Radiator Fan Override Switch (The Easy Way)

(apologies to fyrfytr for cluttering up the thread)
Okay, poking around with the meter, doing some googling. The light blue wire isn't ground, it's power. Ground is black, and unswitched. The relay in the PDC, when energized, puts 12V on the blue wire, fan runs. Taking an educated guess here, but is this how you actually wired it up, Charles? (and sure enough, the PCM doesn't monitor the fan, just the control ckt)

FanPCMBypassed.jpg


Switch off, stock power wire connected to 87a (NC) contact, wire from fan connected to 30, fan runs only when PDC energizes.
Switch on, battery power on 87, fan on 30, stock power wire held open, fan runs constantly.

Have I got it right?
 
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That should work. The blue +12V line should be from an ignition-switched source unless you want to be able to leave your fan on when the jeep is not running.
 
Noted and fixed (I think). You could also use a relay on the battery lead, controlled by a keyed source as well, but it would be a little more work. I like fyrfytr's approach in that it lets the PCM still run things, but I don't see any issues with this since the PCM doesn't care what is or is not connected to the fan wiring.
 
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hubs97xj said:
(apologies to fyrfytr for cluttering up the thread)
Okay, poking around with the meter, doing some googling. The light blue wire isn't ground, it's power. Ground is black, and unswitched. The relay in the PDC, when energized, puts 12V on the blue wire, fan runs. Taking an educated guess here, but is this how you actually wired it up, Charles? (and sure enough, the PCM doesn't monitor the fan, just the control ckt)

FanPCMBypassed.jpg


Switch off, stock power wire connected to 87a (NC) contact, wire from fan connected to 30, fan runs only when PDC energizes.
Switch on, battery power on 87, fan on 30, stock power wire held open, fan runs constantly.

Have I got it right?
Click to expand...
On my '98 I cut the ground wire going to the fan motor an attached the incoming to term 30 and other end to term 87a and wired in an additional ground wire to term 87. 87a is N/C and 87 is N/O. 87a is used for normal use by the AC and 87 is used for manually running the fan for extra cooling of the engine. This setup has worked fine for over 7 years.
 
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CharlesS said:
On my '98 I cut the ground wire going to the fan motor an attached the incoming to term 30 and other end to term 87a and wired in an additional ground wire to term 87. 87a is N/C and 87 is N/O. 87a is used for normal use by the AC and 87 is used for manually running the fan for extra cooling of the engine. This setup has worked fine for over 7 years.
Click to expand...
My apologies for telling you you were wrong earlier then, for some reason I thought you were saying you had wired something in between 85/86 and the ECU and it worked great.

CharlesS said:
Remember the ground to the fan motor is switched by the ECU not the positive.
Click to expand...
At least on a 2.5L wiring harness (what my 4.0 seems to match :huh:) the positive is switched, just did my wiring a few nights ago and the blue wire was definitely routed through the relay to +12V.
 
I checked with a meter, and looked at an FSM. You can see it in the pic fyr posted as well as being covered in a few other places in the FSM. At the fan motor connection, black is ground, and light blue is power (it is switched by the relay, which is energized when the PCM grounds the dark blue/pink wire between the PDC and PCM). Power is switched, ground is not.

Not trying to be argumentative, just trying to keep things clear for those who are also going to attempt this. If someone wants to check against a later FSM (my info taken from a 97, but I'm guessing it's identical for 97-99, maybe 00 and 01 as well), feel free.

8W-15-4 ground
8W-42-7 control ckt
8W-80-75 pinout
 
This worked on my 1998, but I only have 1 DB/WT wire and it wasn't the right one. Checking several of the DB wires I found a DB/OR (OR=Orange?) that had 12 volts on it only when the engine was running. I used that one to power the relay.

Oh and the "Check Engine" light cleared from a prior attempt. :yelclap:
 
Geez, after months of inactivity on this thread it has suddenly come to life. Of course it happened to be during the same time that I was taking my longest break from NAXJA since posting it... Glad to see folks are referencing it. I have some ideas about some of the questions posed here and apologize for not having replied earlier. No time right now (it's about midnight) but I'll try to reply tomorrow.
 
fyrfytr1717 said:
Relay Pin 85 - DB/PK wire coming from your PCM
Relay Pin 86 - DB/WT (ST-RUN) wire
Relay Pin 30 - Ground wire
Relay Pin 87 - DB/PK wire going to the Fan Motor Relay (in the PDC) and the ground signal wire coming from your override switch
Relay Pin 87a - May or may not exist on your relay, but is unused regardless
Click to expand...

ok..... im in trouble again..... now its all messed up! when the switch is on, the fan operates normally with the switch off it is on all the time. and theres a check engine light! just wondering maybe i did the switch wrong? cause it says "the ground signal wire coming from your override switch" is this the middle one? wtf..... im :dunce: in wiring. sry for bringing this up again!
 
poorboy87 said:
Where did you get that series of relay pigtails?
Great write up. Looks to me like it should work on my 91. Ill post up if it does.
Click to expand...
AFAIK you can do whatever you want on a 91 and the ECU will not notice or care. They only got crazy about fault detection around 96 to 97 (depending on the subsystem involved) when OBD-II was introduced.

