need help please

i have a 99 chevy venture and have been trying to get my radiator fans to work. so far i have replaced all of the relays and the temp switch. and no luck. i also checked the fuses and they are all in working order. tried to use the repair manual and no luck there either. any help would greatly be appreciated.

What temp are you letting it get to? Mine kick on at 234*..... Temp switch is near thermostat neck, it also feeds the dash guage.

it got up to 3/4 of the way to the high side and the fans never came on. i just put a new sensor in it and still nothing.

3/4 of the way to the high side may not be hot enough for the fans to come on. Most come on at 228 degrees +/- a few degrees. They should, however come on when the A/C is turned on and the high side pressure build up a bit. Do they run with the A/C on?

fans work. i just have to manualy turn them on. which is getting old as i have to remember to turn them off or wake up to a dead battery.

The temp has to go pretty high until the fans ficks in, it is somewhere about 3/4 on the temp gauge so it's certainly possible that your engine wasn't warm enough. I thought I had the same problem but today driving in the city I could see that they kicked in because the gauge went slowly up to 3/4, then slowly moved back to about 1/2 when the fans were active. I would have wanted to stop and visualy verify that they were operating but it was in middle of traffic and family in the car, but there is no other explanation. I.e. they worked as they should.

If the reading of your temperature gauge is correct that means that the sensor is working. The fans are controled by two signals from the PCM via three relays. You've verified that the fans are working and the relays are new so they should be working. I did like this when I tested the circuits from the relays to the fans:

http://www.abrahamsson.com/IMAG0164.jpg

I.e. carefully remove the cover of the relay and manually press the control arm. It has to be done very carefully to not break the cover and also be very carefull when removing them again to put the cover back as it's easy to break them.

I would suggest the following steps to verify that the fans and the signal that controls them are working:

* Use a ODB-II adapter to verify the actual temperature reading. As Lesterl and procaddytech the fans aren't activated until about 230 degrees Farenheight. Most likely the "problem" is that you don't reach sufficient temperature. If you A/C is working then turn it on as that will activate the fans earlier. I use a bluetooth ODB-II adapter so that I can connect it to my Android phone.

* Measure the signal on the PCM. I never got this far but I believe the control signals are open collector, they are definitly pull down at least (i.e. when they are active they pull the signal to the ground to make a circuit and activate the relay). If the circuit is working then you would be able to measure 12V on these outputs, if it is broken you should have some other voltage (should be 0 but can be some other voltage due to leak currents). The colors of the wires are dark green for the low fan speed control and dark blue for the high speed control. If dk green is low then fans are at half speed, if both are low then the fans are at full speed.

You will NOT read 12v on the PCM side of the relay as the PCM pulls the relay to ground and the other side of the relay is fused at 12v.

ok i will check that tomorrow morning. the other problem i have is that the fans won't come on when the a/c is turned on as well. i just hope it's not the pcm. and thank you all for your replies. i have been beating my head trying to figure it out.

You will NOT read 12v on the PCM side of the relay as the PCM pulls the relay to ground and the other side of the relay is fused at 12v.

When the PCM is not pulling the signal low and if circuit is completed then you'll read 12v on the PCM side. My point is that if circuit is broken then unless PCM has a pull up, which I don't think it would have, then you should not read 12v there. If PCM is pulling the signal low then it reads 0v there for sure.

When the PCM is not pulling the signal low and if circuit is completed then you'll read 12v on the PCM side. My point is that if circuit is broken then unless PCM has a pull up, which I don't think it would have, then you should not read 12v there. If PCM is pulling the signal low then it reads 0v there for sure.

i agree, i just went down this path myself/./ i chased a bad pcm wire...dr blue underneath the fuse box

If you disconnect the engine temperature sensor, the fans should default to on all the time. The computer cannot tell what temperature the engine is, so it provides constant cooling just to be safe.

tried that and they still won't come on. so i guess i have to start traceing wires. how fun. not. thank you guys for all of your input i will let you know what i found out.

There are 2 fan control circuits at the PCM that ground the relay coils to provide different fan speeds. The cooling fan fuses are hot at all times so even with the key off you should read 12 volts on both fan control circuits, with a DMM, until the fans are commanded on and the relay coils drop the 12 volts. If you can carefully backprobe PCM connector 1 (blue plastic), terminals 5 (dark blue wire) and then 6 (dark green wire) with your + meter lead and - lead to ground you should read 12 volts. If you jump them to ground the fans should come on, low speed term. 6 & high speed term. 5. If the voltages measure correct and the fans run when you ground the control circuits, the PCM is bad or your engine temp is not getting hot enough and your A/C is not working properly. Use a small hat pin type of probe to back probe with and clip your lead to it. I don't recommend poking through the insulation of the wire but if you do put some liquid tape or rtv over the hole to seal it.
Link to schematic: http://i36.photobucket.com/albums/e50/twabelljr/fans.gif

Excellent writeup ProCaddy..... :-)

hope this helps too....i would check for a bad ground blk wire

try this again

try this again

i will bet there is a broken wire leaving your pcm on the passenger side under the coolant bottle

I was driving today with the ODB-II adapter connected so that I could see the data in my mobile. I could see that although the temperature gauge was pointed at 100 degrees (celsius), which is where it normally is when driving, the PCM value was 90 degrees. It seems that with the new temperature sender (which I put in as part of the job of changing the LIM gasket) the gauge is indicating a higher temperature. I had noticed before it was indicating a higher temperature when the thermostat opens than earlier. Probably the PCM reading is the correct temperature as it corresponds to when the thermostat should be opening. Either way it's the PCM's temperture reading that controls when the fans are starting.

The guage on the dash is just an idiot light with a needle...... not calibrated.......

well ihaven't had time to trouble shoot it more. but at the moment i am able to get the fans to come on by manualy overriding the realy.