mikekaz1 Posted November 17, 2015 Share Posted November 17, 2015 1991 4.0 mj with no factory AC option. so I'm having an issue with my electric fan and overheating. its not turning on.... even when temps hit 260! i checked all connections and relays and y=they are good. if i unplug the temp sensor on the thermostat housing the fan kicks on, so i know the computer is seeing the input. and i ohmed out the sensor and it tested right where it should and even replaced it with a used one and a brand new one. i can't get the fan to kick on! any input would be appreciated Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 17, 2015 Share Posted November 17, 2015 I went through the same drill with my 91 aux fan. It comes on with the A/C just fine, but doesn't come on until ~250*-260* engine temperature - too high for me. Went through the same troubleshooting as you with a new temp sensor and wiring check and nothing was wrong. Since I had ditched my mechanical fan for an electric, I wanted a fail-safe way to turn on the secondary aux fan in case my primary electric cooling fan went belly-up. When the ECU senses a high temperature from the coolant sensor, it applies a ground to pin 31 which latches the aux fan relay and turns it on. I simulated this by adding a dash switch that did the same by splicing the switch ground output wire to the BLU/PNK wire on ECU pin 31. The new wiring is shown by the dashed lines in the wiring diagram below. Now if I'm stuck in traffic and the engine temp is going up higher than I want, I flick the switch, the aux fan comes on, temp drops, then turn it off when cooled down. Works great. Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 17, 2015 Author Share Posted November 17, 2015 I'm glad someone else has seen this identical issue. so your making an artificial ground with the dash switch correct? rather then an applied power. any reasoning on why the sensor closes so high? I really appreciate that diagram Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 17, 2015 Author Share Posted November 17, 2015 I'm also in the process of replacing the main fan with a stand alone electric fan. as the fan clutch was bad and my shroud was broken also. and my radiator blew last week from overheating so I had everything apart and taken out. Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 17, 2015 Share Posted November 17, 2015 I'm glad someone else has seen this identical issue. so your making an artificial ground with the dash switch correct? rather then an applied power. any reasoning on why the sensor closes so high? I really appreciate that diagram I don't think the sensor is causing the late turn-on since I tried several with no change. I think it's probably the ECU. Maybe built-up internal resistance over time. Also unplug and try cleaning up the 60-pin ECU connector, and look for any recessed pins. That seemed to help a little......... Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 17, 2015 Author Share Posted November 17, 2015 will do! Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 17, 2015 Author Share Posted November 17, 2015 I checked my 60pin connector and it was spotless. Perfect condition. Even had white grease still on some pin tips. Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 17, 2015 Author Share Posted November 17, 2015 So in further research. In reference to your diagram. Pin "2" has 12v and if jumped to "4" the fan runs (good). BUT Neither coil side "1" or "5" have 12v. From your diagram it looks like "1" should have constant 12v and "5" should be a ground through the ECU. Correct? And with an artificial ground wire placed in "5" and the relay replaced then the fan should run. But it doesn't. BUT if that artificial ground is then placed to 12v the fan runs. I checked the voltage at the temp sensor and it shows ~5v. Is this correct? And further more. If I jumped pins "2" to "5/1" (not sure which but it was the left coil receptacle looking down at the relay center.) I herd a clicking noise from this box/relay... but don't exactly know what this is... Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 18, 2015 Share Posted November 18, 2015 Pin 1 is the hot side contact of the aux fan relay relay coil. It's the same point electrically as the ballast resistor 12V input, so the engine must be running to see 12V on pin 1. Pin 5, tied directly to ECU pin 31, is the ground side contact of the relay coil. In normal operation, the ground signal is applied there from the ECU when the engine temp gets high enough to trigger the ECU ground output. You can tie your ground wire from the dash switch to either pin 5 of the relay socket, or pin 31 on the ECU as they are the same point electrically. I spliced the dash switch ground wire to the BLU/PNK wire in the ECU harness since I was installing a Unichip at the time and had the harness unwrapped. In your case it might be easier to tie it to the relay socket pin 5 in the PDC. Relay pins 2 (12V) and 4 (Ground) are the relay load contacts. Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 18, 2015 Author Share Posted November 18, 2015 OK. All my voltage tests were while the truck was runnning. So I'm missing that 12v ballast resistor input "pin1". I'm guessing your talking about the fuel pump ballast resistor, correct? Which is obviously energized because the truck runs. Not sure how I'm missing this input 12v... I'm going to inevitably run a over ride switch. But would really like it to operate semiproperly too. Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 18, 2015 Share Posted November 18, 2015 Roll on............ Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 18, 2015 Author Share Posted November 18, 2015 do you know what that relay/connector attached to the overflow bottle is? I herd clicking from that if I jumped power to certain pins? sorry, not sure what you mean by roll on? Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 18, 2015 Share Posted November 18, 2015 Roll on, carry on. :yes: That's the fuel pump ballast resistor bypass relay. Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 19, 2015 Author Share Posted November 19, 2015 They have a bypass relay for the ballast relay? What's the point? Haha. Well I tried half of the issue. I got my new electric fan installed to replace the clutch fan and the new standalone temp switch for it wired too. And then I just spliced the output of the from switch to also control the stock fan relay coil. So both click on when the temp switch closed now. Did a test drive and climbing a big hill it still creeped up to the 3/4 mark. But never went full bore. This is also with my plow on too. Blocking a lot of airflow Link to comment Share on other sites More sharing options...
mikekaz1 Posted November 21, 2015 Author Share Posted November 21, 2015 Wtf. Lost reverse lights now..... time to track that down. Link to comment Share on other sites More sharing options...
mikekaz1 Posted April 19, 2016 Author Share Posted April 19, 2016 http://comancheclub.com/topic/48847-reverse-light-cooling-fan-electrical-issue/ continuance here Link to comment Share on other sites More sharing options...
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