Strange wiring on 4TR

[Philk]

For the past week, I've been sorting out some messy wiring behind the speedo and rev counter on my 3.8 OTS (caused by the installation of an incorrect indicator switch and some "spur" wiring to get the repeater dashboard lamps to work). That has now all been fixed which left one job to be done.....

I had noted that the green Fuse 7 supply wire to the low fuel lamp had been cut. It now directly runs (via an intermediary cable) to the negative spade on the Lucas 4TR unit (the car had had an alternator fitted when it was renovated in 1985). Does anyone know why this might have been done and what purpose it serves? I am guessing that I can get the low-fuel light running again by spurring into this new cable arrangement but, prior to doing so, I would like to understand how the 4TR should be wired on a 3.8 with an alternator conversion. The car is still positive earth.

Does anyone have a view on this?

Many thanks

[Heuer]

You need to provide a (green) power feed to the low fuel lamp; there will be a white/green which goes to the sender in the tank. It earths when fuel is low illuminating the bulb. Does not really matter where it comes from but it would be best to run it from fuse 7.

No idea about the rest although I am curious about the +ve earth alternator. What model number?

[Philk]

You need to provide a (green) power feed to the low fuel lamp; there will be a white/green which goes to the sender in the tank. It earths when fuel is low illuminating the bulb. Does not really matter where it comes from but it would be best to run it from fuse 7.

No idea about the rest although I am curious about the +ve earth alternator. What model number?

Hi David,

Many thanks for the reply. I'll definitely splice back into the green cable which had been re-routed back across to the 4TR. Re the white/green wire - I was expecting to see that too given it is white/green on the fuel sender unit. However, the other wire which is attached to the low-fuel bulb holder is white cloth covered (like the cables that go the headlight dip switch next to the handbrake warning light). I do not know if it is white/green underneath.

Re your question on the Alternator, please see photo attached - it is a Lucas unit but I could not see a model number. I have also attached a photo of the battery area; as you will see, +ve goes straight to earth (as it also does from the 4TR unit (via an intermediary component)). The -ve battery cable goes to a device which is strapped to one of the chassis members. I've yet to investigate this - if you know what this is please let me know!

Do these provide any further insight? I am guessing that whoever did the wiring wanted a fused supply into the 4TR and took it from the nearest (and most redundant) cable (that being from the low fuel light). The dark blue cable that goes from the -ve terminal on the 4TR goes back to that green cable.

Regards

Phil

[Series1 Stu]

The device on the frame is the starter solenoid. There should be a heavy cable running from there around the engine bay to the starter motor.

I'm puzzled as to how your alternator is wired. Are you sure that the battery hasn't been turned around and the cables connected red to negative, black to positive? This would make it negative earth.

I must say that the wiring doesn't look very professional. It would certainly benefit from a thorough sort out.

Regards

[Philk]

The device on the frame is the starter solenoid. There should be a heavy cable running from there around the engine bay to the starter motor.

I'm puzzled as to how your alternator is wired. Are you sure that the battery hasn't been turned around and the cables connected red to negative, black to positive? This would make it negative earth.

I must say that the wiring doesn't look very professional. It would certainly benefit from a thorough sort out.

Regards

Hi Stuart

Many thanks for identifying the component as the starter solenoid. I would be the first to say that I am not an expert on car wiring; I have been working my way through some of the simplest of stuff ahead of its (much) later MOT. The car arrived in the UK in February and had been running for 32 years (and 34,000 miles) in the current form since its last major restoration in 1985 (I have all the receipts since then and there's nothing there that points to a re-wire in the intervening period). The car was delivered (post import) with the +ve terminal of the battery connected to the red lead (which runs to earth) and everything electrical appears to work on the car as it should as per the 3.8 wiring diagram. I cannot drive it (it is not registered yet on the UK roads) but it starts okay. What is it that you can see on the Alternator Wiring that is of concern? What is not present on the wiring diagram is how a 3.8 should be wired if it has had an alternator subsequently fitted which is where I am "in the dark".

