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MG MGB GT V8 Factory Originals Technical - Painless Wiring fan relays

I've got the motor ready to go in and now I'm sorting out my wiring upgrades. I have an engine driven fan and the factory MG electric fans for auxiliaries. Painless sells a fan relay kit that includes a screw in thermostat. It's available in 185 on/170 off and 200 on/185 off versions. Has anybody tried these yet? My thermostat is 180 degree so I figure that I would want the 200/185 version, otherwise the electric fans would run all the time. I am also considering using their headlight relay kit for my halogens and their auxiliary light kit for my fog lights. Any comments would be appreciated.

Thanks in Advance,
Joseph Lagasse

Joseph..There is a great MGB electrical web site that goes into great detail on all aspects of updating the wiring, including how to wire in relays for the fan and lights. I modified my wiring with relays for everything of high current draw and added an additional power buss to support them. Try this link:

Lots of great tips.


Gordon, we had a thread a while back discussing headlight wiring.

We all pretty much agreed that what you did is a no no. Should that 1 fuse or relay blow, you loose both lights. Only way to do it with regular relays is to have 4 of them, but that gets bulky

Someone said Hella makes a special relay for headlamps. It controls both high and low beam for 1 lamp. I haven't been able to find any information on it though.
Michael Hartwig

I understand the concern, but there are many manufacturers, including GM that put auto resettable breakers in the lighting circuit.

As it turns out, I split the circuit for high and low beam and put a resettable (automatic) breaker and relay on each. It's a compromise but a reasonably safe one. I personally disagree with the concept of unfused high current circuits as being "safe", and will take my chances on fused circuits, along with GM, Volvo, Ford and most other modern auto manufacturers.

About the Relay that controls both high and low beam . . . you simply use a relay that has the 5th terminal, which is the Normally Open terminal. That means that when the system is powered on, the lights come on at low beams (on the N.O. terminal) and when you switch to high beams, it sends the current to the Normally Closed terminal of the relay. Those relays are available from Bosch, radio shack, NAPA and most automotive electrical suppliers. I used one on the 2-speed, high volume Taurus cooling fan I installed along with a dual temperature fan thermostat from a Vanagon Diesel.


The concept of "safe" has more than one consideration. Without fuses in your headlight circuits, you run the risk of setting your car on fire. With fuses, you run the risk of wrapping it around a tree with you, and perhaps a loved one, inside. You may disagree with me, but to me the choice is clear.

You're not comfortable with high current carrying wires being unfused? What about the highest current carrying wire in the car - the main battery cable? Or the wire from the alternator to the battery? Do you have fuses in these circuits? There are other unfused wires in most cars as well, with good reason.

Not all modern manufacturers use fuses in the headlights. None of the Volvos I've ever owned did. In many cases, the manufacturers use fuses because it's the law in their country, not because the engineers prefered to.

If you feel you must protect the headlight wiring, use fusible links, not fuses or breakers. Fusible links will protect against a sustained short, but will not blow on a momentary short. The downside is they are a pain in the butt to install, and even more of a pain to replace if the need arises.

Your suggestion of using form C relays for the high and low beams defeats, to a large extent, the purpose of using relays in the first place. Wiring the relays as you suggested requires running the main power feed through a switch. The only benefit of using relays is to eliminate as many switches and connections as possible. To maximize the benefits, the headlight wiring should run directly from the battery to the relays, and directly from the relays to the headlights. ALL switching should be done by the relays.
Dan Masters

It amazes me that anyone can determine how my car is wired without detailed schematics. As far as whether I "burn and crash" or "crash and burn" is purely a matter of my choice. The fact is, I have never encountered (in 55 years) anyone who has totally lost there headlights during use, but I frequently see evidence that the wiring in MGB's is subject to fire because of the way it's wired and the quality of the wiring and components. Really . . . 4 or 5 primary fuses for an intire car? And your point about requiring a switch prior to the relays is also incorrect. ALL my switching is done by relays. You ASSUME there is no other way to wire the system other than your way, and you're incorrect. Do I have to submit my schematics for approval of this board? No! I'm qualified and CERTIFIED to design and construct my own wiring system and I'm not seeking anyones approval.

This link was begun by a man wanting information about adding a relay to his lights ( and by the way, the Painless relay kit includes a auto-rest breaker)and I responded with directions to an informative link. However, If everyone has to be of one mind to participate in discussions on this forum, then it is not the forum for me.


It was not my intent to start a flame war. You're right, the man asked for advise - you gave him yours, and I gave him mine, albeit in the form of an answer to your reponse to him.

