Author Topic: Overheat at idle, fan suspected (Read 3945 times)

92A2VWJTD · « **on:** June 28, 2008, 02:20:18 am »

I have a 92 TD w/ac (inop, soon to be removed).

Once warmed up, it is stable at just over half of stock temp gauge when cruising with decent airflow. Up long hills and idling for long periods, temp moves steadily to 2nd to last line of gauge. Once it moves past the normal steady state temp, it seems to "run away" if I don't get back up to speed and provide more airflow.

I suspect the fan and/or switch and/or fan sensor is faulty. Would anyone agree that the coolant system is seemingly not the issue, and that if the fan were operating correctly, the temp would remain stable? I haven't yet tested the fan to see if it works or not (don't believe I have heard it though when she runs hot). I have read here that fan operation (low speed) is tied to a/c operation, true? I have a new aux fan, switch and sensor to swap. My concern is about the useless a/c system and any effect it might have on the aux fan's operation. Are the aux fan switches and sensors different than non a/c mk2 td's? If I pull all the a/c lines, will I affect the aux fan's proper functioning? I can't find my bentley to check into this more, Im afraid. Need to buy a new one

Any suggestions would help immensely.

Quantum TD · « **Reply #1 on:** June 28, 2008, 07:11:18 am »

Here's how you test if your motor is good:
1) Turn on AC or Defrost: the rad fan motor should come on at high speed. If not, you have a wiring issue. Even with no freon, the fan should come on when the selector paddle is set on AC or defrost.

2)If the fan does turn on when in those positions, but not when the car is hot, then your fan switch is likely bad. It's a cheap and easy fix.

3) If your fan does NOT come on with the AC or defrost on, then it's time to check the wiring.

If it does not come on, then pull the plug off the fan switch in the rad. Using a test light, determine which (if any) of the 3 wires has power to it. It should have power ALWAYS (i.e. even with the key turned off). Usually, its the Red wire with a White stripe that is the constant power wire. If you have no power at any wire, then look at the fuse.

Ok. So, once you have determined which wire has power to it, take a test lead, and bridge the positive battery terminal to the OTHER two terminals on the fan switch plug (use a solid wire, not the test light. The test lite doesn't support enough current thru the bulb). The fan should operate at low (should be the all-RED wire), and high (should be the Red/black stripe wire).

If the fan does not come on with either of those two wires bridged , then your fan motor is not working. If if only comes on with ONE of the wires bridged (usually the all-Red wire), then you have a fault fan relay somewhere.

Good Luck

92A2VWJTD · « **Reply #2 on:** June 28, 2008, 01:38:18 pm »

Nice

Thanks. I am checking it out now, have already tried the a/c, no fan. Ill jump the fan and see what gives. Also, as per my post, would you agree that it is in fact the aux fan that is the culprit in my overheating scenario? It surely must be a necessary component in the system when there is little airflow through the rad. Thanks again

veedubcanuck · « **Reply #3 on:** June 28, 2008, 05:46:24 pm »

My 91 Jetta fan will not come on with the A/C switch IF the system is low on pressure, If you look at the diagram it is dissabled by the low pressure switch same as the compressor. Just letting you know.

Quantum TD · « **Reply #4 on:** June 28, 2008, 07:13:52 pm »

Quote from: "veedubcanuck"

My 91 Jetta fan will not come on with the A/C switch IF the system is low on pressure, If you look at the diagram it is dissabled by the low pressure switch same as the compressor. Just letting you know.

Good to know. The last couple cars I worked on had working AC, so you may be right.

The fan should definitely switch on when you jump the wires at the fan swtich. If not, then the fuse, the relays or the fan is toasted.

Yes, aux fans are absolutely necessary, especially at idle. If you've got mild compression loss, then its a must. Some cars can idle for a long time before the fan kicks in, but others (esp cars with AC) really use the fan quite a bit. So, yeah, I'd say the hot running is fan-related, and you probably did not do any damage, so long at the needle did not get pinned to the right.

