A4 (B5) 1.

Car: Audi, A4, 1997 - Back to Fix-It - Audi Repair Manuals

Q.engine runs normal for about 20 minutes then temp guage starts going over the mid mark. If I put the HVAC on it cools down a bit.

It appear that overheating is due to the Electric Fan not coming on when the engine is hot. Changed water pump, Thermostat, Coolant Sensor on radiator, coolant Sender at back of engine and Electric fan. Problem persists.

Observation: Electric fan comes on with the AC but does not come on if AC is off and the engine is hot (75% on Guage).

Any words of wisdom as to where to look?
What do you think is the issue, how can I verify the diagnosis and what would be the solution?

Answer Seems like you have a proble with the electrical control mudule, and since you have replace everything, so I sugets you go ahead and replace it you must cut the wires and crimp the new one in see if this help and let me know also make sure you erase all the fault codes out the cluster and the engine control units..

Answer This year model Audi does NOT have a "electrical control module" for the coolant fans. I suggest you diagnose the condition by first testing the fuses. Then if you jump the terminals at the connector for the temp sensor in the lower left of the radiator it should trip the relays and function the fan. Here is the electrical diagram.

Let me know if you need help with current flow of the circuits.

Addition This is an intelligent answer and I think we are on the right track. I would like to keep this channel open until I resolve the issue. There will be a bonus at that time.
Thus far, visually verified fuses #19 and #21 to be "not blown". Will be jumping terminals next and get back to you.
Yes, any additional help with the current flow will be helpful.
At what temp is the electric fan triggered? I have Vag-Com 409.1 and can view running temp. I did not want to create an overheating condition higher than neccessary and damage anything else. I would like to verify my assumption.

Answer It's pretty simple.
I would start at the temp switch connector at the radiator. With the key on, jump between terminals #3 (ground) and #2. This should trip the low speed fan (J26). If not jump terminals #30 and #87 of the J26 this should run the fan.
Back at the temperature sensor connector, jump terminals #3 and #1 this should trip the high speed relay (J101), again jumping the #30 and #87 terminals should run the fan high speed.

Addition Thanks for your response. It helped.
I am having some trouble locating some of the item on the circuit diag with that on the car. I am looking at the fuse & relay assembly under the dash next to the steering wheel and I don't have a way of matching J26, J101 & N39. For S42 and S51, all fuses here is good).
TESTED temp switch connector at the radiator. With the key on, jump between terminals #3 (ground) and #2. SUCCESSFUL: at #2 slow speed and #1 higher speed.
TESTED: temperature sensor connector, jump terminals #3 and #1. FAILED nothing happened. Could not identify J101 to jump #30 & #87.
What would be the next steps?

Answer The relays are on the 8 fold relay panel behind and above that panel. They are located in positions #2 & #3 as indicated on the electrical diagram.

What does not make sense is that you say the low speed fan circuit works. This should be kicking in before the gauge reads 3/4. Do you have a infrared thermometer gun that can read temperature? It would be quite helpful to know the actual temperature of the; Coolant flange at the front of the cylinder head, The thermostat housing, inlet and outlet hoses at the radiator.
If the coolant is getting as hot as you say it is, the switch in the radiator should be kicking in and turning on the low speed before the needle leaves the 1/2 mark on the gauge. Since you replaced the switch, water pump, and thermostat there "should" be coolant flow and the sensor "should" be working.

Addition Tested a few more things:
1) F54: While 3 to #2 and #1 produced the fan turning at low and higher speed jumping #30 to #87 of J26 test light came on but DID NOT run the fan.
2) F18: Jumping 3 to 1 DID NOT run the fan; Jumping #30 to #87 of J101 test light came on but DID NOT run the fan.
3) There is a verified coolant flow.
4) Allowed the temp gauge to go to 80%. The BBQ thermometer said 160F on the thermostat housing and 140F on the top hose of the radiator.
5) Putting the A/C on at idle the fan runs at Low at higher engine speed the fan turned at higher speed.
Hmmm..????

Answer If jumping (bridging with a short length of wire) the #3 to #2 and #3 to #1 produced low and high speed, there is NO need for further electrical testing. Provided the temperature switch is good (you said you replaced) the system functions.
I do not understand what you are doing in "2) F18: Jumping 3 to 1 DID NOT run the fan; Jumping #30 to #87 of J101 test light came on but DID NOT run the fan.", but it is not necessary and I don't see where a "test light" comes into play.
This "4) Allowed the temp gauge to go to 80%. The BBQ thermometer said 160F on the thermostat housing and 140F on the top hose of the radiator." makes no sense. 80% (more than 3/4 on the gauge) should be WELL ABOVE the temperatures listed. The thermostat should open at 87c (188.6f). Possibly your gauge or more likely BBQ thermometer is not Acurate. I suggest you sort that.

Addition 1) Retested to verify that my findings is consistent behavior. (Additionally, I am using a simple tester with a light between the two ends as the jumping wihte. I know it steals some amps but I am also testing that the circuit is closed.)
2) This is the one that really concerns me "F18: Jumping 3 to 1 DID NOT run the fan;" Is this suggesting that J101(443 951 253J) is bad?
3) I agree, thermometer is likely inAcurate.
Is it possible that F18 is acting as it should, only triggering speed 1 & 2 when AC is on and J101 is defective failing to trigger high speed when engine goes over 50%?

