CtrMint Posted February 18, 2019 Share Posted February 18, 2019 Separated from an earlier thread, https://www.lotus7.club/forum/techtalk/start-my-car-today-help-over-coolant-ratioA quick summary of the situation, Reverse light fuse blows if the reverse gear is engaged on the gear lever. The fuse is shared with the brake light fuse. When the brake pedal is depressed the fuse is fine and the brake lights work fine. The reverse light has been disconnected at the rear and the fuse still blows when reverse is engaged. I believe I connected the gear switch adapter to the forward most (black) gearbox connection, although not confirmed. The reverse light ground loom connection has continuity with the ground on the fuel tank. The reverse light positive loom connection does not have continuity with the fuel tank ground, without the reverse engaged. (This might be a pointless test)I'm not knowledgeable on how the reverse switch works, I'm assuming its just a serial switch which closes the circuit to the reverse light, so essentially doesn't have a polarity. Assuming I've connected the loom to the correct plug there's very little that could go wrong there, so limited reason to pull up the tunnel cover.Beyond this I'm stuck. Your help would be very much appreciated. Link to comment Share on other sites More sharing options...
K7 VCT Posted February 18, 2019 Share Posted February 18, 2019 Sounds like you may have a wire trapped or been compromised so how. Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 Based on that information the switch is shorting to earth when reverse is engaged, which for me can only mean two things, fault switch or wrong connection, let’s hope it’s the latter because replacing the switch might be an engine out job on a Mazda box.Edit - maybe not if you work from underneath with a crows foot spanner Link to comment Share on other sites More sharing options...
Wrightpayne Posted February 18, 2019 Share Posted February 18, 2019 Have you got the wiring diagram for your car?On older cars the positive feed went to the lamp unit and was earthed via the reverse switch in the gearbox - opposite to the normal way of auto wiring.Check the wiring diagram and see how the circuit should work. Then, with a multimeter check for voltage at relevant points. I.e. if the lamp has a positive feed check for that. If the switch has the live feed check there.If it is positive feed to the lamp, it shouldnt need an earth at the lamp.First thing check the wiring diagram.Ian Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 I've removed the passenger seat and tunnel top and disconnected the switch loom connection.If I do a continuity test between each of the pins on the loom to the gearbox housing, I get a connection, meaning they are both connected to ground??? Totally confused... Link to comment Share on other sites More sharing options...
Wrightpayne Posted February 18, 2019 Share Posted February 18, 2019 Have you got the wiring diagram?one of those wires should give continuity to earth - the wiring diagram will show which along with the colour(s) of that wire. I would be checking the other wire for shorts. Link to comment Share on other sites More sharing options...
Jim 123 Posted February 18, 2019 Share Posted February 18, 2019 Can you pass a current through the 2 poles on the switch mini loom? With and without reverse engaged.Also try and have a squint to see if the switch has been damaged where it enters the box. Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 There is a loom diagram in the 2015 assembly guide but it only shows connections on the loom rather than a circuit diagram.I've checked both the pins on the gearbox connection for continuity with the car's ground, there isn't any, so open circuit which I assume is good. I was looking to see if the cable down the side of the box was damaged but it seems OK. Link to comment Share on other sites More sharing options...
bosshogg Posted February 18, 2019 Share Posted February 18, 2019 In the absence of a wiring diagram you need to work out were the supply is coming from. If as Ian has said it is going first to the light and then to the switch it gives you an idea of where to start looking for problems. Now you have all the connectors disconnected, can you turn the ignition on and test all the loom connectors to find out where the 12v supply comes from. Alastair Link to comment Share on other sites More sharing options...
Jim 123 Posted February 18, 2019 Share Posted February 18, 2019 Maybe it's how you describe the test you have done, but are you saying the switch opens and closes the circuit if you bridge between the 2 pins in the mini loom and operate the gearshift? Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 Are your switch connections Green and Green Brown? If the are Green comes from the fuse supplying both brake switch and reverse switch, if the Green Brown wire goes to the reverse light. So if your switch wires are giving a good earth when reverse is selected then the switch is faulty. Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 Looking at the loom diagram I think it's something like this. [0 / 0] = Gearbox switch[*LIGHT*] = Light bulb. G37 [0 / 0] GN137 gear box —> GN137 Rev light [*LIGHT*] B13I'm assuming G37 is the ground, and B13 is positive, though I can't find either on the loom diagram.Jim, I'm not quite following you, but continuity test the switch indicates its fine. Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 G37 is the 12v supply (from the brake light switch / fuse box)B13 is earthif G37 or GN137 is earthed good bye fuse Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 The green and brown from the loom is earthed. Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 Remove your brake light bulbs (or disconnect the rear lights) disconnect the reverse light and remove the fuse. Then test your G37 and GN137 wires, are they earth? If they are disconnect the reverse switch, if the wires are still earth, then the loom is the problem, if not then the switch it is. Link to comment Share on other sites More sharing options...
bosshogg Posted February 18, 2019 Share Posted February 18, 2019 Ok this may not be the done thing, but is what I would do to rule out the switch. Assuming you can spare another fuse, get a short length of wire and bodge a connection between the two pins on the loom where it would connect to the switch, thus bypassing the switch and see if the reverse light works. Just remember to reconnect the loom to light connection first. If it works ok then it is definitely the switch. Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 Disconnecting the light will prevent false earth readings through the build filaments, after all the are very low resistance until they heat up. Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 GuysI'm confused, Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 Either my post at 21;21 or boss hog at 21;22 will work ( boss hogs will be easier ) Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 Just to confirm I did it right, I reconnected the reverse light I used a piece of wire across the loom where it would normally connect to the gearbox switch. Turned on the ignition, and the fuse went Link to comment Share on other sites More sharing options...
bosshogg Posted February 18, 2019 Share Posted February 18, 2019 It's not the switch at fault then. Before you completely rule it out, check that there is no way that the piece of wire you used could have touched ground by mistake, otherwise you could be going on a wild goose chase. Now you are going to have to do as Chris C suggested and disconnect the brake lights and check the loom sections for ground. Link to comment Share on other sites More sharing options...
CtrMint Posted February 18, 2019 Author Share Posted February 18, 2019 When you say check the loom sections for ground, what do you mean, there's only two points I can see?? Link to comment Share on other sites More sharing options...
bosshogg Posted February 18, 2019 Share Posted February 18, 2019 If you have more spare fuses it may be worth trying leaving the short bodge wire in and disconnecting the reverse light see if the fuse still blows. (May narrow down the section of loom to look at). Link to comment Share on other sites More sharing options...
ChrisC Posted February 18, 2019 Share Posted February 18, 2019 If you discount the rear light (both) and the reverses light, disconnect the switch, and test the two wires (green and green and brown) if either are earthed you have a loom problem. Discounting the lights and the fuse should isolate the wires. Link to comment Share on other sites More sharing options...
bosshogg Posted February 18, 2019 Share Posted February 18, 2019 I think my money is going to be on faulty led unit in the reverse light. Assuming it is led. Link to comment Share on other sites More sharing options...
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