Am I qualified to give advice on electrical matters? Yes, I am. I'm a degreed electrical engineer, with over 40 years of electrical experience, including 28 years experience designing electrical systems to meet the stringent requirements for nuclear power plants.

I've also written and published a very successful electrical repair manual for Triumphs - have you written one for MGs?

Do I need detailed schematics to understand how your circuit is wired? No, I only need your description of the circuit and an understanding of relays. If you wired it the way you described, then you must indeed power the relays through a switch. If not, then your description is not very accurate. You're right, I can't "devine" how your circuit works, and it appears I can't judge by your verbal description either.

Maybe you've never known of anyone losing their lights and crashing into trees, but I have. But then I'm 62, for whatever that's worth.

We do agree on one thing: If disagreement isn't allowed, then this list isn't for me either. I disagree with you. Quite strongly. If you get so enraged because someone disagrees with you, maybe this list isn't the place for you. Or is disagreement only disallowed for others, but not for you? Odd that you're so in favor of dissenting opinions, but become quite hostile when one is offered to you.

Nowhere in my response did I say you were wrong. I only offered my opinion, and pointed out a shortcoming in your headlight circuit (at least as you described it). You have a very thin skin indeed if you can't take that mild bit of disagreement.

Yes, whether you risk a "crash and burn" or not is your choice, just as it is mine, and just as it is Joe's, but I want to have, and I want Joe to have, ALL the facts before that decision is made.
Dan Masters

Gordon is probably right, i'll apologize to Joseph for crapping on his thread.

The 200/185 stat will work fine. If you want to do something really cool, use a PWM circuit and connect it to your temp sensor. The cooling fan will start off at a low speed. As the engine temp rises, the fan rpm's start to increase. More of a geeky coolness factor than anything else.

I don't know what that painless relay cost, but if you can deal with a single temperature on/off sender, take a look at one of these. It's an adjustable thermo switch. Should be around $10-$15

GM: 25036512
ACDelco: D1879
Borg-Warner: WT331
SMP: TS-11
Wells: TU6
Nieh: DR-134E
Filko: 2050
Michael Hartwig


I just want to be sure I understand . . . everything will be OK so long as everything is done your way?

You can look up my avionics qualifications at



No, you don't understand, not at all. And, it appears, you never will. You're the one who doesn't want to allow a dissenting opinion, not me. As far as I'm concerned, our discussion is over.

If any reasonable person wishes to discuss the issues further, pro or con, I'm available.
Dan Masters

Thanks for the information Michael. The Painless fan relay kit is complete with thermostat and runs about $60. I'll look into the parts that you list and continue to check out some other options.
In regards to the headlight relays, I like to see different opinions expressed in this forum. Granted, my primary question was about the fan relay but I will be making all my wiring mods at this time so the comments regarding the head light relay are also appreciated. I will keep the information presented here in mind when I decide how to wire my car.
Joseph Lagasse

Gordon said "The fact is, I have never encountered (in 55 years) anyone who has totally lost there (sic) headlights during use,"
...and presumably that is because the manufacturers do not try to protect them with fuses...

I have owned cars that don't have that circuit fused, and that hasn't concerned me on most of those cars. Those cars have had better wire quality and were not particularly troublesome. Every MGB I've looked at (I also own a 65 and a 78), the wiring in the underhoo0d area has been severly cracked where exposed to the atmosphere and even some bare wire visible under protective wrapping. In the areas behind the dash, wires are damaged and insulation pierced or split due to the way the wire is routed and supported. or not supported. More often than not, when you remove a lucas rocker switch, it comes apart in your hand. My point is, there is not a level of comfort with the design and components of this car or there wouldn't be so much effort that goes into modifying it (and discussing it).

I've wired my car so that the original wiring is now the primary wiring to activate relays and fused accordlingly, and high current loads forward of the firewall are on new circuits with new wire and relays, wired directly to the main battery wire at the starter. I have chosen to use circuit protection in my circuits. I don't care if anyone else does.
Gordon Elkins

