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MG MGB Technical - Up-grade to cooling… Electric fan/relay concern

Hi
The set I have on a 1974 roadster and I have fitted an electronic fan. The fan unit is off a Peugeot van (motor rating unknown), with the switching controlled by a Kenlowe unit. I am running the power supply from a second fuse box. The fuse is rated at 17amp continuous + 35amp blow.

The problem is that the spade connection onto the box (power) side had got hot and melted it’s plastic cover, the fuse remained intact. To over come this I was thinking I should install a relay to control the fan.

So my questions are -- 1) will this cure the problem? -- And 2) if this idea will cure the problem; what wires should be connected to what relay terminal numbers?

Thanks in anticipation
Ian
Ian Holliday

A good quality 25 amp off-road lighting control relay will not only help cool down the engine, but the wiring as well. It's best to add a dedicated brown wire directly off the starter solenoid, and install either an inline fusible link or spade fuse adjacent to the relay for ease of troubleshooting. Wire it so that the kenlow sensor controls the relay, not power to the fan. Your fan will see more voltage from the relay, and should run at its rated speed even at idle, and with lowered parasitic circuit resistance, there should be far less heat generated in the circuit.
Mark Childers

Even if you install a relay presumably you will still supply the fan with a fused voltage. Sound more like a problem of a poor connection at the fusebox to me.
Paul Hunt

Thanks Pual for the suggestion i will lok at the fues box, it was second hand so it could be the culprit.

Mark, am I correct in saying that you think I should put the relay in line after the connection to the Kenlowe unit? I never thought about that, thanks.
Ian Holliday

If you add a relay it would have to be after the Kenlowe otherwise there is nothing to control it.

I've recently been right through my factory V8 fan circuits cleaning up all the connections. I've also added grounds from each motor to its mounting bracket, and a thick brown from the spare alternator terminal direct to the spare battery terminal on the relay. The result is a 30% increase in voltage at the fans and a very noticeable increase in rpms and speed of cooling.
Paul Hunt

Paul brings up a good idea for powering the big load/current draw items like the electric fans and the headlamps. That second, spare, output terminal is a great source for those two items, and if one were to run a proper gauge wire from there to a separate fuse block to supply the two relays for those two items, one ought to get about the same results as Paul has.

When I added relays and the Bosch alternator to my car, I did mine in a less efficient way, simply running a heavy wire from the second output lug of the alternator to solenoid battery terminal and then returning current from there to a separate fuse block for the relays. Paul's is a better way and I think I will go out this afternoon and revamp my wiring to reflect his better and more direct routing.
Bob Muenchausen

The other thing to mention is that by leaving the original brown wire on the fan relay I now effectively have a 'ring main' for the whole of the brown circuit, doubling its current-carrying capacity but more importantly halving volt-drop. So as well as getting max voltage to the fan it has also boosted the voltage to the fusebox and ignition.
Paul Hunt

Thanks to all of you who have helped me out. I have now looked at the fuse box and it would seem that the female connection was loose, so I have use a different one. Over the weekend I will be adding the relay, but I still don’t know which connections I should be using, can someone please help me, they are all numbered so it should be easy to explain to me.
Paul, in reading your postings, am I correct in understanding that the factory set up runs without the ignition switch cutting of the power supply? I had mine set up so it would run all the time, but I found it would suddenly switch it’s self on if the coolant near the sensor. Now it is turned off when the ignition is.
Ian Holliday

I do not have a fatory car by any means now, but what is the "spare alt terminal" you guys are refering to? My car originally had 3 wires to the Alt (least when I got it) one big brown that traces back to the starter and thus battery, then 2 small wires one runs the dash light and the other I don't know the function of.

I am using 2 stock fans on my car. I plan to duct them and the rad in to increase cooling and the fans efficiency. Adding voltage/amps would also help like Paul mentioned with the speed increase in his, so I would like to know how he set it up.

I plan to buy a purpose made Fan relaey kit from Summit racing, and maybe one of thier thermostats as well. for now my fans are always on with my V8 but I want to change that to off/therm/man override switch on dash AND get the wiring set at the same time...
Larry Embrey

Ian

There is an excellent section on relays showing how they work, the different types and all the connections on

http://www.mgcars.org.uk/electrical/

which is the link "Electrical help" at the top of Thread List on MGB Technical.

Good luck
Peter
Peter Vann

Ian - although the factory V8 fan relay has a brown wire (always hot) to supply the fans, the relay itself is operated by a ground from the sensor and the green circuit (fused ignition), so the fans cut off with the ignition. If you get the original style of MGB relays (shiny rectangular metal case with two mounting lugs) the terminals will probably be marked W1 and W2 for the winding, and C1 and C2 for the contacts. Modern aftermarket relays are usually numbered 85, 86, 87 and 30. 85 and 86 are the winding, and 87 and 30 are the contact. 30 is usually the contact to which the 12v supply would be connected and 87 the load. Some versions of these relays come with an integral fuse, avoiding the neccessity of having to provide something seperately.

Larry - At various times the looms provided two browns at the alt, at others only one. Originally it was one thick and one thin as you describe but both these went to the same place i.e. the solenoid. It was as if they felt they needed a 'sense' wire as well as an output wire. Then they dropped to only one brown but seemed to keep the same alt - i.e. one of the big spades is unused. Later still they went back to two browns again, but these are both thick browns used to double the current-carrying capacity (or more correctly halve the volt-drop) when the cars got more electrical load and bigger alts.

I have an override switch as well, which effectively parallels the thermal switch. Because the switch is illuminated I can also see when the fans have come on from the thermal switch.

Paul Hunt

Larry, also, in the case of an early car modified/converted to a Bosch or Lucas 18ACR, the two large output spade lugs may present one more than was used either originally or in the conversion. In that case, the "extra" lug can serve the purpose and plan as outlined in Paul's comments - a definite improvement for those cars.

If your car has two large browns from alternator plug, then examine your wiring diagram. You may be pleasantly surprised to find that you already have this wiring advantage. The factory was not above improving their product from time to time.
Bob Muenchausen

Paul, thanks for your explanation, I have looked at your web site bit I am a bit slow on the uptake when it comes to electrics. I have several relays in my “bits box” but no wiring diagrams with them.

Thanks also to Peter for pointing me in the direction of the web site.
Ian Holliday

Another good electrical site to supplement Paul's already excellent site is Rick Astley's, http://www.mgb.bc.ca/electrical which is also MG oriented.
Bob Muenchausen

This thread was discussed between 27/05/2002 and 04/06/2002

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