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Automotive/'96 dodge 1500 w/ 5.2 auto

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Expert: - 2/21/2008


Question
truck has a stumble around 2500 rpms, no codes present, no misfires present, rpms on scan tool, not tach, show a fluctuation between 1000-6500 rpms, crank sensor ok, fuel system ok, map sensor, tps sensor, and exhaust back pressure ok, coil, plugs, wires, cap, rotor ok. i think the rpm variance has something to do with the problem but i am about out of ideas. any thoughts would be greatly appreciated.

Answer
It does sound as if something is going "open loop" somewhere, and the controller is overshooting based on the missed information.

If this has been developing over a very recent period, it may be an early warning of an impending failure of some semiconductor device in the controller.  If, on the other hand, it has been a problem for a while, and has not been getting any worse or better, then it more likely is a connection anomaly, probably to a circuit board.  Finding that sort of thing is not easy, but the use of a "freezemist" product (essentially anything liquid in a can and preferably nonflammable and nowadays non-fluorocarbon that goes to a gas when released to the atmosphere) could help to find it by "forcing" the problem by contracting any suspected joints.

You didn't mention any temperature dependency in your note, which was properly descriptive and thorough, so that might not apply here, but it does so often apply in many similar cases that I would give it a good test anyway, just to be very sure in eliminating it if nothing else.  The speed variance aspect is interesting, all the more if the same sensor drives your tach with no problem - but could the tach have a low-pass filter and maybe just not see it, I wonder???  Too, I would imagine that your tach display is moving needle analog, and not digital display; in the latter, again a low-pass filter would apply, I expect.

Is there anything else going on in the overall system at about 2500 RPM that could cause some interference?

Sorry ... I guess that I am posing more questions than answers.

If nothing else, this is one of those problems that makes one feel good when they ultimately solve it!

Good luck ... EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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