Dan and Gordon put forward two differing and, in their own way, valid points. But do we really have to choose only one way and hope that it works out right for us. I have lost headlights due to the way a PO wired the lights, though the sidelights/parking lights got me home. I incline towards fuses (or breakers), especially with old wiring but have often considered protecting either each headlamp or beam separately. Either way you will lose only half your lighting and be able to see any oncoming trees. If you wire it to lose only one lamp then you will see and be seen - but look like a motorcycle to oncoming traffic. This may not be too bad if it is the onside light which you lose. Either way though you stand a chance of inviting the attentions of the police if you continue. Wiring it to lose one beam or the other (high or low), as Gordon has done, similarly allows you to see the tree but makes it difficult to carry on driving if it high beam which goes as this will dazzle oncoming drivers. Of course the ultimate is four fuses/breakers (on the output/bulb side of the relays) as this way you will only lose one beam and can tell the cop that the filament has gone - if indeed you keep driving - not that I would advocate this.
ian thomson

Flamers Dan & Gordon: most cars I've owned use circuit breakers rather than fuses in the headlights. That way, an intermittent short won't start the car on fire, but it won't keep the headlights off either. They now make some relays that have circuit breakers internal to them in fact. You guys need to take a few bong hits ... it's just not that big a deal at the end of the day ...

Hey, If you folks want to beat on someone, take a shot at me. I am just starting to wire my V8 conversion. So far, I am using no relays, and only two fuses (ala 1967). I am using all Lucas components. The car has a mechanical fan and no backup lights, trunk light, no door light switchs. It has a metal dash with the old style guages, but I have used an electronic speedo for accuracy. There are only three swithches on the dash--Lights, wipers, and heater fan. No electronic ignition (Mallory dual point). It is a trip back in time to a simpler life--when life was good--because it was simpler--and folks argued about baseball---and didn't have to carry a factory shop manual in their car--and went on picnics--and listened to mono radios--and had time to look up at the clouds---and---well you get the picture. Have a nice day!!
James Johanski

Do the headlights still illuminate when they short out? I do recall reading in something once about another car that used circuit breakers for the headlights rather than fuses to avoid being left in the dark when one blows. Does this type of headlight circuit breakers reset? How does that work, does it flicker off and on as it trips and resets repeatedly? Back then all I thought was, why are there any fuses in a car if they could all be replaced with circuit breakers. I had no thoughts of hurtling off the road into a tree, but then again trees are few and far between in this nation’s arid zone (Arizona). There are saguaro cacti however that when crashed into break into huge chunks and crush the offending car. I wouldn’t want to do that in a roadster!

James, are you going to put a mono radio in there or keep with the three switches?
George Champion

Another two cents, Most cars do have breakers for the headlights(now anyway) The cost of breakers in production is why there aren't more of them. Every two cents counts when you produce millions of units. I've seen some set ups in Volvos and other euro cars that divide the system side to side. Seems smart to me. Loose one headlight at a time.
As one who has burned up a bugeye, I now have a auto reset breaker off of the main battery feed. Cheap insurance.


Thanks Ted . . . I needed that ....

Gordon Elkins

If I can address the original question, if anyone still cares-

I have always used an adjustable "fanstat". There are several versions available in autoparts stores, also Trak Auto & Pep Boys. My preferance is the type that has a capillary or Bordon tube that is clipped to the radiator fins or inserted into the radiator via an extra fitting. I do not recommend inserting them into the top hose, as they seem to always leak.

I like to let the fan come on a little sooner in the hot weather & let the heat build up a bit more in winter, for obvious reasons. These units are usually under $20.00 US.

My wiring preferance is a relay & 20 amp circuit breaker, as I run a large high draw fan as my only cooling element. In hot weather, it can draw a good bit of power at start up, & has once or twice in the last several years tripped the breaker.

The only fanstat I had that failed caused the fan to run continuously.

I will stick with the three switches.
James Johanski


Sorry I missed you at British Car Day. I arrived late but fortunately missed the lunch debacle. On the topic, I have a drain valve on both sides of my block that appears to be a 3/8" pipe connection. I was thinking about using one of these to mount a thermoprobe for the fans. Is this feasible?


Joseph Lagasse

I would not use these block drains to place a sensor to engage electric fans. The idea is to run the fans when the radiator needs the additional air flow, so this is where the sensor belongs. In a properly functioning cooling system, block temperature should be relatively stable compared to radiator temps, which can fluctuate in a wide range. At speed, your radiator can drop to 170' or less but engine temps can be 200'. In this situation, your fan may run unnecessarily, & could even block airflow through the radiator, especially if the radiator is properly ducted and a spoiler is in place. At 65 mph, a ducted radiator with a front spoiler is receiving more airflow than even a 3000 cfm electric fan can provide.

A second consideration is the location of the block drains. They are only an inch or so from the headers & inserting & removing a probe may require header removal.


This thread was discussed between 19/06/2002 and 22/06/2002

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