Tr4aJim Posted March 13, 2021 Report Share Posted March 13, 2021 Folks, sorry if this has been discussed before, but I didn’t find a topic in the search. My TR4A has a stock electrical system (negative ground) with only an electric fuel pump as an additional draw (with its own fuse). I also have an an alternator. However, I have seen mention in various threads about modifications to the stock wiring, e.g. relays, additional fusing, fusible links, etc. Are there recommended alterations/additions to the stock wiring to make it safer/more durable? thanks Jim Quote Link to post Share on other sites
RobH Posted March 13, 2021 Report Share Posted March 13, 2021 A don't believe there is any recommended scheme Jim - people do what seems sensible to them. The provision of fuses on these cars is scant so there is no 'circuit discrimination' - i.e. a single fault somewhere that blows a fuse will remove power from nearly everything though the car should still run as there is no fuse at all in the ignition circuit. If originality is not too much of a consideration then fitting separate fuses for most circuits is a sensible step as the switches, wiring and insulation of a lot of cars are now getting very old so faults may be expected to develop. It also makes sense to use relays to switch the larger loads, such as lighting, to relieve the switches of that duty and prolong the life of their contacts. Modern relays are considerably cheaper than replacement switches. Quote Link to post Share on other sites
Tr4aJim Posted March 13, 2021 Author Report Share Posted March 13, 2021 Thanks Rob. Originality is not a concern. For example I was thinking of adding fuses to each wire at the fuse block that are currently serviced by a single fuse. However I recall reading somewhere that some fuses are designed as “slow blow” allowing a surge of electricity to occur without immediately failing. I assume that type should be avoided? Where would I add a solenoid to trigger the headlights, the brown wire leading to the switch? thanks Jim Quote Link to post Share on other sites
RobH Posted March 13, 2021 Report Share Posted March 13, 2021 I don't think there is any need for slow-blow fuses anywhere on a TR Jim, you just need to ensure the fuses you use have the proper current rating for the circuit they feed. You should be aware that there is a difference in the way Lucas fuses and modern fuses are numbered. The original glass Lucas fuses as used in TRs are rated for the minimum current they will blow at. A modern glass fuse looks more or less the same but is rated for the maximum current it will carry continuously without blowing - the same as modern blade fuses. Where you place the relays is probably best determined by convenience, and not being too familiar with the '4A wiring layout I can't really suggest the best location. On my '3A I put the headlamp relays under the top lip of the front cowl as that is close to the junction point for the headlight looms and makes the wiring simple. Obviously it will be different for a '4A, not least because there is no front cowl ! The principle should be that the relays are located close to the loads and the relay coils operated using the existing wiring so that the switches only carry the coil current. A new heavy-gauge wire provides power to the relay contacts, to minimise voltage drop. This is a typical arrangement where the existing wiring is shown as blue : Quote Link to post Share on other sites
Bleednipple Posted March 13, 2021 Report Share Posted March 13, 2021 As a non-electrician I'd just add that any upgrades to fusing should use decent quality components otherwise you may create more irritating problems. A lot of the repro Lucas style fuse boxes ("MGB style" etc) are actually rubbish. I've now switched to modern blade fuses which don't even try to look 'period' but at least have a reasonable chance of not creating new snafus. +1 for relays. You can get various neat fuse box-style containers for sets of relays instead of having them randomly scattered around your engine bay. Quote Link to post Share on other sites
Tr4aJim Posted March 13, 2021 Author Report Share Posted March 13, 2021 Great, thanks! Jim Quote Link to post Share on other sites
RogerH Posted March 13, 2021 Report Share Posted March 13, 2021 Hi Jim for our old cars do the simple things first. Cables and looms going through bulkheads need decent grommets to protect the wires from sharp edges. Cable clips that hold the loom down need rubber/plastic covers. Connectors can corrode this causes higher resistance and could produce heat/fire. Do not use significantly thicker wires than necessary to be used. Fuses do a good job but do not allow them to blow and allow the car to stop in a dangerous place (fast lane of motorways) Where do you start. Roger Quote Link to post Share on other sites
