k80rum Posted August 11, 2009 Share Posted August 11, 2009 My car seems to have a strange electrical issue (which is entirely my own fault since I decided to tackle the wiring..so I probably deserve it) I'm using a Geartronics gear indicator (it's the same as the Quaife one) which puts out 12v when the box is in reverse. This is very useful to me because I've no reverse switch, so I want to use the output to switch on the reversing light. The way I've wired it up is that the 12v output from the gear indicator goes into pin 85 of a 25a micro-relay. Pin 86 of the micro-relay is connected to Earth. a fused +12v is connected to pin 30 of the micro-relay The original power supply connection on the reversing light is connected to micro-relay pin 87 The gear indicator works well but the reversing light didn't come on when I selected reverse. Further investigation showed the gear indicator wasn't outputting 12v when in reverse. Discussions with Geartronics led to me sending the unit back because it was decided the output transistor had probably blown. While it was away, I double checked the connections in case I'd made a mistake which I don't seem to have. I was in a rush to try the display when it came back so didn't remove the relay and the same thing has happened. I can only conclude that *something* must be causing the output transistor to blow but can't figure out what that could be.. Any suggestions as to what I could try, to figure it out? It should effectively be pushing 12v from the gear indicator, through the relay to earth. I've certainly got continuity on the earth pin of the relay but haven't checked the resistance yet. Could it be something like that causing too much current to blow the transistor? (my electronics knowledge is pretty rusty ) Darren E K80 RUM Website and Emerald maps library Home of the long-term, supercharged Duratec project Link to comment Share on other sites More sharing options...
Ian B Posted August 11, 2009 Share Posted August 11, 2009 If it's a transistor output driving a relay coil then the back emf from the coil will probably blow the transistor. You should add a diode 'in reverse' across the relay coil, close to the relay, to quench any back emf it generates. There could of course be something else wrong as well! Ian - Slightly Vider SVelte SuperVeloce model 😬 - Joint AR for North Kent Link to comment Share on other sites More sharing options...
Bob_Rich Posted August 11, 2009 Share Posted August 11, 2009 Back emf could be the problem. relays with integral diode are available but if you use one you must ensure the polarity is correct-they are usually marked +Ve and -ve to make it clear. Is the 12V output capable of driving a relay? data on the unit should give the available current that the source can provide or the minimum ohms (resistance) that can be connected. Relay current demand can vary over a wide range depending upon number of contacts and rating. Hope this helps best of luck Bob Link to comment Share on other sites More sharing options...
k80rum Posted August 11, 2009 Author Share Posted August 11, 2009 Thanks Ian That's answered my next question I think. The relay I was using originally did have a reversed diode across the coil. I wasn't sure what this meant so I've also tried one without. Sounds as though the 'plus diode' version is the one to continue using as a safeguard. Just measured the resistance from pin 86 to a chassis ground and it's 0.6 ohms which doesn't look too suspicious. Very strange.. Darren E K80 RUM Website and Emerald maps library Home of the long-term, supercharged Duratec project Link to comment Share on other sites More sharing options...
k80rum Posted August 11, 2009 Author Share Posted August 11, 2009 Thanks Bob 😶🌫️ The pic on the side of the relay shows the +ve of the diode on pin 86. I'll ask Geartronics the question about what it's designed to drive. Perhaps it's a peculiarity around these little mico-relays and i should use a more conventional one. I'll do some digging Darren E K80 RUM Website and Emerald maps library Home of the long-term, supercharged Duratec project Link to comment Share on other sites More sharing options...
tbird Posted August 11, 2009 Share Posted August 11, 2009 from Geartronics website Reverse and neutral outputs can source a current of 200mA maximum. Relays must be used when these signals are required to power reversing lamps or solenoids etc so as long as relay draws less current you are doing correct thing, what is the resistance accross relay coil, or have you spec for current req to operate relay Link to comment Share on other sites More sharing options...
k80rum Posted August 11, 2009 Author Share Posted August 11, 2009 Ooo, thanks Tbird. I hadn't spotted that. A quick of check the coil resistance of the relays I'm using shows the diode-equipped relay has a coil resistance of 127 ohms and the non-diode-equipped one has a resistance of 129 ohms. Adding the measured 6 ohms of earth wiring resistance, I make that a current draw of about 89 milliamps at 12v, just under 100mA at 13v. I think I'll need to send the unit back for repair in any case (Geartronics very kindly did it FoC last time as a gesture of goodwill, but I think it's pretty certain it's caused by the relay). I might ask Geartronics to supply the relay this time just in case. Perhaps there's something odd about these smaller relays. I'll check the output voltage with the relay removed and run the earth directly to the battery. There's absolutely nothing about the high current circuit that could affect the low-current side is there? Darren E K80 RUM Website and Emerald maps library Home of the long-term, supercharged Duratec project Link to comment Share on other sites More sharing options...
