Radiator Fans

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cit5625
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Radiator Fans

Post by cit5625 » 19 May 2018, 19:27

On my Turbo 1 [1985] the two fans on the radiator operate when required but only one fan goes into full action if needed. Anyone got any ideas? Does it make a difference which wire goes to which terminal on the radiator sensor? I'm sure it must but not sure which wire goes where. In normal driving on the motorway where does your temp. needle sit...mine seems to go more than halfway between 80 and 120.Peter

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white exec
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Re: Radiator Fans

Post by white exec » 19 May 2018, 21:42

If your two fans are operated by a thermo-switch and three relays, the operation should be as follows:
- When thermo switch detects Lo (1st) temp rise, the relays should switch the fans into Series, and they will operate at low speed
- When Hi (2nd) temp is detected, relays put the fans into Parallel, and both operate at full speed.

For slow speed to operate, both fans need to be in working order (they're in series).
If one fan is non-functional (open circuit), slow speed will not happen, but high speed will (when the engine/rad gets hot enough).
If one fan is non-functional (a short circuit), low speed will result in just one fan running at full speed, and if the engine/rad gets to the high temp, the fuse should blow.

Suggest:

- Check the connections at the thermo-switch....There are 3 connections....One is either Batt+ or Gnd (depending on the car's wiring diagam), the other two lead off to the three inter-comnected relays. One of these two triggers Lo speed, the other Hi speed.
- The action of the thermo-switch can be checked by removing it and placing its end in heating water. One contact closes first (Lo temp), and then the other (Hi temp).

- These front-end relays can suffer from weathering and corrosion of their base spade connections. Pull each relay in turn, and clean these up. Also worth checking (off the car) that each replay is working properly, and that the internal contacts are also clean. They are usually standard 30/40A automotive changeover relays, and easily replaced.

Someone may be able to post a wiring diagram for your CX fans, which will confirm whether the thermo-switch switches 12v+ or Gnd.

If both the fans can be seen to operate, most likely issue will be the relays. Both fans are identical, and 12v, so they could be tested one at a time, independently.

cit5625
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Joined: 13 Sep 2011, 13:21

Re: Radiator Fans

Post by cit5625 » 19 May 2018, 22:16

Thanks for that. Thermo switch switches ground so it's most likely to be a relay problem....hey ho, I can see the nearside headlight coming out tomorrow.

marc61
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Re: Radiator Fans

Post by marc61 » 29 May 2018, 20:38

If it helps there are 3 fan relays wired to enable the 2 fans to be in series or in parallel. One of the relays is activated by the low temp signal and it puts 12V across the two fans in series, so they each get 6V and turn at a slow speed. With the bonnet closed you should see the two fans turning quietly as the engine water starts to get hot. If you just get one fan belting round noisily then the other one has packed up or its wiring has come adrift.

If you now get both fans turning quietly, then once the engine gets hotter still the high temp signal will activate the other two relays and these change it to a parallel circuit so that each fan now gets the full 12V. The noise level should go up a lot at this point with both fans turning at full whack. If one of them isn’t turning, then one of the two relays isn’t working or it’s a lousy wire/connection issue.

Usually pulling the relays off cleaning up the terminals and applying some grease will sort it. Sometimes it’s a wire that’s broken or the relay itself, in which case be sure to replace like for like relays. I can only remember once when a fan motor had seized up through lack of use.

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Re: Radiator Fans

Post by wurlycorner » 29 May 2018, 22:25

Yep, this is definitely either relay related or wiring in that area/earths from the relay. it's not the radiator thermo switch.

On a quick look at the wiring diagram, check the earth from the high speed relay (pin 5, relay number 772 in wiring diagram), the link wire from pin 2 on high speed relay (772) to pin 2 on the series/parallel relay (773), or loss of feed onto pin 5 of the series/parallel relay (773).

You probably won't find a hard fault, it's probably just verdigree, so as soon as you touch it you'll clear the fault before you've been able to test it :lol:

You shouldn't need to take the headlight out to get at them, can reach down without.
Beware - If the relays are still attached to the relay holder rail (RARE!) the relay rail will almost certainly break on first attempt to remove the relay from it.

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white exec
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Re: Radiator Fans

Post by white exec » 30 May 2018, 06:34

Three things to clean up on the relays
- external blade connectors
- relay socket connections (insert a small piece of folded fine glasspaper, with a squirt of contact cleaner)
- internal contacts of each one (cover should prise off)

If they look in bad shape, just renew all three. They're a standard and inexpensive item; any motor factor (or good electronics supplier) will stock them. Check the wiring diagram on the side, which shoud be identical, five contacts numbered 30, 85, 86, 87, 87a, unit rated at 30 or 40amp.

Very important that all three of these relays are working properly, and that their wiring is not altered. They operate interconnected, and altered wiring can result in short circuits and blown fuses.