Hey guys,

So its my first post here but i've been cruising around the site reading everything and really trying not to start yet another thread about something everyone else has dealt with, but alas i have failed to diagnose the problem and find someone with the same issue.

About a week ago i in my TJ 2001 3.5L magna I noticed the temp gauge needle was sitting a little high, to my surprise i tested oil and saw it was extremely low (thats another problem in itself as the leak cant be found), thought i had it fixed as i filled up the oil...but nope.

So I spent time looking at why the car would be over heating. Found the problem, thermo fan isn't coming on. Great. So i thought i would just go through here and start looking up reasons. Relays work, fuses work, power to the fan works, turn the a/c on and both fans work. So the issue is not the wiring or fuses and i have also made certain that the amount of power going to the fuse box is correct.

Next up, I replaced the temp coolant sensor and thermostat for good measure. Still no change, its just not working. I have however noticed that there is only 4.8 volts going to the temp coolant sensor. Can anyone tell me if this is normal? is there something in the wiring that could be lowering the voltage? For clarification, if i remove the temp sensor connection the fans come on with the check engine light as well.

Any help or suggestions would be tops guys, thanks in advance (im just hoping i dont have to go to an auto elec and pay out the rear, or replace the ecu...)

Thanks guys,

Jamo

IIRC there is a 5V regulator somewhere (back of speedo block possibly) to ensure the gauges get a steady voltage and read consistently with fluctuating system voltage (< 12 volt starting, 14.4V with engine running)

So 4.8V at the sensor sounds ok to me.

EDIT: Car is definitely overheating if you stop after a run and coolant overcomes cap pressure and coolant spews out into the overflow bottle and then onto the driveway. Is it doing that? How high does the needle go?

Haven't noticed overflowing coolant, however I am seeing steam and you can tell when there is some serious heat.

So 4.8 should be fine you think? What about if the car has reached operating temp and still not getting the full voltage? Also forgot to mention that I replaced temp gauge unit aswell.

So perhaps i was given a wrong temp coolant sensor? I'm honestly quite confused by this whole thing.

Is there somehow a way of me testing whether the ecu is possibly at fault? without getting a mech charging me for plugging in a diagnosis tool?

Test the wire output/input from the sensor and ecu?

this is the sender for the instrument cluster

https://g-a.d-cd.net/6335114s-960.jpg

this is the temp sensor for the ECU and the one that 'll trigger the fan. 5 volt supply is correct.
It's the resistance between the 2 terminals of this I'd check. At 80 C 260-360 ohms, as temp goes up, resistance drops so the resistance will need to be below 260 ohms for the ECU to latch the fan relay on.

20 C, 2100–2700 ohms
40 C, 900 –1300

"For clarification, if i remove the temp sensor connection the fans come on with the check engine light as well.
"
that would make sense, infinite resistance so the ECU detects a missing sensor and defaults to FANS ON which is the safe option. that would also seem to suggest the ECU is ok. So you've put a new one of these in and no good?

http://image.cn.made-in-china.com/37f28j01OBdEvNsjPtcZ/%E6%B0%B4%E6%B8%A9%E4%BC%A0%E6%84%9F%E5%99%A8%EF%B C%88%E4%B8%89%E8%8F%B1MD-182467%EF%BC%89.jpg

Steam from where?
If it is from the radiator cap, replace that.

Huh? Isnt the coolant sensor mounted on the thermostat housing? I guess thats the one you replaced?
Anyhow, possible problems are broad.
Things like low water level due to hose or radiator leak,
Collapsing top/bottom hoses,
Water pump failure,
Poor quality coolant, etc. I guess the fluid level is now ok since you replaced the thermostat and topped it up.
Once the coolant is hot the fan should cycle.

How old is the pump? And hoses? (Whats the k's) ?
Yeah,wheres the steam coming from?

Also if it is a fan issue, driving should keep the temp normal especially if on the hwy or major roads cruising around. fan is only going to come into play in stop start city driving.

Radiator cores can block up too.

The cooling fan has 2? speeds - probably the low speed coils have burnt out (my TH did this) - the ECU turns on the low speed fan first, but if that is burnt out the car just gets hotter until the ECU detects an over temp when it turns on the high speed coils and then the temp will drop back to near normal, The A/C turns on both fans at HIGH speed which explains why they seem to both work. I used a diode to jumper the low speed fan relay output to the A/C fan which fixed the temps for more than a year when both fans died - and were replaced.

WOW thanks for the replies guys!

Okay so yes i have replaced the coolant temp sensor, however i wasnt completely convinced it was the correct one as the new and old sensors weren't exactly replica's (not uncommon for aftermarket pieces i know) but i still thought if they knew it was the right part it would be.

Honestly the car has 270xxx km's and i have no knowledge of its service history. My guess. it wasn't treated great. There is a tiny bit of steam from the radiator, where from i dont know, (i know it sounds like i don't know anything but honestly this is impossible to see where its coming from, basically no it's not the rad cap steaming).

This is a stop start issue, only heating up when i stop at lights etc.
I don't think that fans came on at any point, no matter how hot the engine got. Ive been running the engine trying to get a response from the fans but i cant bring myself to let the engine get hot enough for the secondary fans to kick in, rather not play with fire in terms of my engine.

I have tested the fans by adding power to them, both high and low speed function so that means there's no problems with the fans correct? I am okay with electrics but looking at the same things days on end sends me a tad insane.

So what could possibly be doing this? unless I have done my testing wrong what i have learnt so far is: All relays function, all power reaches all relays and fuses, all fuses function, power is reaching all sensors and all sensors replaced.

I may be going in circles now so sorry if i'm not making sense

Take it to an Auto Lec and get it checked out ....... I had a similar issue and needed a new(2nd hand) fan.

Just for what it's worth,
The cooling system uses water as the heat transfer fluid. To increase the heat transfer rate, the water temperature is raised to above 100 degree C by using a pressurised system. So, if you can see steam anywhere, there is a leakage and this will prevent the cooling system from becoming pressurised. The engine will then overheat. Also, the water will leak out of the cooling system requiring constant topping up. Topping up will imply that you are also filling the system with dirt and crud which is in the topping up water, and this will collect in the radiator and other places and tend to clog the cooling channels.
So probably you need to have the cooling system pressure tested as a first step. Normally this is done as a regular maintenance check. Your post #9 gives a clue to the likelihood of this test being missed in the past.
Hope this helps.