Voltmeter Wiring Diagram

[Lee Dutton]

Would anyone advise as to the correct wiring for a voltmeter on a TR4a please, or a wiring diagram would be great.

Best regards Lee

[KnightsT]

Basically you just want to connect any one of the white wires that go to the ignition switch to one terminal, and earth to the other. I tied into the white wire supply to my overdrive relay from the ignition switch, but you can either tie into any of them or just piggy-back onto one of the terminals on the ignition switch that has 2 (or 3 of an o/d relay is fitted) white wires on it. Any good earth will do.

That way the voltmeter only reads when the ign is on.

 

Mick

[Lee Dutton]

Thanks for the info Mick, seems straightforward enough (when you know!!)

 

Will try it out cheers Lee

[ianc]

The easiest place to access the supply switched by the ignition is probably at the fuse box - you won't have to crawl under the dashboard. To find the correct terminal on the fusebox, remove the fuses, turn ignition "on" and look for 12 volts! Don't leave the ignition "on" too long and remember to replace the fuses afterwards!

 

Just to add that if you have fitted an alternator, I have written a description which should explain the mysteries surrounding the alternator kit supplied by Moss. This is being "road-tested" by a couple of guinea pigs (sorry, fellows!) and should be ready for publication in the near future! It includes the fitment of a voltmeter instead of the ammeter.

The bad news is that, with diagrams included, it runs to four pages!

Watch this space.

 

Ian Cornish

[TorontoTim]

Ian, wouldn't it be better to find out which terminal at the fusebox did NOT have 12 volts with the Ignition OFF? Once the ignition is on, don't both/all input terminals, switched and unswitched, have power going to them?

 

Or am I rembering the wiring diagram wrongly?

[ianc]

Tim - thanks for the clarification.

One terminal on the fusebox will have 12V all the time (marked A1 on wiring diagram - to feed the horns), the other (A3) has 12V only when the ignition is turned "on".

I suggested removing the fuses to ensure that the voltmeter would be connected upstream of the fuse (at A3), on the run from ignition switch to coil, so that should the fuse blow, it would not cause the voltage reading to disappear.

Ian Cornish

[ianc]

With the able assistance of Jim Christie, who was fitting a Moss alternator kit to his TR3 at his home in France (hence, a considerable number of e-mails have been exchanged over the last 6 weeks or so!), I have investigated the situation regarding the peculiar behaviour of the ammeter when the instructions provided by Moss have been followed. The result has been a Christie/Cornish article which went to the Office at the weekend and seems likely to appear in the next TR Action (but its size may mean that it is split across 2 issues). My theories have been put to the test by Jim, whose alternator and ammeter now are behaving as one would wish (i.e. like a beefed-up dynamo).

 

Basically, I am saying that one should disregard the instruction to attach the large and medium size brown wires to the starter solenoid (which is fed from the battery) and connect them as prescribed below. To achieve this arrangement of connections, one needs:

4-way connector block with either large blade connectors or large screw connectors, all linked together, or a single, insulated, terminal post

2-way connector block with either medium-size (6.3mm) blade connectors or medium-size screw connectors linked together, or a second (but smaller), insulated, terminal post.

 

The wiring arrangement for the 18ACR and 20ACR alternators should be as follows:

Connector with 4 large, electrically linked, terminals:

large brown wire from alternator;

medium-size brown wire from alternator (see Note below);

wire to ammeter - previously connected to Control Box terminal A;

wire to ignition switch terminal 1 (TR4) or lighting switch terminal A (TR2/3/3A) previously connected to Control Box terminal A1.

 

Connector with 2 electrically linked terminals:

thin yellow (or yellow/brown) wire from alternator;

thin wire to ignition warning lamp (yellow on TR2/3/3A, brown/yellow on TR4) previously connected to Control Box D.

 

Note 1: Since 15, 16 and 17ACR alternators are rated at no more than 34 amps, they require only a single, 35 amp, brown wire.

Note 2: On a TR2/3/3A, link the two wires which went to Control Box E, so as to preserve the earth connection for the dashboards instrument lighting (else you may suffer from dim or no illumination, as Jim discovered!).

 

There's a great deal more in the article (including details on the fitting of a voltmeter/battery condition meter instead of an ammeter), but I hope this may serve for the moment to assist anyone who is concerned about the odd behaviour of the ammeter.

 

Ian Cornish

[badshead]

If you want to keep things looking original, you can use the existing regulator as the connection block as follows:

 

 

 

The 40A fuse is obviously optional, but given the greater capacity of the alternator provides a safeguard against meltdown of ancient wiring in extreme situations e.g. startup with a very discharged battery.

[ianc]

Not a good idea to include a fuse there, Badshead. If it were to blow, the alternator would lose the battery reference voltage and could be wrecked.

 

For the same reason, one should never attempt to re-charge a fully discharged battery on a car with an alternator - there MUST be a reference voltage for the alternator's control system to work.

 

Dynamos are controlled differently, and (if you can get the car started by, say, supplying 12 volts to the coil alone and then bump start or use the starting handle) can happily re-charge an absolutely flat battery.

 

If the battery is very low, it is better to (partially, at least) re-charge first with a mains-driven charger unit so that it's showing about 12 volts at the terminals and then let the alternator take over.

 

Ian Cornish

[badshead]

Not a good idea to include a fuse there, Badshead. If it were to blow, the alternator would lose the battery reference voltage and could be wrecked.

 

 

Ian

 

It's a fair point, but in the unlikely event that a problem occured, a replacement alternator or replacement diode pack is a cheaper/easier option than melted loom. Sometimes it's not easy/convenient to avoid the low battery start attempt, this method means you can risk it without tears. In Jersey last year I forgot to turn off the fan override and went for a drink only to discover my mistake about 90 mins later. Without the fuse I probably wouldn't have attempted the start, but with it I was confident enought to give it a go and saved myself a whole load of hassle/delay getting it charged by some other means. Do modern alternators still use battery reference, I thought machine sensing was now the norm?

 

Bill

[ianc]

The information which I have received from more than one source says that one MUST have a battery reference voltage, and without it there's a very good chance of damage to the alternator.

However, I don't intend to experiment by disconnecting the link from my own alternator, so perhaps someone who did this (or had it happen by mistake) might care to comment?

I believe the racing guys use a special battery isolator which permits them to isolate the electrics with the engine on full chat (which could be the situation if there's a crash), yet avoids damage to the alternator. I have a simple (on/off) isolator, so have to ensure that I stop the engine first.

Ian Cornish