Radiator Circuits explained?

Post electrical issues and questions here

Moderators: kamzcab86, CalAltaDubber

Post Reply
User avatar
Whats that smell?
Posts: 4051
Joined: Mon Mar 21, 2005 5:45 am
What year is your cabby?: 1992
Do you own a Cabriolet?: No
Location: Duluth, Ga

Radiator Circuits explained?

Post by Briano1234 » Wed May 14, 2014 9:49 am

The definitive operation of the Radiator fan.

The first things we need to know is the air flow over the radiator.
In the old days folks weren't worried about safety, and radiators were big, and unshrouded, usually they were cooled by a
belt driven off the water pump.

Well Eurpoean folks took a longitudinal engine/tranny/driveshift/rear differential system and turned it sideways.
So the ideal had configuration wouldn't work as the water pumps were now on the left or right of the car and not
directly infront of the engine.

This lead to increased under the hood temps as the Transmission was now producing heat in the engine bay, and now
the invention of the electric radiator fan.

The Radiator fan was and is stupid. It has to be told when to turn on instead of being run by a direct fan.
So they added a Radiator fan switch to tell the radiator fan when to turn on.

Now your radiator fan is told to run, it doesn't care if your radiator cards are there or not. It is told to run.
It will pull air regardless of where the air comes from.

With the Radiator cards missing, it can and will pull air from the engine compartment back over the radiator, decreasing the effeciency
of the cooling ability.

Missing top card.


To prevent that, VW placed a shroud over the radiator to duct air over the radiator from the outside. These shrouds aren't a tight fit.
VW placed "cards" on the top, left and right of the radiator, the top one while preventing your tools and bolts from dropping infront of the radiator
also keeps the radiator pulling air from the Grille.

Brass top card:


You can make replacements out of sheet tin, brass, formica these are more durable as replacements.

The side baffles or cards also prevent that too, but usually are removed over the years as they get in the way.
If you watch the radiator fan work, you will see that on the Radiator shroud, there are 2 rubber flappers kept in place by nylon
clips with a stopper.


You have wondered what those are for? When the Radiator fan is running, they suck in to close the Shroud off and only allow air to pull from infront
or from the grille. When the radiator fan is off, they are allowed to free open, so that the radiator has air flow across the width of the core.
If they are missing then the Fan can draw engine air so much so that it can decrease the efficiency by half.

Now that we know that Radiator cards and flapper have a direct cause and effect, and if missing it is a bad thing,
lets move on to the Radiator Fans.

VW originally used a single speed fan, that was simply controlled by a switch in the radiator.
They applied 12V+ from the battery to one side of the switch, and the other went up to the fan.
Since current needs a path from ground to a source. VW added a ground wire to the fan, why because the fan is mounted to a shroud,
that is bolted to plastic radiator tanks. These plastic tanks are an insulator, and will not conduct ground.


The above is a/c equipped cars, the radiator fan switch also acted as the "After Run" fan switch Simple is more better.

In the olden days of the first ittereation of fans that I saw, the fans had only a 12V source to them and used the BRASS and Copper
Radiators that were bolted to the frame from ground.

Not so with the advent of the plastic/aluminum radiators...You have to have a ground.

To accommodate the higher temp needs of a a/c equipped car, they added another speed to the fan, hence the 2 speed fan, as well as the
after run switch to prevent hot-starts and a iffy cooling of the engine when shut off.

The ideal thing would be to allow it to run based solely on the switches in the radiator.
Most of the after fan run are on the Low temp side of the switch in the radiator.

Now when the engine is running down the road, air flow over the radiator is sufficient usually to keep the car cool, however
if you run the a/c, it will run hotter, so they added a switch to the fan to turn it on, it is controlled by the heater control fan switch.
if the control selector is in the black area, on non-defrost, then the radiator fan operates as if the a/c isn't there. If the a/c is on, then it turns on the radiator fan. The Radiator fan will come on if you have the selector in the blue, or defrost setting.

When are driving at hi-way speeds, and you pull off the road, the engine temp spikes or rises, as it is trying to cool down, sort of like you after a run or exercise, need to "walk-it-off" or cool down. This engine spike is normal, but controlled by the cooling fans operation.