This mod, however, should work perfectly. It's just likely overkill on a 91.
 
truckntractorhick said:
ok..... im in trouble again..... now its all messed up! when the switch is on, the fan operates normally with the switch off it is on all the time. and theres a check engine light! just wondering maybe i did the switch wrong? cause it says "the ground signal wire coming from your override switch" is this the middle one? wtf..... im :dunce: in wiring. sry for bringing this up again!
Click to expand...

I'm guessing you just have to reverse two of your switch wires. Normally a 3 pin ON/OFF switch with a light has a 12V+ in, 12V+ out (shared with the indicator light), and a 12V- ground (also for the light). Since you are sending a ground signal through, you need to flip flop everything. You should have 12V- in, 12V- out, and what would normally be your ground on that switch should actually get connected to a 12V+ source to make the light come on. Incidentally, this means that if the indicator on your switch is an LED, it won't work as the current will be flowing the wrong direction through the light.
 
I like this idea. Thanks for all the work in laying it out so someone without too much knowledge of relays and such can install it.

I would like to know if it's possible though to add another relay with it's own "switch" to power on the fan. My fan is not coming on when the coolant temp reaches 217 F like my FSM says (I've had my scanner attached and reading live data and reached 226 F and no fan.) When I pull the CTS connector, the fan kicks on and throws a CEL. To me, this seems like the PCM isn't turning the fan on when it's supposed to, identifying the PCM as the problem. Could it be the sensor itself? That doesn't make sense to me. The fan also comes on when the A/C is on.

What I'd be thinking of doing is using a radiator probe relay like the ones from Hayden or others that turn a fan on at an adjustable temp or pre-set one and off again.

Thanks - need to get this figured out to stop the temps climbing on the trail.

Rob
 
97stomper said:
I like this idea. Thanks for all the work in laying it out so someone without too much knowledge of relays and such can install it.

I would like to know if it's possible though to add another relay with it's own "switch" to power on the fan. My fan is not coming on when the coolant temp reaches 217 F like my FSM says (I've had my scanner attached and reading live data and reached 226 F and no fan.) When I pull the CTS connector, the fan kicks on and throws a CEL. To me, this seems like the PCM isn't turning the fan on when it's supposed to, identifying the PCM as the problem. Could it be the sensor itself? That doesn't make sense to me. The fan also comes on when the A/C is on.

What I'd be thinking of doing is using a radiator probe relay like the ones from Hayden or others that turn a fan on at an adjustable temp or pre-set one and off again.

Thanks - need to get this figured out to stop the temps climbing on the trail.

Rob
Click to expand...
By pulling the sensor cable and verifying that the fan comes on, you've verified that the ECU can turn the fan on when it wants to, and that it is receiving some sort of a signal from the temp sensor. The only possible explanations are (a) the temp sensor is defective/worn out/not the right temp gradient or (b) the wiring between the temp sensor and the ECU is marginal. I'd lean towards (a) since I've seen a lot of variation in when the fan actually kicks on from jeep to jeep.

As for how to put in a manual override - sure, add another diode wired up just like the other two in my diagram. Put a switch from its striped end to ground. When you want the fan on, turn the switch on... if you don't want the timer setup as well, you can simply remove the timer and install the switch instead.
 
kastein said:
As for how to put in a manual override - sure, add another diode wired up just like the other two in my diagram. Put a switch from its striped end to ground. When you want the fan on, turn the switch on... if you don't want the timer setup as well, you can simply remove the timer and install the switch instead.
Click to expand...

kastein, I think you're thinking this is the delay timer thread...

97stomper said:
I would like to know if it's possible though to add another relay with it's own "switch" to power on the fan. What I'd be thinking of doing is using a radiator probe relay like the ones from Hayden or others that turn a fan on at an adjustable temp or pre-set one and off again.
Click to expand...

This whole thread is about how to wire up an override switch. Just set up your "probe relay" in place of the override switch so that it triggers the fan circuit relay. If you want to do both, I don't see why you couldn't wire it up so that either your probe relay or the override switch would activate the fan circuit relay.

The best thing to do though would be to figure out why your fan isn't activating like it's supposed to and fix the problem rather than working around it. Then wire up an override switch if you still want/need it.
 
Thanks - I'm going to try a new CTS first...I would rather have it work the way it's supposed to.

On the sensor, RockAuto has like 5-7 choices...ranging from under 20 to over 30...I know I prob shouldn't pick the cheapest and prob don't need the most expensive either...Good brands?

Thanks!
 
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