Automotive/F-150 Dual tanks

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Expert: - 6/5/2010


Question
Ernie,
I just bought an F-150, 1990 with 4.9 L6 automatic. The problem lies with the coolant temp gauge and the fuel gauge. Neither work. I find it difficult to believe both sending units in the fuel tanks are bad. I replaced the thermometer and sending unit for the coolant and NOTHING. The needles on both return to normal position when the truck is not running and ignition is off. Also when I unplugged the switch for the tank selection the gas gauge pegged way past full. Any ideas? Thank You for your help. Dj

Answer
I have no documentation on the Fords built after 1980, and so can say nothing specific.  However, given that there are two gauges which share the same problem, and also because both share the same operational concept - most likely proportional voltage, given that your fuel gauge pinned when it was open-circuited - I would imagine that they are both powered from the same supply-voltage regulator, probably with a 5-volt output as was the case with older Ford instrument-supply regulators.  

If that regulator is at fault, it would appear that yours had failed into a shorted condition, wherein it passes the 13-15V input voltage at all times the controls are energised.  This is quite a common failure mode for semiconductor regulators, and can also happen with the older electromagnetic regulators if the regulating contacts weld together.

Again, I can't speak to the configuration in you - to me- "newer" Ford, but I don't imagine that buying a replacement instrument regulator will pose any problem.  Using something more generic would also work; in the case of my 1959 Edsel, I replaced its failed oem regulator a couple of decades ago with a xx7805 linear voltage regulator, which is a very standard product in the electronics industry.  This can be adapted into place quite easily by those familiar with wiring and mounting practice without destroying anything original.  That would probably be a last resort, if not a purely optional resort, in your case.

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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