Dave Smith Posted August 16, 2016 Report Share Posted August 16, 2016 I'm after some help / ideas After removing and refitting the speedo on my 73 - CR car, the "green" electric circuit stopped working So I isolated the white wire from the ignition switch and and at the fuse box and battery totally disconnected Put 12V down the wire and checked with multi meter - 12V See attached diagram But if I put the earth connection onto the battery - 0V Then lastly I checked the voltage between the car body and the neg battery terminal - 12V Did not quite isolate the white wire from the ignition as there were two wires running to the same lucar connection, the other white wire goes to ignition / oil warning lights, volt gauge and coil / solenoid - see attached diagram. It looks hot on I have a live wire shorting against the body somewhere, is this the general concensus? Next move? Quote Link to post Share on other sites
Peter Cobbold Posted August 16, 2016 Report Share Posted August 16, 2016 (edited) I'm after some help / ideas After removing and refitting the speedo on my 73 - CR car, the "green" electric circuit stopped working So I isolated the white wire from the ignition switch and and at the fuse box and battery totally disconnected Put 12V down the wire and checked with multi meter - 12V See attached diagram But if I put the earth connection onto the battery - 0V Then lastly I checked the voltage between the car body and the neg battery terminal - 12V Did not quite isolate the white wire from the ignition as there were two wires running to the same lucar connection, the other white wire goes to ignition / oil warning lights, volt gauge and coil / solenoid - see attached diagram. It looks hot on I have a live wire shorting against the body somewhere, is this the general concensus? Next move? Dave, Just checking: as drawn,with no earth on battery you get 12volts. Then you connect the batt to earth and get 0 volts? And 12 volts from batt negative to earth? What do you mean by 'battery totally disconnected' - both terminals off, or as shown in diagram? Puzzling how that relates to removing/refitting the speedo.But you would have disturbed the earth battery terminal when you isolated the battery. Might it be a trick of the voltage stabiliser? 'A live wire shorting'........you'd see and smell the smoke immediately.Doubtful. Is there an electronic ignition device connected to the coil that might be generating a voltage? Or anything with its own battery connected to the loom? One for the electricians I think.... Peter Edited August 16, 2016 by Peter Cobbold Quote Link to post Share on other sites
RobH Posted August 16, 2016 Report Share Posted August 16, 2016 (edited) I confess I really don't follow the logic of the test you have done Dave. First of all did you check that the fuse is intact? How did you determine that the 'green' circuit was not working? Was everything dead (instruments, brake light, heater fan, indicators etc)? I suggest you put everything back as it should be, disconnect the wires from the + terminal on the coil so it doesn't get overheated, and then with the ignition switched on measure the voltage to earth at the input to the fuse and then at the other end of the fuse. At least then you will know whether there is a voltage feed to the circuit. What you do next will depend on what you measure. As Peter says its unlikely you have a short circuit to earth as the smoke would tell you where it was..... unless it blew the fuse that is. I take it you are working from the '73TR6 page of this: http://www.advanceautowire.com/tr2506.pdf Edited August 17, 2016 by RobH Quote Link to post Share on other sites
peejay4A Posted August 16, 2016 Report Share Posted August 16, 2016 "Then lastly I checked the voltage between the car body and the neg battery terminal - 12V" I don't follow this at all. Maybe I'm being thick. It's a negative earth car so the body and the negative battery terminal are at the same potential if the battery earth is connected. Quote Link to post Share on other sites
Peter Cobbold Posted August 16, 2016 Report Share Posted August 16, 2016 Even if the battery earth lead is not connected I can't see how 12 volts can appear between the negative terminal and the body. As there is 12 volts across the battery itself we'd have to have 24 volts in total to give that result. Its odd. Peter Quote Link to post Share on other sites
Steves_TR6 Posted August 16, 2016 Report Share Posted August 16, 2016 (edited) You could see 12v on a digital meter if a load is turned on and the negative battery terminal is disconnected. Eg a bulb or the coil. The high impedence of a dvm would allow it to see 12v between the body of the car and the negative terminal. The body would be at 12v relative to the battery - terminal. Is this what you meant Dave? Steve Edited August 16, 2016 by SDerbyshire Quote Link to post Share on other sites
