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Automotive/1991 Toyota pickup cab light

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Expert: - 7/31/2007


Question
I recently purchased a 1991 Toyota pickup 4 cyl 5 spd 4wd. The overhead light
in the cab doesn't work. I tried replacing bulb to no avail. Checked all the fuses
(box under dash as well as under hood) none are shot. Also the 12v outlet
(cigarette lighter) has no power as well. Thanx for any suggestions.

Answer
Time to do a detailed trace!  There are two nonfunctional items, which may or may not be on the same circuit, and no fuses appear to be open.

First, you do not indicate whether you checked out the dome light switch, or switches, for continuity.  In many cases, especially with much older vehicles in which the dome light operates only in response to a door opening, the dome light is switched on the low or common (some loosely call it ground) side by one or more simple momentary contact plunger switches, each of which makes a contact to the vehicle body (connected along with the chassis to the negative terminal of the battery) when the door is opened; any one of them so closing will operate the light.  

The above is very general - as indicated in my notes, I do not maintain any shop manual resources beyond my own specific needs.

To carry out a trouble-shooting trace, you need to identify and highlight the circuit involved.  You want to determine the routing of the wire for the circuit, and this is made easier if it bears a particular identifying trace or colour code.  You also want to know which connectors in the system are involved.  Automotive wiring is typical arranged in harnesses, and quick-connect plug and socket points are used to facilitate passing through fire walls and the like.  Of course, it would definitely speed things up if you were to get a copy of a vehicle wiring diagrams(s).  

Visual inspection can tell a lot, especially if insulation is seen to be damaged or, in your case, the wiring itself has been cut or corroded away.  Often however, corrosion or breaks will remain hidden by insulation or connector housings.  Therefore, the most complete test can only be had by the use of a multimeter or, at the very least, a 12V lamp-type continuity tester; these provide the the best means by which to verify the continuity of each portion of the circuit, and by which to isolate the problem.

The manufacturer's shop manual would have this information, or perhaps an Hayne's manual might be available, although these tend to be general over a range of years and often miss details.  Alternatively, you may be able to approach an auto electrical shop for the specific diagrams, taken from the Mitchell's or other database.

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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