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Question about fans

 
Old 08-30-2014, 04:41 PM
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Default Question about fans

Okay, so during the last half mile on my trip back home after a beer run, my Prelude started running hot (temp gauge was 3/4 full where normally it runs 1/2 full). After shutting the engine off, the fan (fans?) began running for several minutes as I think is normal anyway, however after a couple minutes the fan began turning off then back on in a random fashion. I stupidly did not actually pop the hood to see if BOTH fans were actually running, though it does sound as if only one was running (appeared obvious when it cut out).

My dad and I have so far checked the fan relay there appears to be two circuits (4-prongs). When we jump one circuit, the driver's side fan turns on, but the passenger side doesn't. When we jump the other, absolutely nothing happens.

My question is will both fans turn on simultaneously when triggered, or does one normally run and the other only if needed? Of course, if that were the case, both should've been running since the engine was beginning to overheat. As I said, I didn't actually confirm if both fans were running, but I can obviously check this easy enough. Does seem odd that jumping the relay only triggered one fan and not the other...

The reason I question the fans is I replaced the thermostat this spring (then bled the system of course) when I discovered the old one was stuck shut so I don't really know why else the engine would've started overheating.

Also, is it common for one of the fans to have 5 blades (passenger) and the other to have 4 (driver)?
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Old 08-31-2014, 12:58 PM
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Default Re: Question about fans

Okay, today I ran the engine to see if the fans would cycle and BOTH fans ran as they should, no problem.

So how come when my dad and I jumped the relay yesterday, only the driver side fan turned on?
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Old 09-09-2014, 11:14 PM
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Default Re: Question about fans

I would have expected the other way around actually. The driver-side fan is only for the A/C and should only come on when the A/C is on. The passenger side fan is for radiator cooling. If the car was over-heating, there could be one of several problems:

-air in coolant
-bad fan temp switch
-bad fan relay
-bad fan (unlikely, but possible)
-clogged radiator
-stuck thermostat
-loose wiring causing the fan not to trigger correctly

Since you know the relay for the A/C fan works, swap it with the regular fan and see if that fan turns on. If it does, then you know the relay is bad; if it doesn't, then either the fan is bad or the wiring somewhere else is bad.
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Old 09-10-2014, 04:34 AM
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Default Re: Question about fans

1. Both cooling fans should run at the same time if the engine coolant temperature at Fan Switch A is about 199 degrees F or above (for Si, S, 4WS) or 203 degrees F or above (for VTEC or SR-V) with the engine running. Fan switch A has to be functioning properly for the fans to run.

2. In addition, if the A/C system is functioning properly, when you turn on the A/C; both radiator fan and A/C condenser fan should run at the same time.

There are separate relays for the respective cooling fans. A condenser fan relay for the condenser fan motor and a radiator fan relay for the radiator fan.

Jumping one relay will only trigger the one respective fan, not both. Although you see four prongs on one relay, they are not for both fans. The relay is a normally open type relay. When voltage/power and ground are applied to 2 of the prongs, it energized the relay and closes the internal relay contact. A different voltage/power source then goes through the closed relay contact and out to the fan motor for the other 2 prongs (think of it like a draw bridge, the power can only pass after the contact is closed).

Since you saw both fans running, both respective relays were working. Condenser fan relay is located near the front engine compartment. Radiator fan relay is located in under-hood fuse/relay box.

There may still be a problem with the Fan Switch A if both fans do not run when the coolant temperature is above 199 degrees.

For Si, S, 4WS or similar models, the Fan Switch A is located on the thermostat housing (thermostat housing is connected to lower radiator hose).
If you disconnect the Fan Switch A and jump the connector with a wire or small paper clip; with the ignition ON, you should see both fans running.

Yes, it is normal for the radiator fan motor to have 5 blades and the condenser fan motor to have 4 blades.
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Last edited by tech8; 09-10-2014 at 03:47 PM. Reason: typos
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Old 09-10-2014, 04:02 PM
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Default Re: Question about fans

The only time one fan should run is if the coolant temperature at Switch B is above 223 degrees F, when you turn the ignition switch off (0). The condenser fan will stop, but the radiator fan will remain on until coolant temperature Switch B opens (below 221 degrees F), or the timer unit goes off (about 15 minutes).

Coolant temperature Switch B is located on the water outlet, that is connected by the upper radiator hose.
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Old 09-11-2014, 10:20 PM
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Default Re: Question about fans

Sorry, didn't look at the OP's signature. On the 5th gen's, pretty sure the A/C condenser fan only runs when the A/C is on, but that's obviously different for you 4th gen guys.
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Old 09-12-2014, 04:46 AM
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Default Re: Question about fans

On 1997-2001 Preludes, both cooling fans (A/C condenser fan and radiator fan) should run with the A/C on; or if the coolant temperature at the radiator fan switch is above 203 degrees F.

1997-2001 Preludes have only one radiator fan switch, unlike the previous generation Preludes that have both fan switch A and B.
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