Many thanks

Regards

Phil

[Series1 Stu]

Hi Phil

Electrics aren't difficult, you just need to be methodical in your approach. My comments about the wiring are more about the way it looks. It doesn't appear very professional and you would expect a better standard of work now to what you might have got in 1985. I could be wrong but I don't think those insulated crimp terminals were around in 1985.

3.8's were never fitted with alternators from the factory so that's why it isn't shown in any wiring diagram. The change to negative earth came with the fitting of alternators on the 4.2 litre cars.

I am intrigued to know how your conversion has been carried out, mechanically as well as electrically. I made my own conversion as I did not trust the units sold by the major suppliers. The 3.8's are more involved as they have the duplex drive belt and there aren't very many lengths to choose from when you're designing the brackets​, etc. I had alternator pulleys made to suit and dumped the jockey wheel set up. The alternator has the regulator on board so I made up a loom and used a busbar to connect the wiring. I also rebuilt my ammeter with higher rated internals so the dash would look original. People will tell you a volt meter is more useful, which is probably true, but an ammeter is fine and can tell you if things are going wrong.

I am guessing that your car has probably been fitted with a complete 4.2 installation and then had the regulator wiring modified to suit negative earth.

Regards​

[Heuer]

Your car has to be wired for -ve earth as I don't know of any +ve earth Lucas alternators (and yours looks like an Lucas 11AC); the only ones available are the modern Dynamator. The 4TR will not work on positive earth. Suggest you read this: http://www.coolcatcorp.com/faqs/Lucasalternators.html

The only indication you have that the car is +ve earth are the colours of the battery leads (which could be wrong) so I would start with the idea the car is -ve earth and test from there. The only component that is really polarity sensitive is the radio which will be destroyed if you hook it up the wrong way. The coil is polarity sensitive but getting it wrong will only reduce the spark power. The ammeter needs to have the wires swapped over otherwise it will indicate charge when discharging and vice versa.

You need to get this all checked out before connecting the battery. If you don't have the skills to test the alternator take it to a car electrics specialist along with the 4TR and get them checked.

[Philk]

Your car has to be wired for -ve earth as I don't know of any +ve earth Lucas alternators (and yours looks like an Lucas 11AC); the only ones available are the modern Dynamator. The 4TR will not work on positive earth. Suggest you read this: http://www.coolcatcorp.com/faqs/Lucasalternators.html

The only indication you have that the car is +ve earth are the colours of the battery leads (which could be wrong) so I would start with the idea the car is -ve earth and test from there. The only component that is really polarity sensitive is the radio which will be destroyed if you hook it up the wrong way. The coil is polarity sensitive but getting it wrong will only reduce the spark power. The ammeter needs to have the wires swapped over otherwise it will indicate charge when discharging and vice versa.

You need to get this all checked out before connecting the battery. If you don't have the skills to test the alternator take it to a car electrics specialist along with the 4TR and get them checked.

This forum is great - thank you for the v. helpful suggestions. Unfortunately, I can't take the car on the road to be tested (not yet registered in the UK) so I'm stuck with what I can do in the garage until it is shipped in due course down to E Type UK (who supplied it) for them to do the mechanical work to get it through its MOT.

At the moment, when the battery is connected (as it what when it was delivered) with +ve going to the red lead (which then goes to earth), the ammeter correctly shows a discharge when anything is used on the car. As per the other photo, the +ve output from the 4TR also goes to earth. The -ve output goes to Fuse 7 (which is what started the query above). The output from the Alternator goes directly to the other side of what I believe (via Stuart) to be the starter solenoid. There it directly connects to the same junction as the -ve lead from the Battery (via which it would be charged). Let me see if I can find out more about the Alternator.

[Heuer]

the ammeter correctly shows a discharge when anything is used on the car.

Don't rely on that as the PO may have not wired it correctly.

This is the wiring diagram of the 11AC and 4TR on the 4.2 cars:


This is what the 3.8 dynamo wiring looks like:


With the alternator on the bench you can connect a 12v battery to the F+ and F- to energise it and then spin it using an electric drill. With a digital test meter on the B post and the other to the casing you should get a voltage. If it is +ve then the car is wired for -ve earth, and vice versa. Full downloadable wiring diagrams are available in the Forum KB.