I will say, that I'm finding a really thin mix of rad fluid/water (like 70% water) keeps the temps really low. Apparently water dissipates hear better than ethylene glycol. That also means it absorbs heat really quickly, and freezes really quickly. So, run it in the summer to keep the car cooler, and switch back to 50/50 or 60/40 in the winter (if you want added cooling benefits). The old "tiny drill hole in the thermostat" trick is also good to keep the water moving, but not drop the temps so much as to adversely affect optimum engine combustion temps.

If you want to go gonzo (not recommended), you could always pull the t-stat. Then your needle won't move past 1/4 of the gauge.

Good luck.

The Mad Hatter · « **Reply #5 on:** June 28, 2008, 09:14:37 pm »

I'd been having similar issues (check my posts). I'd say jumper the fan first to make sure its working. If it is (mine was), check the condition of your coolant. If its been a while since its been changed, or it looks greasy (mine was), go ahead and get a rad flush (i did), AND a new fan switch (they get greasy too if the coolant is greasy). That fixed it for me, except i had a bad coolant temp sensor that was driving the gauge bonkers. IF you coolant is greasy check that coolant temp sensor (its in the overflow tank) to see if its covered in sludge (mine was). If you replace it, get a VW one, NOT a cheapie, as the cheapie that i bought leaked and caused my gauge to go nuts, giving me more undue stress!

Just as a side note, my car has A/C, which is non op, but if i put it on A/C the fan does NOT come on. I found that one of the wires at the top of the radiator (drivers side) was disconnected to disable the A/C. Check that, that may be why your fan doesn't come on when the A/C is switched to on.

While you're at it, go ahead and throw a good thermostat (i prefer 180 degree) and a lower radiator hose on it, and you should be good to go all summer! Good luck!

92A2VWJTD · « **Reply #6 on:** June 30, 2008, 01:15:05 am »

Thanks for the advice all

I did in fact find the fan to be working, and it was the switch at the bottom of the rad that was faulty. Phew! The easy way out :lol: I think the series resistor is garbage though, cuz low speed wouldn't jump, so I discon'd it and soldered the wires togethor (a/c is to be removed completely at some point anyways). Now the fan runs on low with a/c or defrost selected, and a test run confirmed that the switch works as well. Nice!

I already have a new fan and rad, so they will go in just to be sure, and of course I will flush and re-fill, as well as pop in new t-stat. Would a garden hose flush work, or does the car have to be running for anything to get past the water pump? I think she's good to go just in time for summer weather.

Anyone know exactly what the series resistor is for anyways?

Cheers.

jtanguay · « **Reply #7 on:** June 30, 2008, 01:16:07 pm »

Quote from: "92A2VWJTD"

Thanks for the advice all
I did in fact find the fan to be working, and it was the switch at the bottom of the rad that was faulty. Phew! The easy way out :lol: I think the series resistor is garbage though, cuz low speed wouldn't jump, so I discon'd it and soldered the wires togethor (a/c is to be removed completely at some point anyways). Now the fan runs on low with a/c or defrost selected, and a test run confirmed that the switch works as well. Nice!

I already have a new fan and rad, so they will go in just to be sure, and of course I will flush and re-fill, as well as pop in new t-stat. Would a garden hose flush work, or does the car have to be running for anything to get past the water pump? I think she's good to go just in time for summer weather.

Anyone know exactly what the series resistor is for anyways?

Cheers.

the resistor is to give the a/c a really low fan speed as to not burn out the fan by continually running it on 'normal' speed. it does kick on to high speed when the temp rises though.

flush the engine out with the garden hose and flush the rad out too. i ran prestone heavy duty flush and some nice brown water came out... :wink: cooling system works flawlessly now! good time to change your t-stat too. be sure to test it to make sure its opening when it should! i got a faulty one that was overheating my car...