dr bob

Greg--

Switch on top of the manifold runs fans at low speed if heat soaking gets the switch above 90ºC. Wired in parallel is a high-temp switch at the auto trans, guessing that it would draw some cool air through the ATF cooler as well as coll the radiator and the ATF exchanger in the tank.

The hood switch protects the fingers. Realistically, the manifold cools rapidly with the hood open so no need for fans then.

The alarm connection on the radio goes to ground when the radio is pulled from the original DIN bucket. It's easy to assume that the lead should be grounded when a radio is replaced, since it looks like it's grounded originally. It's not. So if/when you replace the original DIN sleeve, isolate the alarm wire or the fan controller will ASSume that the hood is actually open and limit the heat-soak protection. The alarm system ignores the hood and radio loop if it's already grounded when the alarm is engaged, so that radio lead really does need to be isolated from ground if it isn't supposed to be reporting the stolen radio. Lots of little idiot-synchrocies, great ideas when they were hatched.

Not that the alarm stuff may only only apply to S4+ cars. For sure the fan info does.

newworld

87s4 aut.

I found the hood switch on right side passenger next to sway bar mount. My hood switch was just dangling there in a hole bracket. Maybe there was at one time some mount inside the hole.

I jumpered the intake manifold switch and depressed the hood switch by hand. Both fans came on. Release switch and both fans cut off.

I ran car till coolant temp caused fan to come on about middle of temp range and hooked up intake switch and depressed hood switch by hand to simulate closed hood.

No fans. Switch is new....either temp is not hot enough....or maybe not properly grounded was my thinking.

I rigged a ground wire to switch and tried same experiment.
Nothing.

My best guess is that this switch only triggers when extremely hot or something I am missing.

Usually on other Porsches on hot days I do trigger cooling fans so I don't know if 928 fan switch from intake is rigged differently....

newworld

Dr bob.
You kind of lost me on the alarm rigging I may have to read the post several times again. If the heat soak fans are rigged to intake switch and trams switch....if trans switch is bad does that affect fan operation?

I have not looked at trans switch but given location and possible tranny oil soaks probably it is bad....

I am just pondering why heat soak fans don't trigger on hot days like we have now.

The positive is that you don't get a battery drain if switch is bad but curious why the car has not triggered heats oak fans in shutoff...

Bill Ball

The fan after run function is described in the 1987 Service Info book.

newworld

The previous owner removed the cooling fan relay and my flaps at set open.

I just read your above post...perhaps this relay needs to be in place for the circuit and heat soak fans to come on.

dr bob

Sounds like the fans did just what they are supposed to, based on your info in post 17 above. You went further to say that you ran the engine to get th etemp up, gauge at mid position. Fans might or might not run then depending on the temperature of the coolant coming back from the radiator. Jumpering the intake switch while the engine is running will NOT cause the fans to run.

It sounds like everything is working as it should. What is it that you expect to happen that isn't?

newworld

Dr b.

87s4aut. What I was expecting is that after hot shutdown with temp from mid to upper range and fans on while engine running. I would expect when you cut the engine off the fans should come on to prevent heats soak.

Each time I shut off engine fans also shut off. If I turn key to first position fans come on.

I have never had the car with fans running after shut off...I just find that odd compared to how other Porsches are wired. We are also having 100 deg temps outside

That's why I was looking into hood switches and other things like cooling fan relay as to why fans don't come on during heats oak.

dr bob

Probably the easist test at this point might be to pop the hood immediately on shutdown, and take a temp reading of the manifold with an IR temp gun. If the cooling system is working OK, the thermostat is regulating block temp at 85ºC as you drive. Fans are running as you drive, air from the radiator up over the engine will be no hotter than that. Consider also that the intake has close to ambient temp air passing inside the runners with the engine running. So the intake metal starts out south of coolant temp. Unless you have a huge amount of heat migration from the exhaust side of the heads, the intake won't be hot enough to need the fans. The IR temp gun will give you an idea of whether the intake really is hot enough to need fans.

FWIW, I worry more when the fans do run on after shutdown than when they don't. If my driving temps are staying pretty much between the white lines on the gauge, I really don't expect the fans to run on after engine stop.

Alan

Guys - it has to be really really hot for the after-running to happen.

In Phoenix in the summer time - it sometimes does it - at 110F - But you must have driven at high speed and quickly come to a stop to see it.

There are many places where realistically this is never - ever - going to happen - unless there are cooling functionality issues This is to protect against extremes - not regular every day driving...

If you want to provoke it - pull off the temp sensor connector on the top of the intake and short together the connector leads. Ignition on/off to set your 45 minute interval - and test away...

The fact that the fans are running on high when you turn off the ignition - likely just tells you that your AC was on - Not that the car was particularly hot...

Alan

BTW if this doesn't kick in within a minute or two of shutdown it likely won't - and 45 minutes was way too long - 10 mins would be more than adequate IMO.

When it runs - it runs for a minute (or much less) - doesn't take long to cool the engine with some airflow.