E-fan help

96 4.0 sport auto. My e fan works when I turn on the AC but will never turn on as the temp climbs. I've had it go halfway between the red marks and the line before the red marks.

What should I look at first in terms of relays of fuses?

Thanks.

The temperature sender in the thermostat housing may be bad, it sends the signal the the computer to turn the fan on.

The computer controls that fan based on inputs. If it is able to engage the fan from the A/C signal wire, then the control part is working and you need to look at the inputs. Specifically the computer gets its temperature input from the sensor in the thermostat housing at the front of the engine, and if the temperature signal to the computer isn't changing then the sensor needs to be replaced.

However another possibility is is that your gauge is displaying wrong info while the sensor is working fine. The quickest way to do this is get an infrared temperature gun and point it at the thermostat housing while comparing the data to the gauge. A better way to do it is to have a shop read the temperature from the computer at the same time, so that the raw sensor data can also be compared to the actual temperature and gauge reading.

Up until mid-96 the gauge used a separate sender at the top rear of the head (look for a single thin wire on the drivers side of the engine at the very back), while post-96 reused the temperature reading from the sensor in the thermostat housing. The gauge uses resistance from the input data to restrict voltage going into a coil inside the gauge, which causes the gauge to move up or down as resistance changes. You can get a bad gauge, a bad input, a little bit of voltage bleed on a wire, etc., and the gauge will hop

But if the sending unit was bad I would not get an increase in temp on the gauge. From what I've read on here the temp sensor sends to computer that turns on the e fan.

I think the gauge is ok because it is always at 210 till I hit heavy traffic.

First question is do you have separate sender for the gauge (at the back of the head) and sensor for the computer (inside the thermostat housing up front). Second question is if the gauge is accurate. At the minimum you need an infrared gun to determine the second question.

If you have separate units and the gauge is accurate, replace the computer sensor. If you have separate units and the gauge is inaccurate, replace the gauge sender.

If you have one unit, then the computer and gauge share the same source data, so things get more complicated and you need to use a scanner to read the temperature from the computer. You could have a faulty gauge (it happens) or wiring that makes the gauge show different values than what the computer is reading from the sensor (and you may have incorrect readings from the sensor as well).

Ok this is help full. I know I've seen the sender at the back by the firewall. I'm going to have to look for the other one on the rad. Thank you

The other one isn't on the radiator, it is on the thermostat housing on the engine where the top radiator hose goes into the engine. This is the one that sends info the the computer and tells it to turn the e-fan on.

quick test(but be forewarned, this will turn on the check engine light-you'll obviously have to clear it, or wait for it to go away.):
with the the engine warmed up all the way and running, unplug the coolant temp sensor(the one in the t-stat housing). the fan should come on within 5 seconds or so. if it does, it's "probably" the coolant temp sensor. if it does not, you have an issue somewhere with the pcm refusing, or not being able, to activate the fan.
this is something i learned from a tech at the jeep dealership i worked at. he used it as a quick test of the fan motor, relay, wiring, etc.
also, for reference, my '96 would turn the fan on when the needle just passed the 210 mark.

Ok that's what I meant, I looked it up and I have two temp sensors listed. I'll be really happy if this fixes it. I bought this baby recently and don't feel comfortable until I fell the cooling system is 100%.

Hellbent, why would the e fan turn on when I unplug this?

not sure, actually. some sort of safety feature in the pcm maybe? or possibly the infinite resistance without the sensor there gets read as full hot(iirc, the resistance goes up as the temp increases)?