Hi there - 1987 Porsche 911 Cab. The interior heater doesn't work. Pulling the red levers up and rotating the switch to any position other than 0 or defrost causes nothing to happen. I took apart the center console between the seats. My first suspicion was the switch being faulty. I used a multi meter to check the connectivity. With the switch in the zero position, the resistance measurement was infinity. With the switch in the 1, 2, 3 positions the measurement was around 4 Mohms. With the switch in the defrost position the measurement was very low resistance, like 0.4 Ohms. So from this I conclude that the switch is probably working ok. The reason that I suspected this was that I have unfortunately spilled coffee down there on more than one occasion so I thought there was a good chance that the switch had failed.

What should I check next? Is the blower motor in the engine compartment the next logical debugging step? I also saw something online about a cable that opens the flapper boxes that also doubles as the electrical connection (https://cdn4.pelicanparts.com/techar...s_med/Pic4.JPG). Any thoughts to help me with my debugging would be greatly appreciated!

 

you should have heat regardless of fan operation.

jack up car . flapper boxes easy to see with wheels off the ground, check to see if they open close when you pull up red handle, they could be stuck open, allowing heat to escape or cable is broken/ disconnected. Will also confirm the heater hoses are connected to heat exchangers

 

^^^ true on the heat issue, once you get all the fans to blow. So if you get the fans blowing and you're blowing cold air, check the heat exchanger flappers.

4 MOhms (4,000,000 Ohms) is effectively infinity. 4 mOhms (0.004 Ohms) is effectively a closed circuit (conducting, on). So if you really got "around 4 Mohms", that's a problem.

In general, speaking only from having replaced all 3 blowers in my '85 Cabriolet, there is one blower in the back on the upper left side of the engine behind the black plastic ductwork with the Mobil1 or Shell advertising stickers, and one footwell blower on each side, behind a hard carpeted panel. The footwell blowers typically fail due to the felt washer losing its lubricating oil; I think better maintenance can help those live longer. And the rear motor makes a scary sounding screeching noise when it fails - the bearing / bushing fails.

But it sounds like you have an issue getting any of the fans to blow. Of course fuses would be first to check, switches second. At some point, you can access each of the fans and see what they do when you apply 12V.

Also, careful on the year when getting advice. I'm pretty certain that between my '85 and your '87 there was a major technological upgrade (LOL) with, for example, automatic temperature regulation.

I think cabriolets, and probably targas too, have their HVAC blowers die earlier than coupes do. They put out a lot of heat, and I'm sure in a coupe, you would not have them cranking away for very long. But driving in cool weather with the top down, I have them running at full heat all the time.

 

Thanks for the advice. Fuses all look good. I used the multi meter again today. I used the circuit test mode instead of the impedance. At certain positions of the switch I can measure connectivity between the leads. At this point I suspect the switch is ok.

I definitely remember hearing a sound not unlike a demon screeching from the depths of hell prior to the blower failing. So that sounds like evidence of the blower in the back failing based on what you said. I unplugged the 2 prong electrical connector to the blower int he engine compartment. I put the multi meter probes into the plug, but I can't measure any kind of voltage drop across the leads. Not sure why. The key was in the on position and the heater control switch was on 1 or 2. I wish I had an external power supply to try the blower motor.

Thinking about buying this motor and replacing it: pelican parts replacement motor

 

Thanks, I will try that. When I pull the red lever nearest the drivers seat I can hear something opening.

 

Look under the rear of the motor and inspect the condition of the heater hoses that connect the heater box to the flapper box.

Stock hoses rot out over time.

 

Thanks. I remeasured with the multi meter, this time on the circuit tester setting. I was able to measure a connection between the wires (multi meter beeped) when the switch was in certain positions. I pulled the wiring diagram from my bentley shop manual. All the wire colors in the diagram are different from reality, not too sure why that is. But some of the connections in the wiring diagram made sense with my multi meter measurements. At this point I think the switch is ok.

