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TR4 electrical system


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I was driving my 63 TR4 and everything was going well when the engine and electrical system abruptly stopped.  Everything just died, the ignition light did not come on. I’ve got the car home in the garage and I’m trying to figure out what happened. 
 

I recently installed an alternator and have run the engine a couple hours since then. The three fuses are good.  Using a multimeter I’m getting zero volts at the terminals with brown/blue wires at the ignition switch (I think I’m supposed to have 12 volts on those terminals all the time). The ammeter has voltage on one side and zero volts one the other side.  The starter motor will turn over using the starter switch on the solenoid but the coil is not energized and the engine will not start. All the other circuits are dead and there are zero volts at the fuse box. The car has a negative ground.

Any thoughts on what the problem is or what to check would be greatly appreciated.

Ron

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Sounds like a bad connection on the ammeter Ron or a broken lead connecting to it. You should get the same voltage reading on both of the terminals.  Although it is possible the instrument itself has failed internally, it is unlikely as the insides are pretty simple and robust.

The easiest way to check is to remove the ammeter wires and connect them together (suitably insulated) and see if the electrics are restored.  If that is ok and the wire terminal connections both look good,  it must be the ammeter itself.

If your meter has a resistance range you could measure between the ammeter terminals - it should read nearly zero Ohms.

When you installed the alternator what did you do with the old dynamo control box connections?

Edited by RobH
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Thanks Rob. I’ll try taking the ammeter out of the circuit and connecting the two leads together with suitable insulation and see if the ignition system and other circuits work. 

When connecting the new alternator, I followed instructions in a Moss Motors video.  I’ve got the terminals hooked up as shown in the video and insulated. It’s not pretty but I’m certain the terminals are not shorting out. I have removed the old control box.

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If the ammeter has different voltages (with respect to earth) then either the ammeter has gone open circuit, or you have a bad connection between it & the connecting tags.

Bob.

 

PS I posted this last night (as the 1st reply) but it was gone this morning. !

This seems to happen from time to time ??

Edited by Lebro
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Thanks for the comments and photos. They help me understand how the ammeter works.  If I'm understanding it correctly: when current flows through the little copper coil it creates a magnetic field which deflects the needle. Amazingly simple.  Also from looking at the wiring schematic all the current (except for the starter motor) flows through the ammeter, so if the ammeter fails none of the electrical circuits will work (except the starter). 

I put a jumper wire across the ammeter terminals and all the circuits work as they are supposed to. I took the ammeter apart and the little cooper coil is melted. Not just the solder as in Roger's picture but vaporized. Basically the copper coil acted as a fuse.

Now I'm wondering what would cause this to happen?

I recently installed an alternator, maybe that's the problem? Could the output from the alternator be too high causing the ammeter to fail? I ran the engine for 2-3 hours between installing the alternator and when the ammeter failed.

Have you guys ever tried to form a copper coil for the ammeter from wire? How about installing a fuse on the circuit to protect the ammeter? Any other thoughts about this? Thanks again for the help.

Ron

 

 

 

thumbnail.jpg

 

 

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The alternator is unlikely to be the cause of the failure Ron. Although an 18ACR can supply a bit more current than the dynamo, that will only be momentarily while the battery re-charges after starting and the wire coil should be well able to cope with that.   Most of the time the ammeter has very little current flowing through it.  (or should have - if the big loads are all connected at the alternator end of it as they ought to be).

Perhaps the fault  started off like the one Roger shows, with just one end of the coil becoming disconnected perhaps through vibration ageing of the solder joint. If that then allows the coil to droop until the free end touches the body of the gauge, there is a short circuit to earth which could cause the melting.  I think it is fair to say that the failure is not a common one. 

It is fairly easy to protect the circuit with a fuse on the battery side. I have done that on my 3A using a 40A blade fuse inserted in the wire which takes power from the solenoid terminal to the ammeter.  

Remaking the coil would be easy enough though I've never done it,  and calibration probably isn't terribly important as the gauge is more of an indicator than a measuring device. I think I would be inclined to provide some form of mechanical fixing for the ends of the coil in addition to the solder.

Of course, if you aren't concerned about originality or the diagnostics that an ammeter can provide, you could fit a voltmeter instead as a number of people have done. 

 

Edited by RobH
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Hi Ron,

If you do fit a voltmeter, It is not a direct replacement for an Ammeter.

You should join the two wires of the ammeter, and then connect a voltmeter between the ignition switch IGN connection and ground (many voltmeters are polarised so you will need to check which end goes where.)

If you connect the voltmeter to the joined ammeter connections (and ground), it will read the battery voltage at all times, and put a permanent drain on the battery, hence the need to put it the other side of the ignition switch.