Peter Cobbold Posted August 16, 2016 Report Share Posted August 16, 2016 (edited) You could see 12v on a digital meter if a load is turned on and the negative battery terminal is disconnected. Eg a bulb or the coil. The high impedence of a dvm would allow it to see 12v between the body of the car and the negative terminal. The body would be at 12v relative to the battery - terminal. Is this what you meant Dave? Steve Ah, I think I get it. With the earth lead disconnected, the voltage would be dropped aross the bulb or coil, the body and the multimeter, in series. As the multimeter has by far the biggest resistance it reads 12 v. Maybe a test bulb would be better for trying to locate dead connections? Peter. Edited August 16, 2016 by Peter Cobbold Quote Link to post Share on other sites
RobH Posted August 16, 2016 Report Share Posted August 16, 2016 Yes Peter. A digital meter generally takes so little current that it can be 'fooled' into seeing a voltage which in practice will not power anything because of series resistance. A bulb on the other hand requires significant current to make it light, so it can provide a better simple test as it is a real load on the supply. Quote Link to post Share on other sites
Peter Cobbold Posted August 16, 2016 Report Share Posted August 16, 2016 Yes Peter. A digital meter generally takes so little current that it can be 'fooled' into seeing a voltage which in practice will not power anything because of series resistance. A bulb on the other hand requires significant current to make it light, so it can provide a better simple test as it is a real load on the supply. Thanks Rob, I could not make head or tail of the multimeter readings but now I see why I was struggling. I think your post #3 and a test bulb might be Dave's best course of action. Peter Quote Link to post Share on other sites
matttnz Posted August 17, 2016 Report Share Posted August 17, 2016 I take it you are working from the '73TR6 page of this: http://www.advanceautowire.com/tr2506.pdf AAW not v useful for those of us with CR cars. Try this: https://www.flickr.c...in/photostream/ Quote Link to post Share on other sites
Dave Smith Posted August 17, 2016 Author Report Share Posted August 17, 2016 Thanks for all your replies #2 - Peter - For test 1 and 3 no connection to battery except for multi meter. Test 2 - I just put the earth connection on. #3 - Rob - Before I did these test I, with everything connected as should be, I checked with multi meter power to both sides of fuse box, which had no power. The voltmeter and the warning lights did work, coming off the same connection on the ignition switch. I obtained the correct wiring diagram from Matt after reading other threads on this forum, after I realised I had a problem. #5 - Peter - Battery was not connected, I hot wired to the lucar connection of the white feed at the ignition switch and then put the multimeter between the earth strap and neg terminal to use was the current going into the body. #6 and #7 - I did electrics as part of my first year of my degree, it was a nightmare for me back then, somehow it clicked in time for the exams at end of year, and now long forgotten. #9 - ok test bulb will be next plan of attack. I have had the car for three years with no problem with the electrical side except for the failure of the Clear Hooters light switch, on my first night drive. Going behind the speedo should not really touched this particular circuit, except for the voltage stabiliser , so I am baffled why it stopped working. I have noticed various alterations that looks to be made, I will try to work out what these are for, but looks like Hazard lights and fuel pump etc. Quote Link to post Share on other sites
RobH Posted August 17, 2016 Report Share Posted August 17, 2016 So from all of that it sounds as though you have a broken connection on the feed wire to the fuse from the ignition switch. I would do an end-to-end continuity (resistance) check on that wire. Possibly there is a poor joint in the spade connector on the switch end which was disturbed while you were working under the dash. Quote Link to post Share on other sites
RogerH Posted August 17, 2016 Report Share Posted August 17, 2016 You could see 12v on a digital meter if a load is turned on and the negative battery terminal is disconnected. Eg a bulb or the coil. The high impedence of a dvm would allow it to see 12v between the body of the car and the negative terminal. The body would be at 12v relative to the battery - terminal. Is this what you meant Dave? Steve Hi Steve, if the MM was placed between the body and the negative terminal of the battery a current would be sensed. There would be no (minimal) voltage there sensed as it would have been dropped to zero by the load. However MM's are strange things and a number may be displayed, but not necessarily 12V And more likely the MM would either blow a fuse or melt. Roger Quote Link to post Share on other sites