Now we have estabilished a few rules.
1. Radiator Shroud, and Radiator cards are necessasary for proper cooling and missing parts only decreases the
cooling ability of the system.
2. The fan needs 12V+ and a Ground to run.
3. The Primary fan switch is in the Radiator or, the after fan run switch if installed.

Lets do some checks.
Here is a simplified version of the 2 speed circuit and it's components.


When the fan is running current flows from the ground on the motor back to 12V+

If your fan or car is running hotter than normal, you have to know where and what to test and why.
In the Bentley or Haynes manual you have to look closely at the circuits and then if you have a/c or not.
because if your car is equipped with a/c you have to look at the a/c schematics and not the normal ones.
a/c equipped even if you have had the a/c removed:
a/c equipped:

non-ac equipped:

A little bit different, and now you know why when we ask does it have or had a/c?

Now that we know which schematics we should look at, lets get the basic trouble shooting steps.

Tools needed:
2 or 3 jumper wires with clips is best.
Digital VOM.

Does the fan run at all?

Well if it is a 2 speed fan does it run on both speeds?

Take the connector off the fan. look at the connector, There are 2 or 3 wires, depending on if there
is a single or dual speed fan. They are Brown, and RED/black, know the position of them relative to the fan motor.

On the fan Jumper the BROWN lead to ground
Keep your hands and wire jumpers clear of the fan blades.

Jumper 12V+ to the RED/BLK wire one at a time to +12V if the fan runs on both hi/low speed your fan is good.

Reconnect the connector, and now measure for a good ground via resistance to the Brown Plug or the Case of the fan.
0 or .5 ohms, then you are ok, infinite, then you better get a new ground.

Lets see if the fan works in all modes via the controls.

Fan switch to 0
Key in, and in the run position.
Turn the heater control sliders to the DEFROST setting, now turn the fan on, do you hear the a/c clutch pick, and the
fan start running in "HI-Speed"? yes or no.

If the Fan works in Hi, then we know it is the low speed side of things.

From the first test we know that the fan in general works.

On the Single speed fan, it is a little different, as there are fewer things to go wrong.

On a single speed fan or older circuit, you have but a FUSE, and a radiator switch and a motor.
Makes it way easier....

If the engine is up to temp, and your fan doesen't engage, Quickly measure for 12V+ to the motor to frame.
If the 12V+ is there, then you are missing the ground to the connector.
If the 12V+ isn't there, then we have to find out where it is missing.

The most common cause that I have seen for the 12V to go missing, on the 90ish's is the Relay on the inner fender near the battery.
this is a non-waterproofed relay that is subject to water spray, and battery outgassing.
If it goes bad, your radiator fan usually won't come on for normal operation, but will if the a/c is turned on.


In the a/c circuit, you have F1 sending power to the low speed cooling relay (fender) and to the radiator switch.

In the a/c equipped car, f1 also sends 12V+ Through the a/c relay, then the radiator switch and to the radiator
fan relay (FENDER). as well as straight to the FAN.

So in reality if the Fan will turn on when the a/c is selected, you have validated everything in the circuit, save
for the low speed side of the Radiator switch.

If the Radiator fan doesn't come on at all when you turn on the a/c, then you are looking at the relay, or the
Radiator Fan thermo-switch.

To see the After Run circuit, this is where you have to refer to the normal schematics and not the a/c equipped ones.
But if the fan doesn't turn on after you have run the engine, then the quick test is to take the wire off the
after fan run switch (between the valve cover and the throttlebody, bolted to a bracket on the valve cover.).

This single wire attached, if you short that to ground, and your fan turns on, then your after-run circuts are
ok, if not replace the after run fan relay(controller). If your fan runs, then your after fan run switch is bad.

To test the Radiator fan thermo-switch you have to drain the cooling system, and remove the switch from the radiator.

Connecting the leads of your DVOM to the switch legs in resistance mode.

Using a non-flammable heat source, or a pan of boiling water, place the switches brass end into the water.

