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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?

 

 

 

 

post-12623-0-71712500-1471364948_thumb.jpg

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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 by Peter Cobbold
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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 by RobH
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"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.

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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

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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 by SDerbyshire
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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 by Peter Cobbold
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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.

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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

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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/

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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.

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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.

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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

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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

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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 by RogerH
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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

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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.

post-12623-0-87174900-1471435203_thumb.jpg

Edited by Dave Smith
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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.

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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

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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 by Dave Smith
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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.

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