bypass switch for fan relay

I have a large OE pusher fan thats setup to come on with the AC through a large relay that is powered by the pressure switch on the AC accumulator. I'd like to wire in a switch that would turn the pusher fan on - regardless of the AC. My fear is another power source that would backfeed and damage the fan's OE relay or the pressure switch. Any ideas? I've searched through the postings and can't find anything similar.

I'd also like to find a source that could make me a silkscreened(?!) symbol on a switch for the fan.

Many fan controls are ground switched. This can be either the relay grounding the fan, or the (pressure etc) switch grounding the relay. (Or both.) In either case, you only have to ground in parallel to the grounding <whatever>. Grounding the pressure switch requires little current so it's easy to get a switch & wire etc. But is this signal used for other things too? Otherwise, a suitable relay (eg - across the fan's grounding relay) controlled by your manual switch (maybe grounding too - use +12V power off the fan's relay and run a single grounding wire to your manual switch). If it is +12V switched, then yes - cross-connecting power feeds can be a problem. But I'd suggest a relay using the same power feed as the fan relay and bypassing it - ie, your relay's heavy terminals (30 & 87) in parallel with the original fan relay. That's the same as the above grounding relay except it's now switching +12V, not GND. But that assumes the fan power is direct from the battery (through a fuse) - but they usually are (ie, does not require IGN on etc), and that other things switched on with the fan power are not an issue (like what? A fan indicator light? - that's not a problem. Something else?). If the fan relay switches +12V and you use a different power source, then I'd recommend a suitable change-over relay (SPDT) inserted between the original relay and the fan. Its NC contact (87a) is to the fan relay (its output) and the NO contact (87) goes through a fuse near the battery to the battery). When you switch on the relay, it changes its +12V feed to the battery. (Solenoid/coil terminal #86 could go to the #87 battery feed; then coil #85 to your manual grounding switch.) In general though it seems fan systems are fairly simple - ie, no EMS or other signals sent. (Though 2-speed fans etc can get tricky...) Similarly for the air-con pressure switch - if it does signal something else, it may be inconsequential (like turning the cabin ventilation fan on).... Though I can imagine handshaking - eg, pressure switch on, vent fan not on, therefore turn off the aircon (which isn't on anyhow) so no problem... unless smartie-pants onboard diagnostics reckons that's an impossible state so it alerts SAC & shuts the vehicle down... Though like I said, they are usually pretty basic... I hope that helps. (It should cover all options!)

Thanks oldspark - and you are correct - it is a ground switch. The relay is an OE 40A - very nice (I used them for directly connecting my headlamps and driving lights straight to 12v). It has 4 terminals: 1)power source; 2)power to fan; 3)ground; 4)to switch. The "to switch" is what comes in from the AC pressure switch, and I believe it to only be a ground (will check this with a DVOM). I think I could tap in a switch to ground to the #4 terminal. The switches are also very basic (its a 20 yr old Volvo) with the switch lights also ground based. I use the fan bypass in heavy stop-go traffic times when I want a little more air flowing.

Dang! I could have kept it simple (omitted the unlikely"hot" switching. But thanks for the feedback. I was factoring that it was a "very new" vehicle... things change, but even the very-news seem to be as simple as ever. Thanks again. Stop here - below is the FYI I wrote BEFORE the last para.... GND switching (aka "Open Collector" etc) has the main advantage that it doesn't matter what voltage supply you are switching. (EG - it might be a +5V circuit grounding a +12V device - viz: EMS & fuel injectors, computer circuits, etc.) Another great advantage is that you can parallel many switches without concern. And finally, it only requires ONE wire to be run to the switch/relay which is then grounded locally (chassis/body etc), Furthermore, it this wire shorts to GND, there is no fire hazard etc (though the switched device will turn on). (Though the latter is dangerous if "arc" to GND gets hot - that cannot be fused or protected and can cause fire. The same in a hot (+12V) circuit should blow the fuse.) A disadvantage is that devices are hot (+12V) all the time - if indeed that is considered a disadvantage - they won't consume power and will be fused; if not fused, don't GND-switch them! But if there is a ground leak, the power starts to flow. The leak could be water, dirt, mud, etc. That's why vehicles with GND switched headlights often had full-time or dim lighting - VW Beetles and Datsun come to mind - eg, with low beams on - Beetles with one dim headlight; Datsun 1600s with all 4 beams on dimly (all being combinations of GND faults or GND switching). But the old Ovlov is good 'ol Swedish quality and won't suffer from such barbarian faults.

oldspark wrote: if not fused, don't GND-switch them!