I do remember something making a strange howling noise when I engaged the heater with the red leavers. Based on what you say above I'm guessing this was the bushing in the blower motor. I pulled the 2 prong plug out of the blower motor. I tried to measure the voltage across those wires (key in on position, heater switch on position 2, red levers pulled up), but couldn't get reading. Not sure why. At this point I think I'm going to try a new blower motor. I found this one on pelican parts, seems like an ok deal.

You mentioned a 12V source - can you recommend an external power supply? Any old one do ok or is there a specific model you would recommend?

Thanks!

 

Before you go buying a new motor, please check them (engine blower and two foot-well blowers) with direct power to see if they run.
The switch in the center console rarely fails.
The footwell blowers will not run if the engine blower is not running.
There are fuses at the front and back (engine compartment) fuse blocks for the system (see your Bentley manual to be sure)
I believe an '87 will have fuses at the footwell blowers too.

There is a heater control unit in the relay panel at the left side of the engine compartment.
This is a common failure point for these aging systems.
New control units are pricey, old units can be modified with "jumpers" to work.
There are some methods that do not work correctly, and some that do.

There are dozens of threads here on Rennlist and at Pelican that address this "unique" heater system.
I'll try and dig up some of the good ones when I get done with work stuff.
Grab a beer or two and enjoy the reading ..

 

In your OP do you say that the system runs in "Defrost" (#3) setting?
If that is the case you might have a bad fuse that you missed.
The high speed (Defrost / #3) runs through a separate relay at the front (trunk) relay panel and a separate fuse.
It still needs the controller in the engine compartment, but runs the (high speed/full amperage) fan power through a round relay & a 25A fuse.
So if the high speed is working then there is a chance there is a bad fuse for the lower speed settings.
FYI...Speeds "0" & "1" are the same speed for the footwell blowers, "0" is not off.

 

Sounds good on the re-measuring. Thinking about it, it would be a pretty specialized meter to measure mega Ohms, so your measurement was probably 4 milli-Ohms, = a closed circuit, so current flows and things should be getting power.

I definitely would not buy a new blower without putting 12V on it to confirm it has failed. Should be pretty easy to do. But the screeching noise is a pretty clear clue. Thing is though, it sounds like if that is the problem, it can't be the only problem you have, so maybe it is not a problem at all.

For a 12V source, one way to do it is to use jumper cables to bring battery voltage back to the engine, then alligator clips to get them on the blower.

There are of course far easier ways as well. I have a spare (old, but not traded in yet) battery I keep in my garage for this sort of purpose, and also to make buying a new battery more convenient (trading this one in for the core rather than the one I'm replacing).

If you have 12V cordless tools, you could do what I did when troubleshooting an issue on my Cayenne. Of course thicker wired alligator clips would be good due the higher current draw on the blower, and I would not run it too long in case that little battery does not like the heavy draw:


Here are some pics (many years old) of the original rear blower I removed. Shows the electrical connector, part number, and also how the bushing failed and the rotating shaft partially ate away at the casting (clear as day in real life, but sorry that last photo is blurry) - that would be the screeching noise. My understanding is that failure mode is common on the blowers.

LOL, I still remember when I first noticed it, I was stuck in very slow traffic on 101, could not get over, and could hear this intermittent noise coming from the engine. Freaked out until I noticed that it went away when I turned the heater off so I could hear better.

 

One of those jump start batteries is really handy as a portable 12V source. You can get a lithium-ion one that is very small and comes with battery jumper cables, USB charging, and a 12V socket. Something like this:
I use mine all the time, including powering a QuickJack.

Mark

 

thanks Mark - I'll try that out!

 

thanks for posting these pics. Those paperclips into the dewalt battery are pretty creative lol

 

I put 12V on the blower motor directly and found that it didn't turn on. Changed the polarity also and same result. Diagnosis: seized blower motor, based on the strange noises it was making prior to quitting and the fact that applying 12V directly won't make it run.

 

Update: I replaced the heater motor and now the car has heat! Thanks for the all great advice I got on this thread. Here are some pics.


The old ceased up motor, disconnected air conduit

two mounting screws on right side, one one left (not shown)

Blower motor removed

Apply 12 V to the motor, it does not function.

Old motor on left, new motor on right

Remove clasping mechanism from the old motor

Install clasping mechanism on new motor

New motor installed