Ian B Posted August 11, 2009 Share Posted August 11, 2009 Darren, Your calcs are OK, you should use the diode equipped relay (or use a seperate diode) but make sure it's the right way round. Have you got a manufacturers datasheet on the micro-relay you're using (or an RS pat number?). If connected properly there should be no interaction between the low and high current sides of the relay. Ian - Slightly Vider SVelte SuperVeloce model 😬 - Joint AR for North Kent Link to comment Share on other sites More sharing options...
tbird Posted August 11, 2009 Share Posted August 11, 2009 You would have hoped that as Geartronics are suggesting using a relay they would jave strapped a diode accross the output as a matter of course, but as said best to get correct type anyway, good idea to get them to supply or at least recomend, then if it pops again you may get a 3rd repair FOC, BTW I pressume your path to earth is .6 ohms not 6 ohms unless you have a verrrrry long run 😬 Link to comment Share on other sites More sharing options...
Harry Flatters Posted August 12, 2009 Share Posted August 12, 2009 I'm no electronics whizz by any means but isn't the Geartronics unit operating contrary to the way most ECU-like units work these days. Ie instead of switching a 12v feed, would it not be more in line with current (oops pun) standards to supply a switched ground? I fully admit that I may be talking sphericals but it would eliminate the sort of problems that Darren is experiencing. *arrowright*Harry Flatters *arrowright* AKA Steve Mell - Su77on Se7ener Link to comment Share on other sites More sharing options...
Paul Deslandes Posted August 12, 2009 Share Posted August 12, 2009 This shouldn't be causing your problem but aren't the reversing lights permanently connected to +12V via the ignition switch and the wire to the gearbox switched to ground when reverse is selected. My '95 car is wired this way. This would mean that your pin 30 should go to ground, not +12V. Link to comment Share on other sites More sharing options...
jonboylaw Posted August 12, 2009 Share Posted August 12, 2009 Paul, That depends if you are using the standard Car geabox switch. In this case the output is from the gear display and it outputs a signal when in reverse (as I read it). On think that may be worthwhile checking is putting an additional load in the low current path to restrict the current drawn by the coil. Placing a resistor or a lamp in series (on the earth side so you know the coil is passing the current) will reduce the current through the output Transistor. Just a thought. Jon Link to comment Share on other sites More sharing options...
Paul Deslandes Posted August 12, 2009 Share Posted August 12, 2009 Just checked later build manual and it looks as though the reverse light wiring is different to earlier models and the lights are permanently connected to chassis on one side and switched +12V the other. Sorry for the red herring! Hope you get it sorted Darren. Paul Link to comment Share on other sites More sharing options...
k80rum Posted August 12, 2009 Author Share Posted August 12, 2009 Cheers folks It does sound as though the reverse light wiring was changed Paul, mine's definitely the later switched live rather than switched earth type. It used to switch through the plunger-style affair in the side of the CC 6-speeder. My new box has no provision for one of these unfortunately but I can provoke the light to come on by applying 12v directly to pin 85 and simulating the supply from the Gear indicator. I'll certainly do as you suggest Jon with some extra-resistance and see if that helps matters. I must admit I was surprised the gear indicator uses a powered output rather than a switched earth as that's what ECUs seem to do.. Darren E K80 RUM Website and Emerald maps library Home of the long-term, supercharged Duratec project Link to comment Share on other sites More sharing options...
tbird Posted August 12, 2009 Share Posted August 12, 2009 Darren, I am sure you know this, but remember you need a cetain amount of current to flow in the circuit to trigger the relay as you calculated earlier you will be drawing around 100mA given 89 ohms and 13 V if you stuff anothe resistor in series yes it will reduce current but most relays seem to need 100mA min to trigger, so you might introduce another problem, while your unit is at geartronics do you think they would test output, to trigger their recomended relay then send you the pair, that would rule out incompatibility, if it popped again then you would know you are looking at a local (but very strange) issue (unless you are keeping something secret about the circuit) 😬 Edited by - tbird on 12 Aug 2009 17:54:35 Link to comment Share on other sites More sharing options...
Mick Day Posted August 12, 2009 Share Posted August 12, 2009 Why do you need a reversing light?........... Link to comment Share on other sites More sharing options...
tbird Posted August 12, 2009 Share Posted August 12, 2009 good point could just keep a really good torch in the car Edited by - tbird on 12 Aug 2009 19:30:01 Link to comment Share on other sites More sharing options...
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