This car originally had the power supply for the fan relay wired directly to the battery via a junction block. I added an underhood fuse box from a later Ovlov that had 8 PAL fuses originally supplied with all 50A circuits (all 8ga. wire - was for heavy duty apps). Originally 2 fuel pumps, relays, power stage, etc was all on a single 25A fuse - that was known to just melt over time. I separated the system into 2 separate 30A circuits in this aux fuse box. I added the high beams on one; the lows on another; the stereo system on another; the fog lamps on another, and then will fuse this fan relay! I had even thought of wiring the switch as a jumper/bypass for the AC pressure switch - also in order to activate the pusher fan. It seems to take awhile and get fairly hot before the fan does finally come on. Other models have a temp sensor in the radiator to activate the fan - in the past I just ran a jumper on the temp sensor through a switch in order to bypass it.

I would have thought the A/C switch would have been immediate if it were a pressure switch. Maybe it's faulty & some other temp sensor is activating it instead (though you suggested a direct single connection)? Or maybe it is a temp switch (doubtful?)? And thanks for naming PAL fuses (flinks)! (I describe my "LOL" way below in the following "Stop Here... don't read on!" sections/s...) FYI: (everything that follows...)     Coincidentally yesterday I got 2 temp sensors for mum's car from my local PAP (Pinch A Part) - the fan is coming on at too low a temperature. I thought the workshop manual was wrong when I read that disconnecting the single-wire grounded sensor turned the fan on. So it's not traditional like and oil-pressure switch which grounded the powered oil dash-lamp to inform of no oil pressure (and NOT inform you if the switch was faulty or disconnected)? Surely they weren't using a "fail-safe" (normally-closed) alarm/control systems? Yes they are! The normally closed sensor energises a relay that pulls the fan OFF its normal(ly closed) power connection. If the sensor goes over temp, the switch opens, the relay de-energises and its NC contact (87a) connects to power the fan. If the sensor connection breaks or vibrates loose etc - or if the relay coil/solenoid fails - the fan comes on. That's reasonable fail-safe - it's better to have the fan on when NOT needed than not have it on when is is needed. Though not a new concept, I would have thought a full time energised relay to have been limited in life. But this circuit has not failed in over 20 years. I know relays are reliable, but 20 years being on! Mechanically they can seize too, though in this case it is most likely to fail in "normal" state - hence the fan would be on. Maybe I will change my traditional normally closed oil-pressure sensor to normally open (meaning that when oil pressure closes the switch, it turn on a relay that disconnects the oil light). Hence no more problems with not realising the oil pressure light is NOT on when I first start the car (because its connection goes bad from coming loose or oiling up etc). At the moment I have a 2 or 3 normal oil-pressure switches in parallel (from different oil-line tapping points) to provide redundancy against bad switches and connections - but I never know if a circuit/switch is bad unless I test it individually).     The above FYI ramble is for your consideration... I like it when a faulty sensor/circuit means that the fan (etc) turns on instead of not turning on. All you need then is another sensor to confirm the fan itself is running.... (TIP: Use airflow - it covers various alarm states which cannot be detected electrically.] Also a LOL! PAL fuses - I've been recommending them for ages (to replace older wire fuse-links (flinks) which I consider unreliable, a hazard, and over "selective" - ie, 13A, 16A, 21A, 26A etc... like I mean, how are mass produced loads that selective?) - yet I can't recall knowing their name! I was dreading PALs were the old ceramic fuses that the Euro cars had, but I think they disappeared during the dark times last millennium. Phew! I am surprised your hot fan(s) were un-fused, though I am used to Japanese car wiring. (I am often amazed by the primitiveness of Euro wiring - not much different to the old English cars. I never liked their "blocky" electric switches and connections, nor their need to note where each wire went BEFORE dismantling... though at least they stated color coding wires at some stage.) But those PAL types are the beez kneez - I love them! The only possible improvement may be circuit breakers for critical circuits (eg, headlights) and circuits that may occasionally overload without damage (eg, fan inrush currents, inter-battery connections). And I suspect that PALs above ~40A should be bolt-in types after having a 40A plug-in type get VERY hot (case melted but flink was ok) - of course that was after I got all my plug-in PALs up to 80A inclusive because I didn't like the delay/hassle with the bolt-in versions (though once trials are over I might use bolt-ins; maybe with spare "bypass" plug-in slots for the fast fix).