This question pertains mainly to OBDII cars, as I have done this mod on my OBDI car without resistors and have had no problem. I am just wondering if they are really necessary if the power comes from a switched power source, or any power source that only functions when the car is on.

Also, if the resistors are necessary, how can I tell which way is positive and negative, and which way do they face in the fan switch install?

As always, thanks for any help

 

A resistor isnt directional like a diode.

Yes the resistors are neccesary to prevent a SES light.

 

So the resistor can face either way, correct? Thanks

 

That is correct.

 

When I was OBD2, the only way I could get a manual fan switch to work w/o a resistor was with the low fan speed (reley #2?) and it would have to switch off when the ignition was turned on.

 

If you dont use the resistors in OBD1 cars do you get the SES light?? Mine dont come on but I hear some do. Just wondering. Thanks.

 

Mine works with no problems. All parts from Walmart.

 

No SES light in my OBDI either.

 

do u guys have a website i can get instructions on how to do this mod to an obd2? thanks

 

Do a search for Shoebox. He has a Tech link in is sig. There is a schematic there for both OBD1 and 2. You NEED the resistors for the OBD2 or you will throw a code. You dont for the OBD1. I have done both.

Craig

EDIT:
Here is the link..

 

I hooked my switch up without resisters. I think you only need the resisters if you want LED's (which I didn't). I can run the fans on high and low with the key on or off. A code is stored in the computer but the SES light does not come on.

Hope that helps.

Mark

 

Does your SES light work? it should turn the light on, did in mine. There are optional resistors but they are for the LED's themselves, some need it some dont depending on what you buy. The 47k ohm resistors in the lines that tap into the fan wires are needed though for an OBD2 car.

 

Yes the light works. It comes on when I turn the key on before I start the car. Doesn't come on when I run the fans. All the switch is doing is grounding the appropriate relays as per shoebox's instructions (no LED's or anything else).

Mark

 

Old post but I figured I would give it a shot.

My fans would only come on when the AC was on so a shop wired the fans so they would come on when the key is in the run position. Problem is that the SES is set with codes p1641 and p1642 from it. I tried using the 1/2 watt 47 ohm resistors like the diagram but the light still comes on as soon as you start the car? Is it because the way its wired or does anyone have any other ideas?


All replies appreciated!

Thanks,
Rich

 

You would need to know what they did to wire the fans on, to be able to figure out a remedy. IMO, it was a poor solution by your shop to wire the fans always on.