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MG MGB Technical - relay to 2spd heater fan

Assuming the weather stays ok tomorrow, i am altering the wiring of my 72 BGT to accept hazard warning lights and if this goes well a 2 speed heater motor from a later car also.

Has anyione fitted a relay to 2spd fan wiring before and could they give any tips?? Is it possible (i am assuming yes to this)? Do i need one or two relays (1 per setting perhaps?) Where to wire it to, starter solenoid possibly (this is where my headlights go to at present)?

At present i have just written out the circuit i am going to use without any thought to relays, but as my 1 spd drains the power somewhat at idle, a 2spd is likely to be worse and therefore i see a relay as a benefit...

Comments, advice or opinions apreciated.

~PHIL

Phil

Phil - 2 relays (one for each speed setting) would be the easiest way to wire up the two speed heater fan. 12V should come, via about 10 gauge wire from a source of power with as few junctions as possible. The starter solenoid is as good as any, except it isn't switched. If you use this source, it should be fused at a level that the wiring is protected in case of any short circuits. Good luck - Dave
David DuBois

Phil,

There is a lot of misunderstanding about the use and function of relays, so I'd like to take your question as a springboard to see if I can clear up some of the confusion.

First of all, we need to realize what a relay is: It is a switch. Nothing more, nothing less. A relay won't do anything a switch won't do. For the normal applications in our little cars, a relay is required if and only if the installation of the relay cures one of the two problems below:

1. The switch you are now using or want to use cannot handle the current demands of the switched load,

2. There are too many bad connections or vastly undersized wire in the existing circuit.

A typical relay is rated to carry 20 - 30 amps, but we don't have loads of that much for any device in our cars. With the exception of the headlight switch, there is no switch on our cars that carry more than 10 amps, and the factory switches are good for that much current.

Unless the relay circuit is designed and installed properly, it is possible to ADD connections to the circuit, rather than eliminating them. This is not as much of a problem in an MGB as it is in an early Triumph, because our MGBs don't have ammeters to cloud the issue as do the early Triumphs. The reduction of connections is the real benefit of adding relays in most cases. If you can run a wire directly from the battery post on the starter solenoid to the relay, and then run a wire directly from the relay to the load, you may benefit from the relay. Looking at the heater circuit on a TR6, for example, there are TWELVE connections between the alternator output and the heater motor ( don't know how many there are in an MGB, but there should be about the same number). By wiring in a relay as above, you will have only 6 connectors in the circuit. If you add a fuse (which you should), you will add 2 or 4 more connections, depending on the type of fuse holder you use. In that case, you've only reduced the number of connections by 2 or 4 over the stock setup. There will be very little benefit to the relay addition unless your new connections are better than the existing connections.

The wire in our cars is somewhat undersized, but not enough to to be a problem with voltage drop, as long as the wire is in good condition and not badly corroded. Using heavier gauge wire when you add a relay is a good idea, but it is not really going to be of a great benefit. Fourteen gauge wire is good for 15 amps, and the heater fan won't draw that much.

As for reducing the load, the addition of a relay will never reduce the load. In fact, the more you need the relay, the more current the circuit will draw after the addition of the relay. Undersized wire and bad connections add resistance to the circuit, reducing the current. It is precisely this added resistance that you are wanting to eliminate with the relay, thus the increase in current.

That's not to say there isn't any reason to add relays, just to say that relays aren't the panacea for electrical problems that may seem to be. Even though the existing switches may be adequate, they do fail, just as relays also fail. Replacing a factory switch requires from $40 to $100, and anywhere from an afternoon to a whole weekend to replace. Replacing a relay takes about five bucks and a couple of minutes to do. By placing a relay between the switch and the load, you reduce the load on the switch to milliamps, rather than amps, greatly increasing the life of the switch. It will wear out mechanically before it does electrically.

Another benefit of using relays is the ability to interlock various loads to each other. For example, you can wire a relay to turn on your headlights any time you have the wipers on, or wire your driving lights to go off when you dip the high beams. Or, if you want, you can wire your headlights to go off when you turn the ignition off so you don't forget and leave them on. Etc.
Dan Masters

David

You're right about the starter solenoid power not being switched, but that isn't really a problem, as the power from the heater switch to the relay coil is switched. Even though power is always on the relay, the relay will only operate when the key is on. The heater fan will turn on and off with the key, just as it does now.
Dan Masters

Dan,
Great explanation!

A few comments, though. I think putting an inline fuse in a new wire from the starter would be a good idea, just from a philosophical standpoint. I just dislike unfused circuits, except where necessary...

Also, I know on some years of 'Bs (my '69 being an example), the power supply for the blower motor comes from the dash harness and thus goes through the voltage stabiliser. This has the effect of triming about a volt off the available voltage and thus means the airflow will be *slightly* reduced. And since every little bit helps.... Of course for cars on which the supply does not go through the stabiliser (and I have no idea which years this may be as I have not looked) my argument is moot.
Rob Edwards

Dan

Good explanation that (i might print it off because its a far better explanation than i could ever give - if i was ever needed to!!).

Just wanted to go over a couple of points.

I realise that the relay fit is not the be all and end all and at first was not going to bother, however just going back over my drawn wiring diagram tonight ready for the morning (checking for errors etc), i started to wonder if it was a benefit for this application. (Your comment about extra connections being added into circuit by 'error' is a very good one!!)

The main "dilemma" i had was to be where to take the power feed from for the relay if added. Original cicuit takes the green fused circuit from volt stabiliser. Couldnt much see the point of taking the power from here and adding a relay as like you say it is just putting an extra item into circuit.

Running a wire from solenoid is about the purest form of voltage the circuit can get, but seems a bit overkill for the situation (i can see the point for headlamps) and of course it will require a fuse of some sort.

Perhaps it would be adequate in this instance to just ensure good connections and ample gauge wire and leave the relay out of circuit? I only truly see the point of doing something like this if the plus points out way the negative. I shall ponder over this tonight.

Many thanks for raising some discussion points, which is what i was after!

~PHIL
Phil

Thanks, Dan, for a great lesson in relays. As you say, they have their place, and in many cases, the "improvement" made may only be a reflection of a different wiring run, not the superiority of the relay type circuit. For my own experience, the two items that seem to have benefitted most from the introduction of relays, were the headlamp's light output, and the longevity of the Mickey Mouse column mounted lighting switches. I think that the switch for my electric radiator fan will probably last longer as well, for the reasons you have outlined above.
Bob Muenchausen

I agree that the supply for some heater blower motors is taken from a green terminal on the voltage stabiliser, but that is not a stabilised voltage, it's 12 volts from the fuse box. The other terminal (I) with the LGG wire is the stabilised voltage. FWIW, Clifton
Clifton Gordon

Clifton,
Just had a look at the diagram -- I stand corrected. That's what I get for shooting off my mouth going by memory! ;-) The pick-off for the fan circuit is indeed on the upwind side of the stabiliser....
Rob Edwards

DAn: you retype that and submit it to BRITISH CAR or B+ magazine for publication. Add a few pics and you'll have a great article.
william fox

It seems pointless to use a relay for a heater fan for anything other than its fastest speed.
Paul Hunt

This thread was discussed between 11/11/2002 and 12/11/2002

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