[Philk]

The only component that is really polarity sensitive is the radio which will be destroyed if you hook it up the wrong way.

Thank you for the diagrams - these are also very helpful. The mystery deepens further given your comment above. The car has an Alpine CD Radio fitted which works perfectly with the Battery connected with the +ve terminal connected to the red lead (earthed). In the boot of the car is an Astron 1212-18 Regulated converter unit which, having looked it up, is specifically to allow equipment requiring negative earth to operate in a positive earth environment. Having also looked again at the 4TR, the wire that connects the +ve terminal to the screwed in earth connector nearby has never been long enough to connect it to the -ve terminal.

All in all very weird and, if I'm honest, considerably beyond both my own knowledge and my own equipment here. Does anyone know a really good mobile automotive electrician who is familiar with E's (both positive and negative earth)? I'm based a few miles away from Woking in Surrey. It would be good to have someone with the right testing equipment to hand look all of this over.

[Heuer]

You could try http://www.winspeedmotorsport.com/ Never dealt with them myself so not a recommendation, just a suggestion.

Someone seems to have gone to a lot of trouble to keep the car +ve earth when there is no good reason to do so. A dynamo equipped car can be easily converted to run -ve earth: http://www.jcna.com/library/tech/tech0023.html

[Philk]

You could try http://www.winspeedmotorsport.com/ Never dealt with them myself so not a recommendation, just a suggestion.

Someone seems to have gone to a lot of trouble to keep the car +ve earth when there is no good reason to do so. A dynamo equipped car can be easily converted to run -ve earth: http://www.jcna.com/library/tech/tech0023.html

Many thanks again - the JCNA article is v interesting. Have now located a Lucas number on the bottom of the Alternator (23504F). From what I can see, this is supposed to be -ve earth (although I've found a number of references to +ve earth alternators that Lucas did produce. I would be surprised though if I have got one of these).

I have no idea why the car is wired the way it is.... however, it looks like it carefully needs to be gone through by someone in the know on these things.

[Philk]

Having spent some more time in the garage last night looking through the strange wiring set-up, I'm wondering if the best option is to replace the alternator (which has been repaired twice already over the past 32 years) with a +ve earth Dynator. That way, I can get rid of the strangely wired 4TR, simplify the wiring and keep the +ve earth arrangement over the rest of the car. It would also make the engine bay look more original for a 3.8. Would that work? If so, does anyone have a view on the merits of a Dynator vs a Dynalite unit in terms of reliability, use, fitting etc?

[Heuer]

Unless you are planning on having a totally original concours car there is little point in retaining +ve earth. You will have to keep the voltage inverter for the radio and you will have problems powering Sat Nav or phones from the cigar lighter socket. Those mock dynamo's are also very expensive for what they are. Consider fitting a Nippon Denso alternator which is small and lightweight with integrated regulation. viewtopic.php?f=9&t=6668

Really depends on what you are planning for the car - show or touring.

[Philk]

Your car has to be wired for -ve earth as I don't know of any +ve earth Lucas alternators (and yours looks like an Lucas 11AC); the only ones available are the modern Dynamator. The 4TR will not work on positive earth.

After a break of a couple of weeks, I'm about to continue the diagnostics around my strange alternator set up. Just following up on the point above, I've found the following on the web for Positive Earth Lucas 10 and 11 AC units. I have to confess that the back of mine looks very similar to this (see earlier photo). Aside from the wiring set-up on my car (which is all still geared for +ve earth), is there any easy way of determining if what I have on my car is one of the rarer +ve Earth Lucas units? From what I can see, this type of unit (23504) was normally fitted on Aston Martins of the same vintage. It is possible that the U.S. restorer laid his hands on one of these at the time of the conversion.

[mgcjag]

Hi Phil...the Lucas part number could well be stamped on the front cover ov the alternator have a good look but you may have to pull the front fan/pulley off.. Steve viewtopic.php?f=5&t=10769