My friend called me yesterday to say that his cooling fan was running continuously. My first thought was to disconnect the sensor on the top right side of the radiator. He did that and the fan stopped.

I went through my parts and found a used sensor that I had saved while stripping out another R/R car. I dropped that in the mail and hopefully he will get it tomorrow.

My question is has anyone here experienced this problem before and did you find the aforementioned sensor to be the problem?

I know the relay controlling that fan is located behind the passenger's headlight in front of the battery, but I didn't have one of those to send with the sensor.

Randy
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Any twenty minute job is just a broken bolt away from a three day ordeal

Your friend can just pull both connectors off and check continuity between the sensor leads If there is continuity, then the switch is faulty and stuck On due to component failure. Had the same problem with my Volvo. Most later models used the ECU to control the cooling fan..

With Regina systems (and as well the early LH systems) where there is a thermal switch inserted in the upper right corner (passenger side) of the rad, about the only way the electric fan can operate continuously when the ignition is on is when the thermal switch on the rad is stuck closed -so your diagnosis and remedy is likely spot on.

If it happened to also run continuously after the ignition is off then both the relay and sensor would have to be gone and both stuck closed, so not a common situation.

For later 900 LH systems where there is no thermal switch in the rad, the story is more complicated where the ECU and an A/C pressostat are both able to control fan operation through the relay. Continuous running in that situation would most likely be a stuck relay -see the FAQ for more detailed diagnosis.

For the early 700s the relays are in the relay tray (front row, leftmost socket). In the later 700s they may be found tied under the right (passenger) side strut tower brace. In the 900s they're up by the right front headlight, as you note.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Thanks for you reply.

I realize now I failed to explain that the car was not running (ignition switch off) and the fan continued to run.

Disconnecting one of the leads on the thermal switch on the radiator caused the fan to stop (the fan would once again run when the wire was reattached).

Isn't the normal position of the thermal switch OPEN and when the coolant temperature reaches a point that the fan is needed that switch closes that part of the circuit and the fan comes on?

If that is in fact how it functions would it be that it is the thermal switch that causes the relay to engage and supply power to the fan?

I have not had the experience of hearing one of those fans continuing to run on a hot day after switching the ignition off, but it would seem that might be the logic of the circuitry.
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Any twenty minute job is just a broken bolt away from a three day ordeal

Post removed.

Yes, the thermal switch is normally open and closes when temp rises above the set point to call for fan operation.

First off, now that you say it's running with the ignition off, I have to ask when the rad has had a chance to cool and the ignition is off, is it still running? If it eventually goes off then the temp switch is okay and it's now almost certainly the relay, which is indeed a common failure point, end of discussion (hopefully).

Now if it's still on when the engine is cold (and the ignition is off), but disconnecting the switch now turns it off then it's almost certainly a bad temp switch as I noted before. With the ignition off, a good relay being able to be switched on and off by disconnecting a bad switch doesn't make sense as power for the control side of the relay originates through the ignition switch in all designs I'm aware of.

Just FYI here, for the single speed electric fan, I'm noting an additional design from what I mentioned before and what's in the FAQ. It does not change the above story though in regard to the problem at hand. In some of the later 740s and early 940s (and this may only apply to Regina systems), the temp sensor in the rad may be used as the relay control. With some of the earlier 700s (and this may only apply to LH systems as I'm looking at a 1989 740 green manual, the most recent I'm able to find for 700s) the relay control is direct from the ignition switch (with the relay always energized in KP-II) and the rad temp switch sitting between the relay and fan motor to turn it on and off.

From what I can tell, it's when they went to the double speed electric fan that the rad temp switch was done away with and the ECU and an A/C pressostat were put in charge of fan operation.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

"From what I can tell, it's when they went to the double speed electric fan that the rad temp switch was done away with and the ECU and an A/C pressostat were put in charge of fan operation."

Then perhaps the 1988 (ish?) 760 is an interesting hybrid between the two with both a rad thermostat and ECC in control of the cooling fan. The A/C pressure switch is connected to the ECC.



This is from the same diagrams of the 760 as in the "Fog lights" thread.

Yes, interesting. In that 760 configuration both the ECU and rad temp switch can run the single speed electric cooling fan. Note that this is a 1980s 760 with the magical ECC (Electronic Climate Control) and not the usual 740/940 ACC (Automatic Climate Control). The B230FT shown there for a late 1980s 760 would have had an LH2.2 ECU and it's shown having the fan control on pin 30 (shared with the rad temp sensor, either one can provide ground to energize the fan relay). I note that for a 1989 740 B230FT with LH2.2 that pin 30 is unused and the rad temp sensor is the only thing that controls the fan, with the temp switch sitting between the energized relay whenever the ignition is on and the fan motor.

I haven't been able to find a full Volvo green manual for a late 700 with Regina, but I just stared in more detail at some 1993 ECU pages for a 940. As expected, the Regina ECU does not appear to be involved in cooling fan operation. My only question remains whether in Regina cars the rad temp switch is between the fan relay and the fan motor (as in the earlier single speed fan LH 2.4 systems), or whether it acts as fan relay control (as in that 760). It could be wired either way at the relay connector and the fan would function the same. I would expect if the relay had been moved from the dash relay tray to up under the hood for Regina that for the sake of wiring efficiency it could easily be wired as relay control (which is what I see suggested in that crude Chilton's early 1990s schematic I mentioned).
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

“ fans continuing to run on a hot day after switching the ignition off”

Often the case, after switching off the car the temperature rises due to no cooling fan running, and the fan runs until the temp drops below the level to trigger the fan.

Curious, have you seen that behaviour of the fan staying on for a few moments after turning off the ignition happening with a Regina car? 740, 760, 940, 960? And you know it has the rad temp switch? I'm only familiar with the 900s being able to do that with the later two speed fan system where there's no rad temp switch and the ECU is in charge of fan operation, only being able to do so when the ignition is on, but with an A/C pressostat switch still being able to keep the fan on for a few moments longer if the A/C system still has pressure. That typically lasts no more than 10-30 seconds after shutdown, at most a minute or so. Anything much more than that could start to drain the battery or become a safety hazard if the fan started up on its own with no warning. The later 700 wiring schematics I'm looking at don't suggest that's even possible when the ignition is off. Unfortunately I don't have access to proper Volvo wiring diagrams for the final 700 years to be able to verify such behaviour is possible.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now