Make Me Smarter on the 3.8 2-Speed Radiator Fan
09-02-2017, 10:56 AM
#1
JK Jedi Master
Thread Starter
Make Me Smarter on the 3.8 2-Speed Radiator Fan
All (and especially Ronjenx) ...
If in a hurry, skip to the last line--it has the germane question.
I've been puzzling out a cooling system problem. To cut to the quick, my fan doesn't seem to work when it's needed. It's a long story that starts when getting off the ferry across Lake Powell during the return trip from my Solar Eclipse Expedition. I don't want to belabor that. But, during research (yes, I used the site's search function, and after all these years it's still not very helpful, LOL) I came across a comment that the early model JKs (with 3.8s) use some magic to create a two-speed fan. I'm beginning to suspect that my fan works fine in low speed mode, which is what the engine almost always needs. But on very rare occasions when high speed is called for, the fan doesn't run. Evidence of this suspicion is during my Beehive B-Sides trip late last year, for about an hour I was having a lot of overheating issues (I monitor the coolant temp with a Nexus 7 over the CAN bus, so have a pretty good reading). Once I cooled it down (running the compartment heater), things returned to normal, and this was the only issue in ten days of this overlanding trip.
Today, while burping the Jeep after replacing the water pump (had a leak so small it didn't even necessitate adding coolant, but changed anyway), the Jeep seemed to be okay. Then, about an hour in it overheated. At 240° the coolant boiled over and the fan would not run. I measured for 12 volts right next to the fan and there was nothing. Before, at lower temps (I wasn't monitoring it on the Nexus 7 ATT), the fan cycled on and off numerous times just fine. But, once it reached the higher temp, it would not run. I suspect that the coolant burped itself into the heater core, suddenly overheating the engine, but the fan couldn't run in high mode since it had no input voltage.
Bottom line: Anyone care to enlighten me on how the 2007 3.8L JK motor cooling fan does that two-speed trickery, and what part(s) might I check that could cause it not to run in high speed mode only?
If in a hurry, skip to the last line--it has the germane question.
I've been puzzling out a cooling system problem. To cut to the quick, my fan doesn't seem to work when it's needed. It's a long story that starts when getting off the ferry across Lake Powell during the return trip from my Solar Eclipse Expedition. I don't want to belabor that. But, during research (yes, I used the site's search function, and after all these years it's still not very helpful, LOL) I came across a comment that the early model JKs (with 3.8s) use some magic to create a two-speed fan. I'm beginning to suspect that my fan works fine in low speed mode, which is what the engine almost always needs. But on very rare occasions when high speed is called for, the fan doesn't run. Evidence of this suspicion is during my Beehive B-Sides trip late last year, for about an hour I was having a lot of overheating issues (I monitor the coolant temp with a Nexus 7 over the CAN bus, so have a pretty good reading). Once I cooled it down (running the compartment heater), things returned to normal, and this was the only issue in ten days of this overlanding trip.
Today, while burping the Jeep after replacing the water pump (had a leak so small it didn't even necessitate adding coolant, but changed anyway), the Jeep seemed to be okay. Then, about an hour in it overheated. At 240° the coolant boiled over and the fan would not run. I measured for 12 volts right next to the fan and there was nothing. Before, at lower temps (I wasn't monitoring it on the Nexus 7 ATT), the fan cycled on and off numerous times just fine. But, once it reached the higher temp, it would not run. I suspect that the coolant burped itself into the heater core, suddenly overheating the engine, but the fan couldn't run in high mode since it had no input voltage.
Bottom line: Anyone care to enlighten me on how the 2007 3.8L JK motor cooling fan does that two-speed trickery, and what part(s) might I check that could cause it not to run in high speed mode only?
09-02-2017, 11:28 AM
#2
Super Moderator
The 3.8L fan is really a single-speed fan that is made into a 2-speed fan by the use of a voltage divider. High speed is really the fan running WITHOUT the voltage divider in the circuit. So if you have low speed but no high speed, then the fan motor is okay. When in high speed mode, the circuit uses a relay that sits directly behind your driver headlamp. This is your most likely culprit and the first item I would replace as it's pretty cheap. It's a bugger to remove from the mount it's on though.. takes a bit of patience.
09-02-2017, 11:59 AM
#3
JK Jedi Master
Thread Starter
The 3.8L fan is really a single-speed fan that is made into a 2-speed fan by the use of a voltage divider. High speed is really the fan running WITHOUT the voltage divider in the circuit. So if you have low speed but no high speed, then the fan motor is okay. When in high speed mode, the circuit uses a relay that sits directly behind your driver headlamp. This is your most likely culprit and the first item I would replace as it's pretty cheap. It's a bugger to remove from the mount it's on though.. takes a bit of patience.