How EXACTLY do I wire in a fan over-ride switch?

[Ireland]

My car started to overheat in traffic earlier today.

I think it was due to a bad contact in the jack plug for the fan which I've cleaned and will replace but I also want to wire in a switch where I can manually switch on the fan if I ever see this happening again.

My question is regarding where to pick up the supply from.

Those of you that have done it -

- did you wite it straight from the battery to the switch or did you take a switched feed so the fan can't accidentally be left on and drain the battery?

- where exactly did you connect to?

- what size fuse did you use?

- did you wire in one or two switches (be able to switch on one or both fans)?

- where did you mount the switch(es)?

- anything else I need to do?


Thanks in advance.

[Allandwf]

I was going to do this,but never got round to it, but thought the easiest way, for me, would be to fit a switch that open circuited the temperature sender to the ECU which would cause the fans to run. This would negate the use of seperate feeds, relays fuses and the like.

Edited by Allandwf on Sunday 1st August 18:15

[SimonSparrow]

I did this this quite a few years ago. I took a feed from the battery using heavy cable and then used relays mounted near the radiator to switch this feed to the lights and fans. The relays are triggered by the original TVR wiring. This takes the load off the TVR wiring, which is marginally sized, and also minimises voltage drop to the lights and fans - it certainly made a difference. The fan relay can also be tripped by a switch inside, so if I see a big queue of traffic coming up, I can turn them on.

I have two relays for the fans, but have jumpered them together so they are tripped by the lower temperature TVR wire.

[brogenville]

Cleanest way to do his is to run a wire from the fan relay in the boot into he cabin. Connect this to a switch that has a ground wire to it coming from somewhere in the cabin (think I used the handbrake mount). Simple and neat, and no messing with any wiring in the engine bay.

[ro_butler]

Allandwf said:

I was going to do this,but never got round to it, but thought the easiest way, for me, would be to fit a switch that open circuited the temperature sender to the ECU which would cause the fans to run. This would negate the use of seperate feeds, relays fuses and the like.

Edited by Allandwf on Sunday 1st August 18:15

But this makes the car fuel conservatively (runs rich) and also activates the MIL light. Can cause damage to the cats if you still have them in.

Edited by ro_butler on Monday 2nd August 10:09

[ro_butler]

SimonSparrow said:

I did this this quite a few years ago. I took a feed from the battery using heavy cable and then used relays mounted near the radiator to switch this feed to the lights and fans. The relays are triggered by the original TVR wiring. This takes the load off the TVR wiring, which is marginally sized, and also minimises voltage drop to the lights and fans - it certainly made a difference. The fan relay can also be tripped by a switch inside, so if I see a big queue of traffic coming up, I can turn them on.

I have two relays for the fans, but have jumpered them together so they are tripped by the lower temperature TVR wire.

This sounds like a good way. You can also just wire the fans directly to the battery via a switch (with no relays) assuming you use suitably rated switch and cable. As Simon says the original TVR wiring is marginal and if you use a shorter run of good cable (ie. not run from the boot) then the voltage drop will be less. The small advantage of this (or simon's secondary relay route) is that if the fan relays in the boot fail you can still switch the fans on by hand. Simply mount the switch on the cowling under the steering column.

Don't worry about leaving the fans on and draining the battery, they make such a racket that it is impossible to miss the fact they are one once the engine is no longer running.

[ro_butler]

brogenville said:

Cleanest way to do his is to run a wire from the fan relay in the boot into he cabin. Connect this to a switch that has a ground wire to it coming from somewhere in the cabin (think I used the handbrake mount). Simple and neat, and no messing with any wiring in the engine bay.

Personally I would bypass the TVR wiring altogether, its cr@p!

[shoggyraminator]

ro_butler said:

Allandwf said:

I was going to do this,but never got round to it, but thought the easiest way, for me, would be to fit a switch that open circuited the temperature sender to the ECU which would cause the fans to run. This would negate the use of seperate feeds, relays fuses and the like.

Edited by Allandwf on Sunday 1st August 18:15

But this makes the car fuel conservatively (runs rich) and also activates the MIL light. Can cause damage to the cats if you still have them in.

Edited by ro_butler on Monday 2nd August 10:09

Hello ro_butler, Can you explain how this is as I don't see why effectively tripping the over heat sensor will cause the ECU to over fuel the engine. Surely this would only be the case if you were "fooling" the cold start sensor? (If there is one).

[FarmyardPants]

At a guess the ECU would treat the out of band input as an error from the sensor. The ECU then reasons that if the sensor isn't working, it has no information about engine temperature (it could be stone cold) and hence fuels as though cold to be on the safe side.

[shpub]

FarmyardPants said:

At a guess the ECU would treat the out of band input as an error from the sensor. The ECU then reasons that if the sensor isn't working, it has no information about engine temperature (it could be stone cold) and hence fuels as though cold to be on the safe side.