Tr4aJim Posted March 13, 2021 Author Report Share Posted March 13, 2021 Roger, I pretty much went right down your list! Over time, I have replaced all the grommets and I put shrink tubing on all those metal tabs holding the wires. Any wire repairs I’ve done (the POs had made some very “creative” additions/changes), I’ve tried to match wire sizes as best I could. I also have begun replacing all the wire connectors, cleaning the bullets and coating lightly with dielectric grease. Now I’m looking at the fuses. If I’m reading the wiring diagram correctly, it appears that the ignition system is independent of fusing (“north” of the fuse block). So if I keep my changes “south” of the fuse block in the diagram, hopefully I won’t impact drivability. Jim Quote Link to post Share on other sites
peter clarke Posted March 14, 2021 Report Share Posted March 14, 2021 Install a smoke extinguisher switch in the bulhead (battery isolation switch) that you can operate from the drivers seat .......self defence for connections north and south of the fuse block. Quote Link to post Share on other sites
AndrewMAshton Posted March 14, 2021 Report Share Posted March 14, 2021 Hello Roger, I am going through a similar process at the moment and to add to Peter's comment above, I am fitting an isolation switch in between the battery and earth, I believe Austin Healeys used to fit them and period rotary switches are available. There is a reason to fit it on the earth side but I can't remember what it is! Cheers, Andrew Quote Link to post Share on other sites
Hamish Posted March 14, 2021 Report Share Posted March 14, 2021 As many in here know I don’t understand electrickery so I read with interest Rogers comment “Do not use significantly thicker wires than necessary to be used” I thought the issues would be using thinner wire ?! Quote Link to post Share on other sites
RobH Posted March 14, 2021 Report Share Posted March 14, 2021 1 hour ago, AndrewMAshton said: There is a reason to fit it on the earth side but I can't remember what it is! Using metal tools it is easy to inadvertently connect something to earth (chassis). If you isolate the battery on the live side, accidentally earthing the live terminal while working can be catastrophic. If you isolate the earth terminal then accidentally earthing anything has no effect. 51 minutes ago, Hamish said: I thought the issues would be using thinner wire ?! There is no detriment from using a wire which is a bit thicker than necessary, except to your wallet and perhaps in difficulty with fitting it to the connector. It is certainly not a good thing to use wires which are too thin for the current being carried - much better to err the other way. Quote Link to post Share on other sites
Lebro Posted March 14, 2021 Report Share Posted March 14, 2021 Down side of isolating the earth side is if you have a modern type car radio, or anything else (tracker etc) which needs to be powered up all the time. Easy to do if you isolate the live side, not so easy if you isolate the earth side. Bob. Quote Link to post Share on other sites
Z320 Posted March 14, 2021 Report Share Posted March 14, 2021 In my opionion most electric improvements on this classic cars look unsuitable because of the careless acting of their owners. Quote Link to post Share on other sites
AndrewMAshton Posted March 14, 2021 Report Share Posted March 14, 2021 Hello Bob, I have already thought of that, in my case an alarm and a 12v supply in the boot to power my warning light ( https://www.ebay.co.uk/itm/Durite-0-442-00-LED-R65-Amber-Warning-Light-Flashing-Strobe-Warning-Lamp-12-24v/133115361722?hash=item1efe4b39ba:g:tEoAAOSwBnZatOVp ) much more effective than the indicators, so I have separate earths that run to the appropriate side of the isolation switch. Cheers, Andrew Quote Link to post Share on other sites
RogerH Posted March 14, 2021 Report Share Posted March 14, 2021 1 hour ago, Hamish said: As many in here know I don’t understand electrickery so I read with interest Rogers comment “Do not use significantly thicker wires than necessary to be used” I thought the issues would be using thinner wire ?! Hi Hamish, my logic is simple. It sounds sensible to use more than enough heavier gauge wire, as that will not burn out with the new load. The new load should be able to live with the old loom If the new load works with the heavier wire but is too much for the 'old' loom then you have trouble. The old loom will not be as good as a new loom 'like for like' so keep the wire gauge as near as possible to the old wire. Roger Quote Link to post Share on other sites