When it gets up to temp, it should "click" and measure 0 ohms resistance. Remove from the heat source and validate that it "clicks open when cool, then repeat for the "other" side if a 2 speed switch is installed. Also leave it in the water a bit to make sure that it isn't flaky and that it stays in the closed or 0 ohm mode...

Now if you have a lower temp thermostat installed, then your fan operation may be off or non-functional. If you are missing the t-stat, then all bets are off, as
your radiator is cooling all the time and the temperatures are not constant to operate the switches.

Radiator Cards or baffle templates:

Thank you Kammy.

Trimming your radiator shroud:
http://volkswagenownersclub.com/vw/show ... tor-Shroud

Hope this settles things a bit for ya.

Yes as matter of fact, I have the Luck o'the Irish...everything I touch turns to fertilizer of the bovine variety.
You can lead a user to a link, but you can't make him Click.... :screwy:

Posts: 163
Joined: Thu Jan 26, 2006 11:30 pm
Do you own a Cabriolet?: Yes
Location: 313

Re: Radiator Circuits explained?

Post by tolusina » Fri May 16, 2014 6:33 pm

Some additions;
On a two speed fan system as Brian has been describing, with the AC off, the fan on low speed is normally barely audible (if at all) from the driver's seat with the hood closed, as in normal driving.

If all is well throughout the cooling system, low speed should be adequate cool the radiator down, high speed should mostly never be able to activate.

Stationary idling, stop and go traffic and city traffic in general, low speed should cycle on and off.

At freeway speeds, airflow should be adequate enough that the fan does not come on.
Until you exit the freeway. Now there's a sudden drop in airflow and a whole lot of heat to dissipate due to continued and relatively high engine loads at freeway speeds. You might get high speed while still on the exit ramp.
Electrical comments;
Looking at the genuine schematics Brian has posted above, on AC equipped cars, low speed does not use a relay to carry current, it is switched directly by either the radiator fan switch or the after run control unit, the after run unit may have an internal relay though.

Current to run the high speed is always carried by the radiator cooling fan relay.
The radiator cooling fan relay gets its control signal from the radiator cooling fan thermoswitch and/or the AC system.

On non-AC cars as shown in the diagram above, there is no cooling fan relay, the radiator thermoswitch carries fan current direct for both high and low speeds, the after run controller operates low speed.
The after run system's primary function is to cool the engine compartment after running on hot days and so prevent fuel percolation and vapor lock on DigiFant equipped cars.
The after run control unit gets a signal from a thermo-switch mounted on the back of the valve cover between cylinders 3 and 4.

The after run system is not used on CIS equipped cars, fuel system pressures are high enough that fuel percolation and vapor lock are essentially impossible, a non issue.
If the radiator is hot enough to switch on the fan as the engine is switched off, the fan will continue to run until the radiator cools down. After run heat soak can also activate the radiator thermo-switch, low speed runs after shut off until the radiator cools.
This after run radiator cool down condition is normal for both CIS and DigiFant cars, it is separate and distinct from the after run system which is intended to lower general underhood temperatures.


User avatar
Queen and Popette of Cabbydom
Posts: 1277
Joined: Wed Sep 11, 2002 3:54 pm
What year is your cabby?: 1986
Location: Arizona

Re: Radiator Circuits explained?

Post by kamzcab86 » Tue Jun 03, 2014 1:39 am

To clarify what Cabriolets came with what:

1980-1987 = One-speed fan, regardless of A/C (parts stores that list 2-speed fan components being required are wrong!)

1988-1993 = Two-speed fan, regardless of A/C, plus after-run system
tolusina wrote:The after run system is not used on CIS equipped cars
Incorrect. The after-run, 2-speed fan system was installed in Cabriolets beginning with the 1988 model year, which is CIS-equipped. What the '88-'89 cars do not have, but what the '90+ cars do have, is the "Cooling Fan Time Control Unit" in the relay panel.

Briano1234 wrote:Radiator Cards or baffle templates:

Thank you Kammy.
Don't thank me, thank Thomas for creating those files. :wink: The AutoCAD link at http://www.cabby-info.com/cooling.htm#Radiator contains the actual size templates that a print shop should be able to print for you, so that all you need to do is trace the outlines.

Post Reply