Answer 1) Retested to verify that my findings is consistent behavior. (Additionally, I am using a simple tester with a light between the two ends as the jumping wihte. I know it steals some amps but I am also testing that the circuit is closed.)
NO, this is NOT a valid test, PERIOD! It will "steal" 1/2 of the voltage. Also if you did this type of test on a control module output you COULD fry the control module driver as the seemingly low amperage that the bulb uses and overload may drivers in a modern control module circuit. Use a jumper wire (in line fuse optional) to jump. And a volt meter to check voltage.
2) This is the one that really concerns me "F18: Jumping 3 to 1 DID NOT run the fan;" Is this suggesting that J101(443 951 253J) is bad?
See #1) above.
3) I agree, thermometer is likely inaccurate.
Is it possible that F18 is acting as it should, only triggering speed 1 & 2 when AC is on and J101 is defective failing to trigger high speed when engine goes over 50%?
A/C comes in on it's own circuit.

Addition I retested jumping the manner you suggested.
1) F54: While 3 to #2 and #1 produced the fan turning at low and higher speed jumping #30 to #87 of J26 DID (different than before) run the fan.
2) F18: Jumping 3 to 1 DID NOT run the fan; Jumping #30 to #87 of J101 DID (different than before) run the fan.
What can we deduce?

Answer "If jumping (bridging with a short length of wire) the #3 to #2 and #3 to #1 produced low and high speed, there is NO need for further electrical testing."
So, if this test of jumping the terminals at the fan switch works.. your done. Electrically the system works.
Either the temperature gauge is not working properly and it's not overheating. You need to check into this before you get deeper into it than you already have. The best way is with an infrared temperature sensing gun.
Or there is poor coolant circulation and the hot coolant is not flowing past the temperature sensor.
Or the temperature sensor has failed. I know you said you replaced the temperature sensor so it's not likely, but it is possible.

Addition I was able to measure the temperature when he guage showed 60% with IR Thermometer: Coolant flange 218F; Thermostat housing 225F; Radiator top hose 198F and bottom 215F. Definately overheating.
Looks like the new thermal switch is not operating properly. Will order the part myself and take it back to the tech to install and see what happens.
I am still unsure about the observation of "2) F18: Jumping 3 to 1 DID NOT run the fan;" and wondered if I misread what is #3 & #1. On the wire connector it is numbered 1,2,3 &4 and I used this #3 & #1 to test. However, looking at the color of the wire to these leads does not match the circuit diagram. It show #1 - Brown/Light Blue; #2 - Blue/Brown; #3 - white/gray; #4 - Brown. According the diag #3 should be Brown and #1 should be red/green.
Is it possible I am jumping the wrong wires?

Answer You have 4 terminals and wires at the coolant fan temperature switch?
Is it not a 3 terminal switch like this mounted in the left radiator tank?

Addition Yes, the Coolant Fan Thermal switch (F54) is a 3-terminal mounted on the left radiator fan like the picture says.
In addition, I understand that there is a 2nd coolant temp sensor F18 that works in combination (also, known as the ignition hall sender that controls the console) which is a 4-Terminal mounted at the at the back of the engine, just infront of the Battery. Is this correct?

Answer No, F54 and F18 are both part of the three pin radiator fan switch. See the previously sent wiring diagram.
G2 and G62 is the thermal switch at the back of the motor that controls the gauge and sends the temperature signal to the Engine Control Module (ECM).

It wouldn't hurt to change that one to make sure the gauge is Acurate.
Coolant Temperature Sensor -G62- (CTS) #059 919 501A
CTS O-ring #N 903 168 02
CTS retaining clip #032 121 142

Addition Problem persists after changes. After starting for about 3 minutes the engine goes up to the halfway mark and stays there. After about 20 Minutes temp guage keeps going up.
Otherwise, engine purrs smoothly.
1) Thermal Switch on radiator was replaced.
2) Thermal switch at the back of the motor was changed at the beginning of this issue.
3) Clutch fan changed - decided to do this while the tech is going to work in that area and eliminate any possibility.
After the test run the tech observed temp of 205F on engine, 198F on top radiator hose & 168F on bottom radiator hose.
At this point, we changed:
1) Thermostat,
2) Temp switch on Rad (Twice),
3) Temp switch for guage,
4) Electric Fan,
5) Clutch Fan,
6) Water pump
Any ideas?

Answer "After the test run the tech observed temp of 205F on engine, 198F on top radiator hose & 168F on bottom radiator hose."
These readings are all within normal specifications. What was the gauge reading at this point? It should have been straight up. I suggest the coolant pipe at the back of the cylinder head where the G62 sensor is located be checked (temperature). This is the temperature sensor for the gauge. Also, after a test drive if the gauge was straight up with these readings. If the engine is allowed to idle does the temp climb with the A/C off? The low speed fan should kick in before the gauge moves more than 1-2mm off of the center mark.
If these are the readings while the gauge is at the 3/4 mark (75%) the gauge is not Acurate.
If the vehicle continues to overheat (anything above the 1/2 mark) there is the possibility of a cracked head or head gasket failure. I have never seen it on a 1.8T, but imagine it is possible if the vehicle were overheated in the past. To confirm this have the mechanic remove the coolant expansion tank cap (CAUTION) and test the air in the top of the tank with an exhaust analyzer. Check the hydrocarbon level, it should not be above 20-30 ppm or so. If it goes straight to 100 ppm or so and above one of the two have likely failed.

Addition Eureka! It's fixed.
I made the illogical decision to change the radiator and after that everything was back to normal. It has been running good for a week now and still can't fully explain it.
The guage is straight up and engine purrs like an Audi.

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A4 (B5) 1.