Hamish Posted March 14, 2021 Report Share Posted March 14, 2021 Ah ok. Thanks guys. Quote Link to post Share on other sites
Z320 Posted March 14, 2021 Report Share Posted March 14, 2021 (edited) A cable never rules (limits) the current, it is designed to loose 1-2% of the voltage. You have no real benefit in using wider cables by loosing "only" 0,5-1% voltage. Original cables normally are not to small at all for the related current - they are simply old and the problem is corrosion of the cables themself and the connections. This increases the loss of voltage and heats the cables up (P=U^2 x R). The hope about bigger cables is always the same for us men. If you understand what I mean.... Edited March 14, 2021 by Z320 my bad english writing Quote Link to post Share on other sites
Harbottle Posted March 14, 2021 Report Share Posted March 14, 2021 6 minutes ago, Z320 said: A cable never rules (limits) the current, it is disigned to loose 1-2% of the voltage. You have no real benefit in using wider cables by loosing "only" 0,5-1% voltage. Original cables normally are not to small at all for the related current - they are simply old and the problem is corrosion of the cables themself and the connections. This increases the loss of voltage and heats the cables up (P=U^2 x R). The hope about bigger cables is always the same for us men. if you understand what I mean.... You will end up on the naughty step if your not carefull Quote Link to post Share on other sites
Z320 Posted March 14, 2021 Report Share Posted March 14, 2021 Yeah I know Quote Link to post Share on other sites
RobH Posted March 14, 2021 Report Share Posted March 14, 2021 (edited) A 1 hour ago, Z320 said: Original cables normally are not to small at all for the related current - they are simply old and the problem is corrosion of the cables themself and the connections. Very true but there are complications in replacing like-with-like because early TRs used imperial size cable which is not now available. The modern metric cables do not correspond exactly and it is best to go to the nearest larger cross-section. Nearest equivalence I can find for the smaller sizes is: 14/.0076 (i.e,. 14 strands of .0076 inch dia) -> 0.0.5 mm^2 23/.0076 -> 0.75 mm^2 40/.0076 -> 1 mm^2 70/.0076 -> 1.5 mm^2 110/.0076 -> 2.5 mm^2 (Beware of the data on the web regarding cable sizes and current rating as most of it relates to house wiring not automotive use.) There is further complication because as well as standard cable you can also get thin-wall cable in the same sizes. Because of the thin wall this gets rid of heat more readily, so has a higher current rating for the same size of conductor. Edited March 14, 2021 by RobH Quote Link to post Share on other sites
Z320 Posted March 14, 2021 Report Share Posted March 14, 2021 (edited) Hi, the problem are the electric consumers we add on this classic cars AND the higher current going via the ignition switch! To avoid this I added an second Lucas fuse box (on top) and a small relais (behind) - the top fuse is constant 12 V via the 2,5 mm² red cable (black wraped) from terminal A of the RB102 - for the hazzard switch and charge socket - the one below is 12 V via the 2,5 mm² white cable switched by the ignition and relais from terminal A of the RB102 - for other consumers Other consumers are: the radio, my anti run on valves, also the heater fan and one spare cable hehind the dashboard. This keeps the original classic design, reduces the current via the ignition switch and the main white/red cable. I let all other cables (new from 1992 due to original specifications) untouched. I only use quality connectors and a suitable quality crimping pliers, not that pre isolated DIY market scrap. That works well since years. Ciao, Marco Edited March 14, 2021 by Z320 Quote Link to post Share on other sites
Tr4aJim Posted March 14, 2021 Author Report Share Posted March 14, 2021 I have fitted one of those twist knob style shut-offs onto the earth terminal of the battery. Per this discussion I will look into a similar switch that operates from inside the car. Jim Quote Link to post Share on other sites
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