Good luck

TT

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Most alternators (all?) rely on sensing the battery voltage to control their output. 

If the ammeter goes open-circuit, or a fuse protecting the ammeter were to blow (go open-circuit), the alternator would lose the reference voltage and would increase its output to maximum (whatever that might be!) and the alternator would likely self-destruct. 

Ian Cornish

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Apologies for a major bit of thread drift at this point.

1 hour ago, ianc said:

Most alternators (all?) rely on sensing the battery voltage to control their output. 

This seems to be a common misunderstanding at least for the Lucas units. 

The voltage sensing if there is any (and the common ACR alternators do not have a separate input for that ) is only there to compensate for the voltage drop on the cable between the alternator and the battery. It is not used to set the basic output voltage but just to tweak it up a bit if necessary.

The output is set by a resistive potential divider which is connected from output to earth and which feeds the base of the first comparator transistor and it is the base-emitter voltage of this transistor which provides the basic reference. This is known as 'machine sensing' - effectively the regulator is looking at its own output. If the alternator is run without being connected to the battery, nothing catastrophic will happen and the voltage will not rise appreciably. 

This is the basic regulator circuit. Note that the two B+ points are the same connection which feeds the battery:

 

Untitled.jpg.3f6fde6507feaea99724d910320331ad.jpg

(At this point it should be noted this is NOT a linear regulator, it is a switching one in which the field current is continually being switched on and off at somewhere between 30 and 60 Hz so the output voltage is an average of a PWM waveform ).

It is the chain of resistors on the far left which set the output voltage. The zener is there for temperature compensation since its characteristic is opposite to that of the transistor base-emitter one. 

This is a regulator with an external voltage sensing connection:

Untitled2.jpg.cd66854be9f7eb3fa4b2e41ea0b892de.jpg

You can see that the resistor chain is still there and connects to the output to set the basic alternator voltage but the 'voltage sensing lead' also connects part way down the chain and so will raise the output slightly if the voltage at the battery is too low. The alternator will still work without this connection but there will be an appreciable increase in output but not enough to damage the alternator. 

 

I think a lot of the legend about fragility of alternators may stem from the very early days when the rectifier diodes were less robust and  were marginal for voltage spikes and over-current.

 

Sorry for the digression. 

 

Edited by RobH
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Rob - thank you for this detailed explanation.  I bow to your greater knowledge.

However, would you (personally) insert a fuse in the line to the ammeter, were you to continue to use an ammeter (as I have done with the alternator installed 27 years ago in my TR4) rather than change to a voltmeter?

Ian Cornish

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I've retained the ammeter with an alternator too Ian and I have fused it on the battery side. I'm not sure of the wiring layout on a '4 but on my 3A  the power feed from the battery to all the car circuitry is taken from the starter solenoid by a single lead which is quite easy to get at. I inserted a link via a 40A fuse  (which also gave me the opportunity to take a separate fused and un-switched tapping to an auxilliary cigar-lighter socket as I bought a twin-fuse block ). Not the neatest installation in the world, but it serves:

IMG_0575.jpg.e215717e9b15f5f75edc7922f61b9c46.jpg

 

 

I didn't fuse the alternator feed as I figured that a fuse with high enough rating to handle peak charging current would out-live the alternator rectifiers in the event of a short. ( You will see that I still have the dynamo control box - which is now just a convenient junction point and which houses a six-way auxilliary fuse block.)

 

 

Edited by RobH
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Rob,

Very informative. I was aware of the internal sensing of the alternator, and had promised myself that one day I would look at how it works, I figured it would be pretty much as you explained, but your explanation was very helpful, and makes total sense.

Thanks

TT

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Wow, I didn’t completely follow the discussion of regulators. But here’s what I’m planning on doing.

I’ll assume the ammeter died of old age.  In other words, whatever happened probably won’t happen again.

I’ll install a 40amp fuse on the battery side of the ammeter to protect the wiring just in case. I’ll take a look at the wiring coming off the solenoid, sounds like I can install the fuse in that area.

I’ll put a jumper wire across the ammeter terminals; the gauge will be there just so I don’t have a hole in the instrument cluster.

I’ll install a volt meter between the IGN terminal on the ignition switch and ground; so I’ll know the battery voltage when the ignition circuit is energized.

Does that sound about right?  Any other suggestions? Thanks for the help.
 

Ron

 

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Excellent question Roger, can I get back to you on that one?

My goal is to make the car more reliable so I can use it as a daily driver.  I want to keep everything original if I can. I also don’t like getting stuck on the side of the road because the ammeter failed.  So my car, like many things, is a balance between those two forces.

Roger, whose opinion do you value more my priest or my wife?

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