weird cooling problem

[mikes3]

Help needed please.

93L Chim, Rover gearbox 4.0 type.

I had a heating incident last weekend (fans didn't cut in as expected in heavy traffic). A belch of steam from the pressure valve (and expletives from yours truely). Sat by the roadside for the car to cool down for a bit and gingerly got myself home once the traffic had cleared.

got a bit of wire and 'shorted' the otter switch and the fans came on fine. Had a look in the oil filler cap and dip stick etc for evidence of 'bad things' and no mayo or general nastiness.

My conclusion was a duff otter switch, got a replacement from Fernies and fitted this evening. Took the car out for a jolly (including an idle outside the garage afterwards until the heat built up). Fans cut in and out as planned. Took it for a bit more of a jolly and on arriving home again, left the car to idle outside the garage (another test), this time the fans didn't cut in and the temp went up past where the fans should cut in...

Left it as long as I dared (not much, bit of a whimp) and still no fans! Turned it off and let it cool for an hour and carefully opened the swirl pot lid and started the engine up. There was a good flow into the top of the swirl pot and when checked, the rad didn't need bleeding.
There seems to be good flow around the cooling system and I'm thinking it may be an electrical fault somewhere as it operated fine, then not after cutting in (stuck relay? Duff earth?)

Anyone got any ideas (please don't mention head gaskets...too close to Le Mans and I've checked the bits I can and will get it looked at this week)

Cheers

Mike

[2 Smokin Barrels]

I just suppose it's possible the second switch has failed?

[mikes3]

Cheapest option, like your thinking!

Found a post on an earlier thread where someone (true, they were in Hong Kong) had fitted their otter switch with a heat shield to stop it getting cooked...

No idea how you would go about it but may be worth giving it a go.

Any ideas on how to test an otter switch (in the 5 seconds before all the coolant piddles onto the garage floor!

[2 Smokin Barrels]

mikes3 said:

Cheapest option, like your thinking! Found a post on an earlier thread where someone (true, they were in Hong Kong) had fitted their otter switch with a heat shield to stop it getting cooked... No idea how you would go about it but may be worth giving it a go. Any ideas on how to test an otter switch (in the 5 seconds before all the coolant piddles onto the garage floor!

Swap it with your mate's!

>> Edited by 2 Smokin Barrels on Wednesday 11th May 22:26

[sotons2]

Shorting across the Otter switch showed that the fan worked OK so its not the fan, nor is it the 12v supply or fan earth.

I'm not overly familiar with the V8 (yet )but are you sure the pipe that houses the Otter switch is getting hot ? If not, airlock or faulty thermostat maybe ?

I think the internals of an Otter switch is simply a metal strip which 'springs' when it gets to a certain temperature and makes a circuit. You can check the switch is working by measuring the resistance across the terminals with the switch in a pan of water on the hob. Open circuit at low temp and closed circuit at high temp. Saves running the risk of overheating the Chim.

I'm no mechanic and someone more knowledgeable than me is bound to come up with a more definitive answer. The above checks are fairly easy to do though and with only a finite number of component parts within the fan circuit there's not much that go wrong really.






>> Edited by sotons2 on Wednesday 11th May 22:55

[GreenV8S]

Now you know where it is, it should only take you ten seconds to pop the bonnet open and short across the otter switch with a screwdriver. If the fans come on, you know the rest of the circuit is ok (the ignition has to be on but the engine doesn't have to be hot for this test). If the fans come on reliably with this test then the fault has to be the otter switch or connections to it. I suggest you test the otter switch you removed, to start with (pan of boiling water and a resistance meter).

It does sound from your description as if it is an otter switch problem and most likely the temperature gauge is fine, but bear in mind the remote possibility that the temp gauge is lying and the reason the fans don't come on is the engine is not hot enough to bring them on.

[mikes3]

I guess that the temp reading on the guage could be wrong but the 'steam bath' I was treated to on Saturday certainly pointed to a fair bit of temperature in the cooling system. The pipe to/from the otter switch is bloody hot (no fingerprints now!).

Is the otter switch a simple bi-metallic strip or some fancy electric rheostat or thyristor (grasping desperatly at O level physics memories)? If the latter I could understand it getting cooked (the back of the switch is very close to the exhaust pipe assembly at the front of the engine).

[sotonS2]

MGRV8 website said:

When this switch, part number BHA5252, packed up on my car I found it very difficult to get a replacement as the dreaded NLA was against the part number. I eventually found a garage that had one in stock. It looks a little like a small bowler hat! Instead of throwing the old switch away I tried to repair it with great success. The principle of the switch operation is the same as the air temperature control - a bi-metal spring that makes the switch earth when it gets to a certain temperature. To make the repair, take the lead of the connector, undo the three small bolts carefully, prise open the three lugs holding the top and pull out the centre of the switch. Wipe between the points with a bit of rough paper, but do so very carefully because if you bend the points, you will find it will not come on at the right temperature. When you have done this put the centre back, fold down the three lugs, use a new gasket (part number 236022), coat with Golden Hemetite, and then tighten down the three small bolts. Finally connect the leads and hopefully the Otter switch should work. Fortunately I now hear this part is now available again.

This is as I understood the Otter switch to work.

Simple to check the functionality of your two switches in a pan of water. Have you got a cheap and cheerful multimeter ? If not, they're less than a tenner from Maplin and are one of the most useful pieces of kit in the toolbax, particularly considering TVR wiring .

[mikes3]

I'll get the multimeter out and spend this afternoon scolding my fingers...

[rpulley]

When you fitted the new otter switch are you sure you bleed out all the air from the radiator. If you fail to bleed out all the air it collects in the radiator and eventually will prevent a correct flow of water. This can then mean that the otter switch is not seeing the true temp of the engine and therefore does not switch on the fans. The best thing is fill the system and bleed using the bleed valve on the drivers side of the rad. Then run and get it hot then lossen the bleed valve slightly (do not remove) and any air in the system will come out. Do this a couple of times and there will then be no air in the system. The other thing is do not overfill the swirl pot other wise it is possible to get it to self siphons when hot and you loose loads of water and then air is put into the system.

[spend]

Did you clean the female spades that connect to the otter switch? Personally I would replace them, it is quite possible after 10+ years of abuse they dont provide a good contact any more, and replacing the switch just made a short lived good connection.

Dave

[mikes3]

Good ideas, I think I'll replace the connectors, bleed and test this afternoon. If that fails I'll whip out the replacement otter switch and 'test' it in a pot of hot water, with a multimeter.

Hope this works out OK, Le Mans is just around the corner and cooling is my biggest stress factor!

[mikes3]

changed connectors (and the one on the temp sender as well) and bled the system (again) and all is well!

Fans cutting in and out as planned, still think I may fit the over-ride switch, more as a feel good than anything else (I can be a real stressed eric when it comes to overheating!)

Thanks to all for your help, always nice to know others thoughts rather than thinking the worst!

[spend]

Bit strange I know, but I always get the car up on ramps so the nose is high when filling the coolant and never have a problem. I change summer/winter stats religiously so do this pretty often.

IMHO if you fit an O/R switch just splice into the Green/pink wire as close to fuse box as poss and switch it to a convenient ground. I dont see the point of of an o/r switch that relies on the existing (possibly faulty) wiring so make it as redundant as possible. If you have twin relays just use a double pole switch.

Dave

[kerberus]

HI RE OTTER SWITCH, I MOVED OTTER SWITCH ON MY 94 CHIM, IT IS NOW LOCATED IN THERMOSTAT HOUSING.