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Discussion Starter #1
My dad's 95/96 405 diesel estate has a mysterious problem. The temperature gauge stays on minimum and the cooling fans are on all the time the engine is running. There's plenty of coolant in the system and the engine feels quite cool. The connections around the thermostat housing all look OK.

The air bag light is also on. Could that be a related problem if it's an electrical fault?
 

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ngolian said:
My dad's 95/96 405 diesel estate has a mysterious problem. The temperature gauge stays on minimum and the cooling fans are on all the time the engine is running. There's plenty of coolant in the system and the engine feels quite cool. The connections around the thermostat housing all look OK.

The air bag light is also on. Could that be a related problem if it's an electrical fault?
For the cooling fans.
cars without a/c, and later ones with, have a thermal switch in the left side of the radiator which controls the fans. to check the switch you can unplug it to see if the fans stop, and bridge the two contacts to see if they run. if they still don't run check the contacts, one should be live and the other to earth.

there are also relays (3 i think)would only take a problem with one of them in the system to cause the problem, these may be located behind the fuse panel.
i hope this is of some help.


As for the airbag, try disconnecting and cleaning the connectors under the seats then reconnect them this is sometimes all that is needed
 

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Discussion Starter #3
It's a late model for a 405, with A/C. We can't see a connector on the left of the radiator but my dad said he thinks he can see one on the right but can't reach it. There are 2 or 3 connectors on what looks like a thermostat housing sticking out of the cylinder head. Pulling one of those off with the ignition switched off caused the fans to run again; I know some cars run the fans for a while after switching off a hot engine to avoid heat soak, so that must be what that connector's for.

I suppose the most logical explanation is that there's a shared temperature sensor for the gauge and to activate the fans, which has failed, and the fans are running as a fail-safe. And the air bag fault must be a coincidence.
 

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ngolian said:
It's a late model for a 405, with A/C. We can't see a connector on the left of the radiator but my dad said he thinks he can see one on the right but can't reach it. There are 2 or 3 connectors on what looks like a thermostat housing sticking out of the cylinder head. Pulling one of those off with the ignition switched off caused the fans to run again; I know some cars run the fans for a while after switching off a hot engine to avoid heat soak, so that must be what that connector's for.

I suppose the most logical explanation is that there's a shared temperature sensor for the gauge and to activate the fans, which has failed, and the fans are running as a fail-safe. And the air bag fault must be a coincidence.

the sensor in the radiator is acting intermitantly. check/ change the thermostat and the fan sensor switch. I don't know how much they would cost £10 - 20 ish?
 

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It doesn't seem to be an intermittent fault. The trouble is we don't know which sensor to replace; lost the Haynes manual :(. Do you mean replace the wax thermostat? It always did run a bit cold if the gauge was to be believed, and it doesn't feel very warm under the bonnet now.
 

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ngolian said:
It doesn't seem to be an intermittent fault. The trouble is we don't know which sensor to replace; lost the Haynes manual :(. Do you mean replace the wax thermostat? It always did run a bit cold if the gauge was to be believed, and it doesn't feel very warm under the bonnet now.
This is long winded so bear with me-
Cant find any diagrams on this model so can only offer general advice.
I will assume this car doesnt have a engine management system, also that there are 2 sensors, one for a/c and cooling fans, one for temp guage. This early type of guage can suffer from needle changes due to battery volts variations, so a regulator is put between battery and guage and guage is supplied with a set 10v say. Route below:-
[ign +12v]------[10v regulator]------[+guage-]------[sensor]-----[gnd]
If you can put a digital voltmeter between point +guage and ground and measure the voltage change when guage goes hot, you can determine where the problem lies. If voltage stay stable or falls, fault is in guage or sensor or line between. If voltage rises fault is in regulator or wiring between guage and regulator.
If you measure voltage between guage- and gnd and check when needle goes high. If the voltage drops sensor reading higher water temp or sensor faulty.
 
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