Electric fan won't turn on
 

I am finally at the end of my conversion of a 2004 LQ4/ 4L60E into my 1977 Jeep Cherokee S Chief. Right now I am stuck on the electric fan. I have hooked up 1 aftermarket electric fan to a relay as illustrated on lt1swap.com and ran relay to wire labeled Fan 1 on harness. I start and let the vehicle run for 20, 30 minutes and nothing happens, but the temps are pretty high judging from the feel of the radiator and hoses. I currently do not have any working gauges. The thermostat is working because no water flows when engine is cold and once warm water is flowing good. I went ahead and replaced temp sensor hoping this was the answer, but fan still doesn't work.

If I physically ground the relay,, then the fan kicks on. I have also tried wiring the relay to the wire labeled Fan 2 and fan still doesn't work. I have checked a few websites to confirm my relay is wired properly. I do have the aftermarket temp switch, but I would perfer to use the PCM.

BTW...my harness was done and PCM flashed by Wait4Me...if that matters. Any help is appreciated.

You really need a temp gauge, it could be that the engine hasn't reached the temprature required to kick the fans on. It's pretty high!

I will be getting some gauges soon, but I would think that the fan should kick on within 20-30 minutes of just sitting still.

You can read live info using most OBDII code readers if you don't have gauges yet...it will tell you what temp the computer is reading through the temp sender in the driver side head. Consider the Aeroforce gauge or a scanguageII. I kinda wish I hadn't bought my autometer temp gauge and just ran an OBDII gauge in its spot.

Those Aeroforce gauges look nice! They almost replace all other gauges. I'm thinking of the Aeroforce gauge and a fuel gauge, but hat will be a pretty empty dash panel.

If the PCM you are using was originally on a belt driven fan engine the fan relay outputs need to be turned "on" via a PCM tune.

Also you might check out Equus gauges. I picked some up for $8 each on amazon for oil pressure, water temp, and voltage.

I had the same issue on my LM7 conversion into an '86 C10 P/U and haven't been able to resolve it. I made sure that I recieved the -12 VDC signal when the engine gets hot enough from both Fan #1 & Fan #2 leads from the PCM, but the signal isn't strong enough to close the coil on the relay. As soon as I get the -12 VDC signal from the PCM and it attempts to close the relay, the voltage drops down to 1.5 VDC. I've tried to install a smaller relay and an additional micro relay with lower coil ratings but nothing has worked. I also double checked all of the ground leads and the issue hasn't been resolved.

The guy that programmed my PCM thinks the internal "drivers" inside may be damaged. I went ahead and ordered another PCM to see if it fixes the problem. I'll keep you posted on my findings. Let me know if you find a solution as well.

By -12VDC are you saying the ground signal or power signal? because the pcm gives a Ground signal for the fan relays and a power signal for the fuel pump signal.

This^^^^^^

get ahold of someone with HP Tuners, go into "System" and turn "Fan1" and "Fan2" on as well as adjust the temp you want them to come on at. usually on 215 in high side and 190 off on the low side.

It may be likely that someone wired a relay incorrectly sending an unloaded 12v to the fan trigger and smoking the traces inside the PCM... But a standard relay only requires 150mA to activate the coil. That much of a voltage drop on the power side of the coil would be more likely indicative of a bad power supply than the ground. Can you manually ground the coil and have it activate?

The Equus guages look nice also...I've been looking at tyhose also.

The PCm is supposedly tuned to include the efans. The wire harness also includes the labeled wires. The harness and tune was done by same place. I received feedback from the tuner saying that the driver might be bad or something, to send it back to them to check and fix. It is an inconvenience to mial it back to them and wait, but I don't have much of a choice.

I know that the relay is wired correctly. When I run a direct ground lead to the relay coil (in lieu of the signal from the PCM), the fan kicks on without a problem. This confirms that the +12VDC power supply to the relay is also good. FYI I have a 40A relay with a coil rating of 90mA.

When I measure the ground signal from the PCM across the battery, i get a full +13VDC. As soon as I connect it to the relay, the voltage drops to +1.5 VDC (It won't even turn on a bulb).

I am certain that the original PCM drivers are bad (maybe through my mistakes) but hopefully the new PCM will fix the issue. The last thing i want to do is install an external t-stat to control the fan operation.

Yes, the -12VDC is a ground signal from the PCM to turn on the fans, opposite of the +12VDC signal from the PCM to turn on the fuel pump.

Well that's strange. I sure hope my PCM works when I finally get it back. Ha ha

Also I apologize if I came off as though accusing you of not knowing what you were talking about. Lol. Automotive electronics are my job, so I deal with a lot of people who don't and its a bad habit of mine.

Go pick up a hand held thermal temp gun for $10 at the parts store. 180 degrees feels really hot by touch. Fan may be programmed at 190 or 200

Digging this topic back up since I still can't get mine to work....

I just installed the second PCM and am still having trouble engaging the relay coils for the electric fans.

Same problem as before:

1.) The PCM sends the ground signal when the engine reaches the temp setpoint. If you measure the voltage of the signal across any positive lead, you get a strong +13VDC.

2.) As soon as you connect the PCM ground signal to the relay coil ground terminal, the PCM signal drops down to 1.5VDC and the relay coil does not engage.

I've tried swapping out the relays, installing additional microrelays, etc etc. The fans engage without any issue when you send the relay coil a ground signal from anywhere else on the truck!!

Has anybody else encountered this problem?! The entire engine swap went smoothly except for this issue...

supergeess

I for some answers.

Ensure the ground terminals have a good connection on the backside of the cylinder head and where the pcm needs a ground. Though the PCM will send the signal to the fan relay, if it does not have a proper ground it it will close the circuit to nothing.

1) Ensure good grounds - Ohm check each ground wire at the head and pcm to chassis ground.
2) Make sure programming is set right in the PCM. Truck PCMs that had manual fans need to be turned on and the activate and deactivate temps set.
3) Condemn PCM.

Did this get resolved?
When you posted about proper engine temperature activating the fans, were you reading a mechanical gauge, or was the temp being read on HP (or AutoTap, or other program) from the engine coolant sensor?
Sorry if I missed this...I wasn't sure.