radiator fan not coming on

[jluhrsen]

Hi,

I have a 94 Honda Civic DX that is overheating. after troubleshooting a bit, I think the root cause is that my radiator fan is not coming on when the temp is high enough.

running the A/C will turn the radiator fan ON, so I know the fan and
fan motor are ok.

I replaced the switch just today, but this still didn't solve the problem.

I suppose I can hardwire it so that the fan will always stay on, but I'd
like to get to the bottom of this.

any ideas to track this problem down? how can I verify that the new
switch is attempting to turn the fan on when the engine temp is high enough? there are some fuses and relays to worry about, but all seem
to be working as I've swapped them out for new or known working ones. maybe the wiring is corrupted, but at least some of it has to be ok, since the A/C will bring the fan on.

hopefully someone can help me.

Thanks,
James

PS. I'm assuming the water pump is ok, since I saw no leaks.
I'm assuming the thermostat is ok, since the upper rad hose is hot, while the lower hose is cool when I'm running the engine. Either way,
I know the rad fan needs to come on when the engine is hot, and it's
not. that's a fact.

[hondaracer]

ummm.....well there has to be some kind of electrical problem there. First of all the car shouldn't get that hot anyway for the fan to have to switch on. Mine mostly only comes on when I stop driving after 45 minutes. My car stays very cool. Well the only thing I could have thought of is the thermostat, but you said its ok. If your car is overheating than check the coolant levels too, but sounds like you got a weird problem.

[i_a_n112784]

I always say this, but check the relay. Its easy to replace, cheap, and they've gone bad in cars that I have had.

[Ricochet]

Not to threadjack, but mine overheated the other day too and the fan won't kick on either. I was completely drained of coolant, but couldn't find a leak anywhere. I filled it back up and I'm fine, but does the stuff evaporate or something? Maybe I put too much water into the mix...?

[jluhrsen]

well, I think I'm all fixed up now. the problem was misinformation. at least, maybe I turned it into misinformation after I read it.

I was under the impression that if my lower radiator hose was cool,
while the upper hose was hot, then my thermostat must be passing
fluid properly. I was also under the impression that if the engine heat,
as represented on the dash was HOT, then my radiator fan switch should kick in, which it did not.

So, with those assumptions and observations, I went the route of
replacing the switch, and leaving the thermostat be.

the problem still existed, and not until I replaced the thermostat did I get back to normal.

as I understand it now, the actual switch sits just before the thermostat, which is all just after the fluid is leaving the radiator cooled.
So, if the thermostat is frozen shut, the switch is getting its readings from standing coolant that is not being cycled through the engine. however the engine (using it's one uncycled coolant) gets hotter and hotter. The switch still reads a cool temp, and doesn't turn on my fan.

So the test of "feeling" the hoses for cool vs hot to determine the thermostat functionality doesn't seem like a good one. And it's only $10 and 15 minutes to replace it, so I'd suggest always doing that first.

anyway, just my experience in case anyone can benefit.

James

[Ricochet]

So the thermostat only costs $10? I figured being a Honda part it would be at least $100 or something... thanks for the info, I may just replace it because it's a swapped motor and the more new parts the marrier.

[jluhrsen]

actually the OEM thermostat I bought was less than $10.00 you can go for different temp thermostats if you want it to open at a cooler or warmer temp, but I just bought the stock.

BUT, the thermo switch I also replaced was a honda part and was $45.00 Turned out not to be the problem too, so that's a little disheartening.

aparantly from what I've read online, both the switches and thermostats commonly go out on these civics after 100K miles or so. no big deal to replace though.

james