what does everyone know about volvo fan relays?

[swynx]

installed a volvo fan relay and it keeps turning the fans on and off every second. the old relay setup would turn it on, till it cooled down then turn it off, and the fan almost never ran.

 

[RonD]

Relay just does what its told, in this case told by the Temp switch, Volvo relay has 12v with key on, then temp switch will ground the low or high speed relay in the two relay pack

If voltage to the 12v relays coil is low, bad + or bad - wire, then relay can't "hold", so will close and then open again

So either Temp Switch is "telling" relay to open and close quickly, or relay is bad or voltage to relay is bad

 

[swynx]

i do have the relay wired to, a probe in the radiator; was wondering if maybe thats the issue? i cut the probe off a junk setup i had laying around. i do have another volvo relay around somewhere. ill try plugging that in.

 

[RonD]

Temp sensor for the Volvo Relay need to be more like a "switch" than just a variable resistor or you will get what you are seeing

The Relay's coil gets 12v with key on, and has no ground so stays open
Adjusting ground contact is the same as adjusting positive voltage, that's how the IAC Valve works, computer adjusts ground
The temp sensor is the ground
Lets say the relay's coil needs at least 8 volts to be a strong enough magnet to pull the relay closed against the spring holding it open
As the temp sensor warms up it becomes a better ground, this has to do with the temp sensors "rating"
When ground gets to 7.5volts the relay may start to click closed and then open, it must get warmer to get to 8volts or higher

Now some temp sensors have a non linear resistance, pass thru Ground doesn't go up steadily, it will reach its Rated temp and then go to almost no resistance
So in example above it might go to 7volts at 180degF and then change to 11volts(almost full ground) at 185degF
So more like a switch than say a light dimmer or volume control which has linear resistance thru out its range

 

[swynx]

what you are describing makes sense. when i had spliced in the probe, i was worried i would have some sort of issue. seeing as tho the probe came from an adjustable setup, and the volvo relay setup is not adjustable. originally i had the volvo relay hooked up to the red/white wire for the temp guage at the coolant temp sensor. but it caused the fans to turn on and stay on. so i switched to the probe. perhaps i should try hooking it to the temp sensor wire for the ecu.

for what its worth this is on a 97 sohc 4.0 swapped into a 94 truck.

 

[Curious Hound]

Don't do that. Bad idea. Bad, bad idea. You don't want to interfere with anything going to or from the ECU.

It needs to have it's own temperature switch. The Mercedes switch mentioned in the tech article for the Volvo fan conversion works well. You can get an aluminum tee to splice into the radiator hose very inexpensively on eBay or from Summit or Jegs. Just make sure the tee you order has the right size hole for the switch.

 

[RonD]

Eric is correct, ECT Sensor is not a good idea and won't work in any case, it isn't a ground reference sender like the temp gauge sender

One other issue you can run into, the whole point of the fan coming on is to COOL the engine, well when it does the temp sensor shuts off the relay, then temp goes up and relay is on again.
The Probe type relay has a circuit that sets a Range, so if you set it for 180degF fan might not come on until 185 but will stay on until temp is below 175, so a 10deg range, maybe 15deg
But thats in the circuit not the probe, a Volvo unit is just a relay, well 2 relays, no electronics

So you need the correct type of temp sensor to use it

 

[swynx]

ive read that article before and totally forgot about those style temp sensor. now i feel stupid, had i remembered about those, this would not be a thread.