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
Symptoms
speedometer moves off of zero with the vehicle stopped and in gear
check engine light (EGR fault code, unrelated)
transmission shifting problems
Fix
replace the alternator (failed rectifier diode)
Tests shown
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
Tools used
Autel MS908 MaxiSYS Automotive Diagnostic & Analysis System - USA Version
https://www.jbtoolsales.com/autel-ms9...
PicoScope 4425 4 Channel Standard Diagnostic Kit
http://www.aeswave.com/cart.php?m=aff...
Playlist
(Chapter 1) Misfire ID, Compression Tests, Vacuum Leaks, Fuel Trim, Head Gasket, Restricted Exhaust, Low Power, DMM
• (Chapter 1) Misfire ID, Compression T...
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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!
Disclaimer:
Due to factors beyond the control of ScannerDanner LLC, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. ScannerDanner LLC assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. ScannerDanner LLC recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video. Due to factors beyond the control of ScannerDanner LLC, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not ScannerDanner LLC.
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