Peter Cobbold Posted August 17, 2016 Report Share Posted August 17, 2016 Hi Steve, if the MM was placed between the body and the negative terminal of the battery a current would be sensed. There would be no (minimal) voltage there sensed as it would have been dropped to zero by the load. However MM's are strange things and a number may be displayed, but not necessarily 12V And more likely the MM would either blow a fuse or melt. Roger Roger, Not when the battery earth lead was off. The current would be tiny, and flow through the load ( eg coil), the body and the Multimeter. Of those the meter has by far the highest resistance and so that determines the current. With three impedances in series the voltage drop will be far and away greatest across the meter, because it has the greatest resistance. Rob and Steve above explained it above. Peter Quote Link to post Share on other sites
RogerH Posted August 17, 2016 Report Share Posted August 17, 2016 (edited) Hi Peter, I'm sure you are right. But!!. With the battery -ve disconnected the earth connections to any load (the body/chassis) is floating. The load has a +ve input and nowhere to go. By connecting the battery -ve lead to the battery via the MM then all the load current will try to go through the mm - no where else to go!!! I must have read the first question wrong. Roger Edited August 17, 2016 by RogerH Quote Link to post Share on other sites
Peter Cobbold Posted August 17, 2016 Report Share Posted August 17, 2016 Hi Peter, I'm sure you are right. But!!. With the battery -ve disconnected the earth connections to any load (the body/chassis) is floating. The load has a +ve input and nowhere to go. By connecting the battery -ve lead to the battery via the MM then all the load current will try to go through the mm - no where else to go!!! I must have read the first question wrong. Roger Roger, But the load current is limited by the series resistance that includes the MM's. As the MM impedance wil be Megohms the current is miniscule. I had to sketch the circuit to see it. Peter Peter Quote Link to post Share on other sites
Dave Smith Posted August 17, 2016 Author Report Share Posted August 17, 2016 (edited) After tracking along the cable, this looks like the problem, see attached photo. I could not peel back the black covering, but it looks like a white wire has split with exposed wire. By the way the test bulb indicated very lttle power to light up the bulb, although the meter suggested 12V, before uncovering the above. Update: The white in the photo is part of the cable covering and not the cable as I first thought, managed to have a better look at there appears to be one burnt out white cable. Edited August 17, 2016 by Dave Smith Quote Link to post Share on other sites
stuart Posted August 17, 2016 Report Share Posted August 17, 2016 After tracking along the cable, this looks like the problem, see attached photo. I could not peel back the black covering, but it looks like a white wire has split with exposed wire. By the way the test bulb indicated very lttle power to light up the bulb, although the meter suggested 12V, before uncovering the above. Update: The white in the photo is part of the cable covering and not the cable as I first thought, managed to have a better look at there appears to be one burnt out white cable. If thats the original loom then it looks like its time for a replacement. Stuart. Quote Link to post Share on other sites
Dave Smith Posted August 17, 2016 Author Report Share Posted August 17, 2016 Yes it is Stuart. Just looking at new harness! How much soldering is involved? Anyway thanks to everyone who's took te trouble to reply Quote Link to post Share on other sites
RogerH Posted August 17, 2016 Report Share Posted August 17, 2016 Roger, But the load current is limited by the series resistance that includes the MM's. As the MM impedance wil be Megohms the current is miniscule. I had to sketch the circuit to see it. Peter Peter Hi Peter, if the MM was in the Voltage setting then it may take more (or less) current than in the current setting. I still think it is odd. Roger Quote Link to post Share on other sites
Dave Smith Posted August 17, 2016 Author Report Share Posted August 17, 2016 (edited) This cable would had been cooked years ago when it fed the fuel pump, which I think was changed to a seperate system / relay a long time ago. Strange how it functioned for so long until I disturbed it, which in some ways its good I found it now. Edit Never jump to conclusions with old cars - the cooked cable was the auxilliary cable from the ignition switch, the clue was that someone had wrapped its end in insulating tape. Edited August 17, 2016 by Dave Smith Quote Link to post Share on other sites
stuart Posted August 17, 2016 Report Share Posted August 17, 2016 Yes it is Stuart. Just looking at new harness! How much soldering is involved? Anyway thanks to everyone who's took te trouble to reply On a 6 none with a standard loom as they will have all the relevant connectors already fitted. Stuart. Quote Link to post Share on other sites
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