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Automotive/2006 Trailblazer Gas Gauge

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Expert: - 8/30/2007


Question
My Trailblazer has only 33K miles on it, but the gas gauge doesn't want
to work. It either fluctuates from 1/4 to 3/4, or it will read empty
(even on a full tank). Could the sensors be dirty, or do I need a new
gauge?


Answer
I think that the first order of business would be to check the connections to the tank senders; they could be loose or otherwise intermittent.

Beyond that, it is rare for the resistive element in the tank to get dirty - unless there has been some fueling misadventure - but, over time, they can begin breaking down.  Sections can open up, and little or no sensing current can flow only when the float-operated grounded wiper reaches those sections.  Reading empty on a full tank means no conductivity at all through the sensor, assuming the gauge and rest of the circuit is good.  Disconnecting the wire from the sensor, grounding the sensor (for safety) and then connecting the removed wire to the gauge circuit to the vehicle ground via various values of resistor (in the range of 33 to 470 ohms, depending on the resistance range of your sensor (mine given above is what is used by my E250) can verify that the gauge and its connected wiring are good, and that the gauge reading increases as resistance is reduced.  If that all happens predictably and with no unexplained breaks in continuity, you could then safely assume that the sensor is at fault and simply replace it.

Do be careful ... one thing you DON'T want to do is to apply a 12V power supply lead to the sensor terminal.  Applying a power supply lead with little or no resistance between the battery and the tank sensor which may or may not be at vehicle ground potential can cause an arcing short circuit within the tank.  Normally, the sensor voltage is limited to millivoltage range, and this keeps the current similarly low and thus not able to strike an arc.  An aside: the reports are that the 747 that was flight 800 (1996 NY to Paris) exploded due to an insulation failure that put the aircraft's 550V power bus into contact with the low-voltage wiring for the tank sensors.  This caused the sensor to flame in the "dry" but fume filled center tank of the airliner, and exploded that tank, along with the airliner.  That is why I say: ground the sensor terminal, and test the lead that normally would connect to the sensor using resistors to chassis ground OUTSIDE and AWAY from the tank.  Good practice.

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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