rjb888

My issue: I have a 1993 Audi 90 (non quattro) v6 5sp which I recently acquired. The car overheats due to the fans not turning on. At first I was loosing coolant because the reservoir was cracked, I replaced that and found that the car was still overheating and noticed the fans do not come on. Also, the heater core is bypassed (in case that matters for anything). So far I have checked: the fans- when connected directly to power they run just fine; the relays for the fans that are present have been tested and are functional. (though there are some suspicious spaces in the relay panel where I am not sure if there are supposed to be relays there.); the temp switch in the radiator was tested and is working as well. I thought about trying to bleed air out of the system but don't know where. I let it run with the reservoir cap off for a while, but I don't know if that will bleed the air or not. I have run out of things to check.
There are a couple anomalies which I am not sure if they are a factor or not.
First: there are several missing relays in the relay box. Some of them (according to the pretty picture on the top of the box) are supposed to have something to do with the cooling fans, but I can't find documents/schematics that call out what those relays are and if they are actually necessary in this vehicle.
The other is a mysterious red plug on the passenger side right next to the radiator that doesn't plug into anything. I have looked for a mating plug and wire harness for it, but it is not there... again, not sure if this is required for anything or if it is a provision for another configuration of the car.
I uploaded photos of these as attachments (i hope... not sure if they made it)
I have tried to be as thorough as possible with my description here, now I ask for someone who knows these cars to help me out. I have run out of options, and do not know what to check next. If there are other components to check, please let me know, with photos if possible, where they are at. I am new to the audis and am not familiar with their components.

Hybrid_Hatch

that red plug is part of the AC system, i believe it is a high pressure switch. It is very easy to jumper the fans and make them run to avoid over heating, and that might be the best option for the time being. the relay panel looks right. the same relay/fuse box was used for SEVERAL different engines and configurations. You said you tested the switch, but the problem could very well be a blockage issue in the radiator, and if it blocks off the area where the fan switch is, it isn't getting proper signal. That would be my first guess is that there is some sort of coolant blockage. The fact that your heater core is bypassed means it most likely failed, that could be due to many factors including wrong coolant. If the heater core failed, it isn't a far stretch to think that the radiator has failed somewhere internally as well as they are all flowing the same fluid though them. Where are you located? if you are in Utah, i'd be happy to give you a very very cheap v6 radiator.

Rusty Spokes

Hybrid is right..if the radiator is original or the coolant is old..plan on putting a radiator in the car. The by-pass heater core is clue as to the condition of the cooling system. Many overheating problems are caused by duff radiators. The radiator fan thermoswitch threaded in the bottom part of the radiator may not be getting hot enough because of the plugged up radiator or is covered in rust and debris.

The good news..rockauto has a Nissens radiator for the 90/cabriolet for $167..NISSENS 60469A

I did my radiator..it is fiddly but can be changed without removing the condenser.

Use the code: 2427941222572124 in the space "how did you hear about us" and get 5% off..that helps with shipping charges.

How old is the water pump, timing belt and thermostat?

rjb888

Thanks for the replies guys. The radiator does not appear to be plugged up. When I removed the temp switch water flowed from the hole, so water is getting there, and when I run the car I can see the water circulate through the reservoir once the car heats up so it it passing through the system. I have read something about a fan control module? Is that something used on these cars? If so where is it and what does it look like? Other possibilities?

rjb888

Would by-passing the fans make them burn out? I used a temp switch set up to run the fans to get home when I first found out it was overheating and the wires got really hot, so I started to just cycle the fans while I was at stoplights... seems like they use a TON of current.

Rusty Spokes

If your going to by-pass the thermoswitch..don't wire the fans direct to power. Use the resistor board that is under the car left front and power though the low speed fan resistors. Put power on the red-green wire.

See the site: Page 34. Should be the same or very close to the wiring on your 93 90

http://12v.org/urs/1996_Audi_Cabriol...g_Diagrams.pdf

rjb888

Great diagrams! Thanks. Any chance it could be that resistor board that's messed up?

Rusty Spokes

Could be. Check to see if you have power to the board on the red/green wire with the A/C on. Fans should run at low speed with the a/c on.

You can jump the connections on the plug for the thermoswitch with a length of wire with 1/4 blade connectors crimped on each end. Remove the plug connection. One of the three connections will be 12v with the ignition on. Jump 12v to the other adjoining slot..you should have a second speed..then jump to the next slot and you should have super turbo speed third speed.

If you jump the thermoswitch and the fans do not come on..check for 12v at the board..if you have 12v at the board and not coming out to the fan wires..there is your problem.

I have seen the boards miss wired if someone was not paying attention..that can burn up relays and wiring.

CRAZY_RUSSIAN

his AC is unplugged....

what color is your coolant? if its anything but pink, follow the next steps

i would start of with this if you have another car to drive, replace your heater core and re-seal the door inside the heater box. This is a weekend job, its sucks, but you need to get it done (unless you don't need heat in the winter then skip to the next step)

get some G12 or G12+ coolant from AUDI/VW dealer (1 gallon will do, but get 1.5 if you want 50/50 mix) and about 1.5 gallons of distilled water, and get the o-ring for the engine coolant drain plug, N-904-520-02. the drain plug is located in the driver side next to the bell housing and it has an opening in the oil pan where you can reach it with a (IIRC) 8mm allen head (does anyone have a link to this procedure? i don't think i can find it if i follow my own direction) I would highly recommend removing that plug, you'll be surprised on what comes out of that hole. disconnect the two radiator hoses, get a garden hose, flush the radiator out, then disconnect the heater core (in your case the connecting peace of pipe) flush the heater core with a garden hose, then flush the back side of the hoses while that drain plug is removed. basically flush everything you can get your hands on, that included disconnecting the engine oil heat exchange lines and flushing those out too. that will clean out allot of crud from your engine that's bean accumulating over the decades in your engine block and your pipes. if you notice that water is having hard time passing thrum the radiator, its clogged, replace it!
when you are installing the hoses back on, i smudge some oil on the pipes, just to help it slip on a little bit. install everything back on and mix your coolant to what ever ratio your hart desires, or your temperature permits. fill her up, you have a small plastic screw (at least should be plastic) on one of the heater core lines, that's your bleeder screw. slightly open it when your engine starts getting warm, all the air will escape and you will have to fill up your reservoir at least once. you can also let the air escape thrum the reservoir cap, by slightly loosening it (dont open it when that temp gauge starts showing past 1/3, personal experience with that) also be careful with the screw as well at that temperature point.

under no circumstances should you use any other coolant in these cars other then G12!!!!!

how is your thermostat/water pump/timing belt situation?

x19

Cooling system must have full of coolant with no trap air in the system and no external and internal leaking with good electric wiring circuit. if coolant in the system was less and (or) radiator clogged , thermo switch on the bottom of radiator won't sense the hot temperature because, no coolant circulate. If your electric circuit was Okay, you can check the system with pressure tester on cold morning and see the leakage from water pump, hose connection or head gasket etc. You car should be Okay if above test passed.