rulloyd Posted August 24 Report Share Posted August 24 Hello all During the process of retro fitting overdrive to my GT6, I wired in an idiot lamp. I simply ran a wire from the solonoid connection on the relay (which is inside the car) to the demist bulb on the Speedo, which doesn’t work anyway. I kept the same earth already wired to the bulb holder as standard. Seemed to work fine, ie when I switch the column switch into od, it lights up (only in 3rd & 4th as it should). But I noticed when I took it out in the dark today that in fact the bulb is dimly lit even when not connected. Any ideas why this would be please? The bulb also blew the other day, I didn’t notice under what conditions. It does maybe look brighter than it should! All advice appreciated! many thanks Rich Link to comment Share on other sites More sharing options...
ahebron Posted August 25 Report Share Posted August 25 (edited) For anything electrical on old cars always make sure you are getting a good earth. You could try a temporary one straight back to the neg terminal on the battery to see if the same issue occurs. Edited August 25 by ahebron Link to comment Share on other sites More sharing options...
johny Posted August 25 Report Share Posted August 25 I wonder if the dim light you see at night is light filtering in from the backlight of the speedo? Not sure on your model if you can turn off just the instrument lights while driving to test this? As for the bulb blowing this could just have been coincidence or just possibly a voltage being generated by the solenoid coil. As you might know when an electromagnetic field collapses due to its power supply being turned off high voltages can be generated. An example of this is when the points open supplying your main ignition coil and a condenser has to be used to absorb the sparking produced at the points contacts.... Link to comment Share on other sites More sharing options...
JohnD Posted August 25 Report Share Posted August 25 Rulloyd, according to your profile you have a GT6. Is the O/d on that, because I've never seen one with a "Demist" bulb in the speedo! If your O/d warning light is glowing when the O/d is off, then volts are getting to it. Use your multimeter to see if there is, and which end its coming from. Then you can diagnose why. John Link to comment Share on other sites More sharing options...
Colin Lindsay Posted August 25 Report Share Posted August 25 The Mk3 at least has the Demist, the red lamp in the tacho. As long as it's just the two bulb connections and the original demist connection has been removed then it won't be running through any other circuit which may cause the leakage. It should also be 12v, I'd assume the demister circuit uses much more power than the o/d so that shouldn't have overloaded the bulb. Where have you taken the feed from - the solenoid? I'd splice into the lead to the overdrive a short distance from that to ensure there's no leakage (what's the electrical term that means cross-contamination?) from the other cables at the solenoid; and see if that illuminates too, which means the leakage is in that circuit alone; it could even be crossing over inside the solenoid even when disengaged so a replacement solenoid might cure that. 1 Link to comment Share on other sites More sharing options...
JohnD Posted August 25 Report Share Posted August 25 Thank you Colin, I bow to you more extensive knowledge! And agree with you connection comments. John Link to comment Share on other sites More sharing options...
rulloyd Posted August 25 Author Report Share Posted August 25 2 hours ago, JohnD said: Rulloyd, according to your profile you have a GT6. Is the O/d on that, because I've never seen one with a "Demist" bulb in the speedo! If your O/d warning light is glowing when the O/d is off, then volts are getting to it. Use your multimeter to see if there is, and which end its coming from. Then you can diagnose why. John Sorry, yes I meant taco. Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 25 Report Share Posted August 25 i guess the demist lamp was for warning your heated rear screen was in use make sure there are no odd feeds from historic wiring lurking and do make sure the jewel is shrouded from panel lights intruding Pete Link to comment Share on other sites More sharing options...
rulloyd Posted August 25 Author Report Share Posted August 25 Many thanks all. For those who haven’t noticed my various cry’s for help over the past months, I have just retrofitted Overdrive to my car. I fitted a new loom last year, so everything electrical is 90% new. Having finally finished putting the car back together yesterday, I took the car out, first time with OD in the dark… Anyway, it turns out that Johny & Pete were right! The back light in the taco is illuminating the demister jewel. It never did this before, however, I just had the taco repaired and its maybe something they’ve done 🙄 I look the demister bulb holder out and the jewel is still lit (dimly) when dash lights are on. Does anybody know if there is some kind of internal shield that may have been disturbed? The indicator jewel is not affected. As always, thank you for the sound advice. Much appreciated. Rich Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 25 Report Share Posted August 25 most jewel shields tend to be a sleeve of stuff like shrink wrap etc to shroud the bulb holder over the years it goes brittle and degrades , any tube would work be it paper or plastic i dont recommend chewing gum ha ! pete 1 Link to comment Share on other sites More sharing options...
johny Posted August 25 Report Share Posted August 25 yes as Pete says dont think the repairers did a very good job with the internal shielding in the instrument... Keep an eye on the light from the bulb when switching the OD out to see if momentarily it gets any brighter due to my crazy idea of back voltage from the solenoid 😁 1 Link to comment Share on other sites More sharing options...
Wagger Posted August 26 Report Share Posted August 26 (edited) When a solenoid is switched off, there is a very large voltage spike caused by 'Back EMF' as the magnetic field collapses. It could blow a bulb. This can be absorbed by connectng a diode or capacitor in parallel with the solenoid. Diode cathode to +ve end. Some relays have this diode built in. My overdrive used to cause a loud click on the radio when turning off. (Back in the AM days). Edited August 27 by Wagger 1 Link to comment Share on other sites More sharing options...
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