I have a 2000 model pete 379 the engine fan runs all the time manual fan switch checks good I changed water temp sensor and had it hooked to a computer all day I can turn it off with the computer but its leading me to a dead end also tried two more ecms to all do the same thing
Hi. Thanks for using JA! I will try and help you out.
When you say you had the computer hooked to the engine, I assume you were using Cat ET correct?
If so, what was the status of the fan when it was on? Did the screen say output on or off?
Also, when you say you can turnit off with the computer, are you activating the fan special test?
yes it was a cat scanner and its showing it to be on and I can turn it off by doing thefan test
If the status of the fan is on, the ECM is detecting something that makes it want to turn the fan on.
The ECM turn the fan off by sending out 12 volts on pin 11. When the ECM wants to turn the fan on, it removes the 12 volts from pin 11.
Peterbilt's fan strategy takes 12 volts to the solenoid to turn the fan on. So Pete installs a normally closed relay to the fan control valve. This way when the ECM send out power, it opens the relay. The relay is normally located on the fire wall of the truck.
There are a couple of things I would check. First would be the AC pressure switch. If it is faulty, it can cause the fan to stay on. You should also check the wiring as well. Your scanner should be able to show the status of the AC switch.
Next I would check the fan relay I was talking about. I have replaced many of these for this problem.
Please let me know if you need more help or have more questions.
I relaced fan override switch already iand im unable to find a relay not in fuse panel ,firewall nor inside dash anywhere
What about the AC pressure switch? Are you able to read the staus of the switch with your scan tool?
not able to read the ac pressures wit my scan
I see. Thanks for the information.
It is very possible that the switch could be the problem. Reading the status is the best way to confirm. You could try to unplug the switch and use a jumper wire as well to see if the fan will cut off. If it does, you will know the switch is faulty.
Also, here is a list of items that will cause the ECM to turn the fan on. If the scan tool says fan on, one of these is happening.
The ECM will turn on the cooling fan, if any of the following conditions are met:
Please let me know if you need more help.
the engine was at idle speed,manual fan switch is in the manual position the water temp was 185 and intake air was 87 degrees tried a paper clip in the ac fan override switch plug and it didnt unlock it
Thanks for the information.
If you used a paper clip to jump the AC switch, it is most likely not the problem. However, there could be a problem in the wiring to the AC switch. I have seen this before. I hate that you are not able to read the status of the AC switch, if you could, you would know quickly if it was keeping the fan on.
Also, as far a finding the fan relay, you should be able to follow wire/pin 11 from the ECM truck connector and it lead you to the relay. The only reason I mention it again is because I have replaced many of these over the years for this problem.
Is there anyway you can get someone with Cat ET to read the ECM? I hate to say it but it may be the best way to find the problem. Sorry, I wish there was an easier way.
may have to take it to cat to get it checked out there but if I swap the air on the fan silinoid it will unlock the fan and it cut in and out right by the ac override switch but then the fan will not come on with the engine temp at 220 degrees nor manual switch in side truck
I see. Thanks for the information. I think having the ECM read with Cat ET will help find the problem the quickest. Without seeing all of the stays parameters, there could be something in there that i obvious and then you will know what the problem is. Without doing that, it is kinda a guessing game. Sorry. I wish there was an easier way.
can I check all the other swiches such as water temp and air intake with a paper clip to and whete does pin 11 usualy run to from point a to b
You will not be able to check the water temp and intake air temp with a clip. They are an active analog sensor circuit. As soon as you unplug or short them out, the ECM will turn the fan on. Pin 11 will run directly to the air control solenoid or fan relay (if the truck has one). Sometimes it will run throughout he manual override switch. Peterbilt can wire these things up several different ways.
ok ive already unpluged both of those ,but I need to check pin 11 for voltage and or a broke wire between ecm and manual fan switch and air silinoid
Ok, sounds good. Let me now how you make out.
ok thanks very much
You are very welcome.
going back on what I told I can stick a paperclip in ac fan override and then fan goes off
I see. Thanks for the information. It looks like that is the problem. Great job. Now you do not need to take the truck to get Cat ET hooked up.
Hey. I just wanted to see how things were going. Please let me know if you need more help or have more questions. I noticed that you have not hit the accept button. Just wanted to make sure you did not need anything else. Thanks for using JA!