zero Posted July 9, 2014 Share Posted July 9, 2014 I just learned something new about my 1988 Toyota. Something I guess I never had to think about before. I just installed a mechanical transmission temperature gauge. All that worked fine. I also hooked up the light in it (for night driving) into the dash-light circuit. Took it out last night and tried to use the little dash-light dimmer control and it no longer worked. They stay at full brightness no matter what I did. So I had to do some research and "Occam's Razor." Ends up Toyota has the dimmer control varying the negative (chassis ground) power to the lights and not the positive. When I hooked up the light in the new gauge -that little light socket was fully grounded because the gauge-bracket is grounded. So that back-fed into the entire dash-light circuit and made them all full brightness. I had my 11 year old boy with me - because I've been teaching him some electric theory. He nailed the fix right away. Make the hole bigger in the gauge where the light-bulb plugs in, wrap some tape around the OD of the bulb assembly so it no longer touches vehicle-ground - and plug it back in. That was a good fast fix and now all works as it should. Quote Link to comment Share on other sites More sharing options...
Vanman Posted July 9, 2014 Share Posted July 9, 2014 Toyota does funny electrical things like that. Aside from the OD lite ckt that has been discussed here already I found when replacing the front turn signal bulbs with LED's the LED's would not light. Turns out the housing is plastic and they wired the base to hot and the center to ground. Bulb does't care about polarity, LED does. Been working on cars for a long time, never saw that before. Thanks a lot for that post, I'm gonna add some gages to mine one of these days. vanman Quote Link to comment Share on other sites More sharing options...
zero Posted July 10, 2014 Author Share Posted July 10, 2014 Toyota does funny electrical things like that. Aside from the OD lite ckt that has been discussed here already I found when replacing the front turn signal bulbs with LED's the LED's would not light. Turns out the housing is plastic and they wired the base to hot and the center to ground. Bulb does't care about polarity, LED does. Been working on cars for a long time, never saw that before. Thanks a lot for that post, I'm gonna add some gages to mine one of these days. vanman Yes. I've had similar problems with LEDs. They've gotten so common now - I sometimes forget that even though they are used as lights - they are still, in essence diodes acting as one-way electrical check-valves. Besides the polarity problem- I encountered something else. I recently had to wire an entire house with 12 volt DC power and LED lighting. Had a few "mystery" LED lights that would never fully shut off. I discovered that LEDs can have an ability to make some light at extremely low voltage. Some switches "leak" enough that incandescents and CFLs work fine- but LEDs will glow a bit. I had two lights that when all else was pitch dark - you could see them glow slightly. These were 12 volt LEDs. I checked voltage at the bulb while "off" and glowing and it was 1/20th of a volt. Kind of amazing. Quote Link to comment Share on other sites More sharing options...
Vanman Posted July 10, 2014 Share Posted July 10, 2014 I found the same thing when I was checking the current draw & voltage drops in the RV. With the fuse pulled some of the LED's would lite up dimly. Turns out the ckt on the Progressive Dynamics fuse/connection ckt board has a red LED that lights up when a fuse blows that bypasses the open fuse and feeds voltage to the "open" leg of that ckt. I was gonna disconnect that and forgot about it till yesterday. There's no end to my to-do-lists. vanman Quote Link to comment Share on other sites More sharing options...
linda s Posted July 10, 2014 Share Posted July 10, 2014 I have a dash light dimmer? Where. I didn't think these trucks had them Linda S Quote Link to comment Share on other sites More sharing options...
zero Posted July 10, 2014 Author Share Posted July 10, 2014 I have a dash light dimmer? Where. I didn't think these trucks had them Linda S 1988 has a separate rotary dash-light dimmer control down low on the left of the steering wheel. 1978 has the dash-light dimmer built into the headlights switch (also rotary control). Quote Link to comment Share on other sites More sharing options...
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