It actually substitutes a value which is around 97C IIRC which then triggers both fans on. The MBE does this substitution for all the sensors so that if one fails it substitutes a value and carries on in a "limp home period"

The only sensor you really need is the crank sensor as without it, the ECU thinks the engine is stationary and doesn't turn over.

I have an additional panel on my LS Cerbera with a low oil level light, a low water level warning light and two leds indicating the fan status. There is also an fan override switch. I hacked into the ECU wiring to do it so that the switch either connects the relays to the ECU (normal) or to ground. The LEDs reflect the relay feed and come on if the relay is activated. Don't ground the relay feed from the ECU as you could blow up the ECU. With the switch I have the ECU is disconnected before the relays are grounded.

The oil level is courtesy of the LS. The water level switch is built into the standard Cerbera expansion tank but is not connected as standard.

Edited by shpub on Monday 2nd August 11:12

[crackedfinger]

(Assuming the cerb has one like the RV8 engines) Would it not be a lot easier just to run a single wire from the otter switch terminal and switch that to ground in the cabin? No need to monkey around with power supplies and relays etc, as they are used as per normal.

[brogenville]

[quote=shpub]Don't ground the relay feed from the ECU as you could blow up the ECU. quote]

Worked for over 2 years on my car with no problems.

[Ireland]

My simple idea of a wire from the battery to a switch and then from the switch on to the fan almost seems a bit "agricultural" now compared to the idea of wiring into sensors / ECU / etc

Does it realy need to get complicated or will my agricultural solution be OK ?



I don't see any need to complicate it.

[SimonSparrow]

Ireland said:

My simple idea of a wire from the battery to a switch and then from the switch on to the fan almost seems a bit "agricultural" now compared to the idea of wiring into sensors / ECU / etc

Does it realy need to get complicated or will my agricultural solution be OK ?



I don't see any need to complicate it.

It will work (as far as I can see), but it may be worth beefing up the earth wire for the fans while you're at it. The TVR wiring basically goes from the battery to the boot and then back to the front of the car for the headlights and fans, so there is probably about 12m of wire between positive and negative on the battery and the fans. The wires aren't huge and you will get some voltage drop along them.

I've done the relay mod on two TVRs now and it does make a good difference.

[ro_butler]

Ireland said:

Does it realy need to get complicated or will my agricultural solution be OK ?



I don't see any need to complicate it.

It will be fine. It has the advantage of not affecting the ECU as this will continue to operate normally. Simon's solution is slightly more complicated in that it uses separate relays (i.e. the switch is not directly connected to the fans) and is therfore probably better as you don't need to worry so much about the rating of the switch or the switch wiring.

[ro_butler]

FarmyardPants said:

At a guess the ECU would treat the out of band input as an error from the sensor. The ECU then reasons that if the sensor isn't working, it has no information about engine temperature (it could be stone cold) and hence fuels as though cold to be on the safe side.

Yep, that is what I had heard (but never experienced first hand). The temp sensor is effectively the 'cold start' sensor.

[shpub]

brogenville said:

shpub]Don't ground the relay feed from the ECU as you could blow up the ECU. quote said:

Worked for over 2 years on my car with no problems.

I've done it in the past and also got away with it. Equally I've blown up kit as well so I now tend to stay on the side of caution, especially as getting MBE's repaired is not cheap.

[shpub]

Ireland said:

My simple idea of a wire from the battery to a switch and then from the switch on to the fan almost seems a bit "agricultural" now compared to the idea of wiring into sensors / ECU / etc

Does it realy need to get complicated or will my agricultural solution be OK ?



I don't see any need to complicate it.

The fans take about 30A each when they are first switched on so it can be hard to find a switch that will cope. Use a switch to control a high current relay that connects thge fans to power and your method will work.

[Ireland]

Thanks Steve.

I was looking on the fans themselves today to see if there was a data label on them to see what sort of amps they draw but there was nothing on them.

That's exactly how I'll do it now that I know what sort of load they take.

[ro_butler]

shpub said:

Ireland said:

My simple idea of a wire from the battery to a switch and then from the switch on to the fan almost seems a bit "agricultural" now compared to the idea of wiring into sensors / ECU / etc

Does it realy need to get complicated or will my agricultural solution be OK ?



I don't see any need to complicate it.

The fans take about 30A each when they are first switched on so it can be hard to find a switch that will cope. Use a switch to control a high current relay that connects thge fans to power and your method will work.

Err, only for a VERY short period, same as most circuits with motors in. Sorry to be an anorak but the big fan draws 18.7A and the small one draws 7A at steady state according to the spec sheets for each one. It is these numbers that are far more important.

Edited by ro_butler on Tuesday 3rd August 10:41