Intermittent Heating System operating

[NathanR]

Last night I drove the Carrera to a PCA event and the windshield got nice and frosted by the time we were ready drive home. I started the car and pulled up the heat levers. Rear Heater blower activated, no problem. Turned footwell blower rotary knob from zero and footwell blowers activated (were audible) until I hit setting "3". Then all three fans shut down.

Figured I must have blown a fuse. Revved engine to 1.5k RPMs to get the engine cooling fan to force a little air through the system until I could see. Started driving through parking lot and once engine RPM was a little higher, whole system kicked back on.

When waiting at a light with engine idling, entire system shuts down. Once underway, system kicks back on.

Related info:

- Both headlights and rear window defrost also on. Electrical system voltage level (measured with escort radar detector) was 12.8v at idle

- I've previously retrofitted a non-OEM rear heater blower motor and one of the two footwell blowers I replaced with an AC condenser blower motor.

- I've read lots of threads as to why the footwell blowers might not work without the OEM rear blower motor in place -- that's not my problem. ALL THREE BLOWERS are cutting out and cutting back in simultaneously.

If time permits, I'm going to go poke around and see what I can see today. If anyone has run across this before, or has some informed speculation as to what to check, would love your insight.

I'm contemplating removing the OEM heater system control module altogether and replacing it with the 2-relay solution posted in a few threads.

Thanks,
Nathan

[86 911 Targa]

pm sent.

[Duc Hunter]

Sounds like a lack of amperage with everything one to get the footwear blowers amperage sensitive relay to kick on.

[DRACO A5OG]

"Non-OEM" Blower Motor, DING DING DING.

I did the same, and it caused Havoc. I swapped to factory motor, issue is no more.

A few of us are un-lucky when it comes to non OEM blower motor.

[NathanR]

I was recently reminded of the theory that the engineers wanted the heat exchangers to be under positive pressure -- I've heard before and find a bit curious. Here's why: Think of what the back-pressure in the exhaust manifold must be compared to the static head generated by the rear blower. My sense is the exhaust manifold sees MUCH greater pressure. So, if there is a crack in the manifold, it would leak into the heat exchangers regardless of which fans were running. Just my sense of it.

[NathanR]

@ Draco -- this will be really easy to test, and seems very plausible. I will report back once I have observational data

@Duc -- in theory the rear blower is running and either drawing enough current to close the reed switch, or not, impacting only the footwell blowers. in my case, none are working.

[DRACO A5OG]

If you do get an OEM motor, keep the receipt handy, They are not built like the original and may fail within two years. Ask me how I know.

Purchased from our host, 2.5 years later if failed and was not under warranty, oh the Joy.

[wwest]

"Refresh" the fuse connections in the engine compartment.

[wwest]

Originally Posted by NathanR

I was recently reminded of the theory that the engineers wanted the heat exchangers to be under positive pressure -- I've heard before and find a bit curious. Here's why: Think of what the back-pressure in the exhaust manifold must be compared to the static head generated by the rear blower. My sense is the exhaust manifold sees MUCH greater pressure. So, if there is a crack in the manifold, it would leak into the heat exchangers regardless of which fans were running. Just my sense of it.

The concern is to not intake the air in the wheelwells that by happenstance might be contaminated with exhaust fumes. Setting idling for a period of time, for instance.

[aj88cab]

This is an edited post after I re-read and realized entire system is shutting down not just FW blowers...if you didn't read my first pass...never mind

Look at the engine blower, as discussed if that stops the system will not work. Also it could be the controller relay in the engine compartment.

Check all fuses and connections as previously mentioned.

[wwest]

There have been instances wherein at idle the 12 volt supply is just a tad under the level required to drive the engine compartment blower at full speed, not enough current flow in the reed relay to allow the foot well blowers to operate. In this case the engine compartment cabin heat blower runs but only rises to full speed with higher engine revs which raises the alternator output voltage.

Just say'n.

[NathanR]

I ended up not having the time/gumption to really work on this last weekend, but THIS weekend I'm all over it. Will report back when I can

[NathanR]

Had time to poke around and see what the problem could be.

Could not duplicate original symptoms. I *think* I was mis-perceiving what was really happening.

With the engine off, I did achieve the well-known footwell blower shutdown with low voltage, but rear blower stayed on throughout. I'm chalking this up to not being able to really hear the rear blower with the engine on. At the time, I figured the light flow at idle was due to the engine fan.

Instead, what I think was really happening is what so many others have experienced. At the time, with engine at idea and a full electric load on (headlights, rear defroster, stereo, etc.) there was not enough current through the rear blower to activate the reed switch in the heater control module. When I revved the engine, the footwells would come on due to higher output from the alternator causing higher system voltage, causing higher current flow through the rear blower, causing the reed switch to close.

So, I tried the reed switch bypass and it worked like champ. Problem solved.

Thanks for your patience.

Cheers,
Nathan

[wwest]

I would suggest adding another winding to the reed relay rather than having, risking, carbon monoxide in the cabin.