Hello,
Here is an excerpt from the official Dodge Caravan Factory Service Manual related to the Cooling Fan Relay.
There it mentions a high speed ground relay and high speed fan relay.
There is a Fan Control Relay. It operates under a PWM controller that varies the voltage to the fans based on coolant temp. I know of no other relay on the vehicle that controls the radiator fans speed or on/off cycles.
I’m not sure how the two differ. Maybe you do?
The relay has 4 wires. 2 are 12Ga and two are 14Ga.
The 12Ga wires provide current to power the fan motors.
The smaller wires are ground and the PWM pulse from the PCM.
I bolded the part that confuses me. Can you help explain this?
SOLID STATE FAN RELAY—PCM OUTPUT
The radiator fan runs at a variable speed depend-
ing on coolant temperature and A/C system pressure.
The radiator fan circuit contains a Solid State Fan
Relay (SSFR). Refer to the Group 8W for a circuit
schematic.
A 5 volt signal is supplied to the SSFR. The PCM
provides a pulsed ground for the SSFR. Depending
upon the amount of pulse on time, the SSFR puts out
a proportional voltage to the fan motor at the lower
speed. For instance, i f the on time is 30 percent, then
the voltage to the fan motor will be 3.6 volts.
When engine coolant reaches approximately 102°C
(215°F) the PCM grounds the SSFR relay. I f engine
coolant reaches 207°C (225°F) the PCM grounds the
high speed ground relay and high speed fan relay. If
the fan operates at high speed, the PCM de-energizes
the high speed relay and high speed ground relay
when coolant temperature drops to approximately
101°C (214°F). When coolant temperature drops to
101°C (214°F) the fan operates at low speed. The
PCM de-energizes the low speed relay when coolant
temperature drops to approximately 93°C (199°F).
Also, when the air conditioning pressure switch
closes, the fan operates at high speed. The air condi-
tioning switch closes at 285 psi ±10 psi. When air
conditioning pressure drops approximately 40 psi, the
pressure switch opens and the fan operates at low
speed.
The SSFR relay is located on the left front inner
frame just behind the radiator (Fig. 42).
In any event, my vehicle doesn’t not seem to operate this way.
The fans only come on when coolant temp reaches 225F. And even then, they are at low speed.
it seems to me they should be at high speed until the coolant cools to 214F but that is not what happens.
They stay on low speed until the temperature reaches 214F and they turn off completely. This cycle repeats. The fans may come on at a higher speed if the temps gets above some other higher temperature but I am not willing to let the coolant get that hot.
Maybe the PCM is never sending the high speed signal which would be a longer pulse width resulting in more on time and higher voltage?
Do the fans run any faster if you turn on the air conditioning?
Do you have a volt meter? You may be able to detect the presence (or lack thereof) of the PWM signal by setting it to read AC and probe across the relay control input. It will depend on the response capability of your meter and the frequency of the control signal. I’m guessing you will get a reading if it is running and see it fall to zero when it is not, if the PWM signal is getting to the relay.
The high speed signal is a solid grounding of the control line (the other side is bus voltage). This should be easy to read with a meter.
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The way I read it… the PCM turns the ground on and off to control fan speed. More time ON than OFF = higher speed. More time OFF than ON = lower speed.
That works the relay pretty hard.
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You posted the correct information in your other thread; low speed fan operation engages @ 220F, high speed engages @ 230 F.
High speed varies from 31% to 99% duty cycle depending on coolant/transmission temperature and a/c pressure.
Fans switch off @ 214 F.
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Forgot about that. Thanks.
I was reading it as if the low speed fan could come on at some coolant temp below 225F and increase and it climbed.
Do the information you just referenced agree with the excerpt info I posted and I’m just reading it wrong?
The temperature values you posted don’t seem right, is that for a Caravan?
Yes.
This one is a 1999 Grand Caravan.
I also have a 2006 but have not checked to see when the fans come on and off on that one.
which part doesn’t sound right?
Do you have a Caravan?
The prose is very confusing, I can’t understand it. I might be able to offer up a little help about the nomenclature. “SSFR” means solid state fan relay. A regular relay uses a magnetic field to mechanically move a contact. There’s a limit to how fast it can be switched on and off, the inertia of moving the contacts for example. A solid state relay uses a silicon controlled rectifier (SCR) (sometimes called a thyristor), which accomplishes the same thing but there are no moving parts, so can turn on and off very rapidly. Imagine you had a garden hose connected to a faucet that you wanted to control the flow. But say the faucet only had two positions, full on, or full off. If you could turn the faucet on for 30% of the time over the course or a second, b/c the flow rate at the end of the hose is averaged out by the volume of water in the hose, you’d get roughly 30% of the full flow rate out of the end the hose. So that’s basically how it works, the % of time the SSFR is on changes the average voltage power input to the fan, which changes its speed (rpm). The fan presumably is a DC motor, and DC motors generally run at a speed proportional to their voltage input.
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It’s solid state electrical conduction path versus mechanical contacts in conventional relay. No wear and tear in switching. But this one may be even more integrated if it converts to an analog voltage based on pulse width input to drive the fans. The description, to the OP point, is not really clear…
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With the absense of an actual schematic diagram, I suspect it works like this.
The fan has two parallel circuits. When both are active it is called high fan. Both ground through the SSFR. At 102°C the SSFR becomes always on, letting one fan circuit operate continuously. At 207°C, it closes another relay to switch in the second fan circuit, putting the fan in high mode. It also has mechanical relay called the high speed ground relay in parallel with the SSFR to reduce the diode drop of the SSFR and take the load off it when the fan is in high mode.
Not sure how it works on OP’s vehicle, but I’ve seen some engine compartment fan circuits configured so the fans can be powered in parallel or in series. They run fast in parallel; in series each gets just half the power, so they run slower.
When I have tough puzzles on my Plymouth and Chrysler minivans, I post at the allpar.com minivan forum. Often an experienced voice answers.
The fan relay is a large transistor, fan speeds are controlled by the PCM and completely variable depending on heat load.
The fan relay is duty cycled @ 30% for low speed, 31 to 99 % is considered “high speed”.
