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TR2/3/3A tr3 electrical question

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sp53

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Hi all I have an electrical question about a different vehicle other than a tr3. Why it is that modern vehicles like a tr6 use relays in the wiring circuits. I am sure that it sounds like a dumb question to those who know, but my tr3 does not use them, and it has work well for over fifty years. Do the relays help protect the system or make it operate smoother?
Sp53
 
Some would argue that the TR3 should have had relays, which can help take the load off of other components such as switches in high-current-draw circuits such as headlamps or horns. But like you, I've had hands-on experience for many, many years with orginal electrical equipment on various British cars...equipment that continues to function as it did when new.

And I can compare a number of cars with otherwise almost identical components. I've Heralds and TR3s with no relays in (for example) horn circuits, and the horns work as well as they do on Spitfires that did have horn relays. Over the years, I've also dealt with any number of such cars that had been, uh, "modified for safety" with addition of relays and goodness knows WHAT else. Not being an EE myself, my usual approach is to undo whatever Rube Goldberg-type "fix" /ubbthreads/images/graemlins/hammer.gif was applied and return things to original...after which they work just fine again. /ubbthreads/images/graemlins/grin.gif

Sometimes I wonder which is worse: well-meaning but sometimes poorly thought-out additions, or the classic "wood screw replacing a fuse"? /ubbthreads/images/graemlins/eek.gif
 
[ QUOTE ]
Sometimes I wonder which is worse: well-meaning but sometimes poorly thought-out additions, or the classic "wood screw replacing a fuse"? /ubbthreads/images/graemlins/eek.gif

[/ QUOTE ]

I've heard that a .22 caliber cartridge works too.

Part of the charm of a TR3 is it's simplicity and the absence of relays supports that without any downside I've encountered. It does mean some beefy wires for some applications (most notably horns) but properly maintained the electrics are quite trouble-free.
 
Thank you for your comments Andrew and Geo. I have been working on a Saab (I know, but it is European) lately and it has a ton of wires and many relays. When I am pondering the wiring problem, my basic reference always defaults to tr3 wiring circuits because I have owned one since basically high school. Moreover, these relays are very costly and I do not know if they can be tested or if I have to just replace and go with new one and hope the relay was the problem.
Sp53
 
When I installed a 12" Kenlowe fan in my TR3A, I connected it directly to the hidden switch under the dash and burned out about 3 switches. Then I hooked it up through a relay because the starting current for the fan is about 25 volts and the running amps is about 5. That solved the switch problem. But I had trouble with 2 relays and had to replace them. This spring I had another relay problem where the heat in the relay caused one of the spade connectors to get loose in the molded plastic casing.
 
[ QUOTE ]
Not being an EE myself, my usual approach is to undo whatever Rube Goldberg-type "fix" was applied and return things to original...after which they work just fine again.
Sometimes I wonder which is worse: well-meaning but sometimes poorly thought-out additions

[/ QUOTE ]

Interesting thread. To me anyhow since I am and EE. The relay allows you to use switches and wires in the dash that are rated for lower current and thus can be smaller (and sometimes longer lasting). The thing we have with our TR3s is the wires and switched were designed (more or less) to handle the full current of the devices they operate (be they lights or horns). So for our cars there is no need to put relays in there "to fix it". So I agree with the approach of returning it to original (unles of course you are putting in lights that draw more current, in which case you should add a relay).

Unfortunately the converse is not true. If your car was dsigned with a relay then chances are the switch and or the wires can't handle the full load. So you should keep the relay in there.

To answer the original question of what a relay does. The simple description is the relay replaces the switch. A regular switch uses your finger to turn it from one prosition to the other. A relay switch uses electricity to turn it from one position to the other. So you could look at a relay as an electricaly operated switch (that uses much less electricity to run then the items it is operating use).

Yes you can test a relay and (once you know how it is a simple test). You can test the winding coil with an ohm meter for continuity. If that passes then you can use the ohm meter on the output side and feed the input coil with the appropriat voltage and see if the contacts on the output open and close with the application of voltage to the coil. Much harder to describ by typing then to do.
 
Thanks Adrio I think I understand how to test them. When I get home tonight, I will give it a try.
Sp53
 
So then Adrio are relays rated in amperes and can they kinda sorta be moved around. Are they interchangeable? This Saab I bought has got a row of them installed next to the fuse box. They all look very different and I am guessing that is because of the amperage draw ?.
Sp53
 
FWIW -- I do think the (unrelayed) headlamp switch in the TR4 is marginal for the job at hand. If conditions are less than ideal (poor connectors, corrosion at the connections) that switch can heat up. They went with a relay (and the same switch) on later TRs.

The TR3 switch OTOH is of stouter stuff... I think it's the same one they used for the hot seat at Sing Sing.
 
Yes relays are rated in amps (to simplify). They also have different input current draws depending on how hard they want to pull the contacts together. So if you need a 10A relay, you can use a 15A relay BUT ONLY if the input current the 15A realy draws is not more then the circuit feeding it can handle. In short there is some slight interchangeability there but you have to consider the whole system and make sure you are not fixing a problem downstream but creating one upstream. I suspect that is why they used different form factors on the relays, so that they could not be interchanged by mistake (or on purpose) I hope that explains it.
 
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