying excessive AC ripple in a sensor voltage waveform
- how to eliminate a failed alternator by disconnecting the control circuit
- checking AC ripple at the alternator vs. at the battery
- Autel MS908 MaxiSYS Automotive Diagnostic & Analysis System - USA Version
- PicoScope 4425 4 Channel Standard Diagnostic Kit
- (Chapter 1) Misfire ID, Compression Tests, Vacuum Leaks, Fuel Trim, Head Gasket, Restricted Exhaust, Low Power, DMM
- How to test an Alternator (alternator not charging from a blown fuse) - Mazda
- How to test an alternator (Chrysler Dodge Jeep)
- Computer Controlled Alternator Testing - 2006 Mercury
- Battery keeps going dead (what to test before replacing)
For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available at www.scannerdanner.com as an eBook or paper book.
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On ScannerDanner Premium I will bring you right into my classroom at Rosedale Technical College. You will find page for page lectures taken right from my book as well as exclusive classroom type case studies. What is so special about these classroom case studies? I pull live problem vehicles directly into my classroom and we troubleshoot them in real time, using and applying the theory and testing procedures we learn during the classroom lectures. There is no better on-line training of how to troubleshoot automotive electrical and electronics systems anywhere!
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