Try to put heater back: rear air blower, which wires?

[Vincent 911]

Almost ready for the spring. After a top end rebuild, suspensions ,brakes, turbo tie rods and engine upgrades I am trying to put the heating system back in my 78 SC ROW. It is still chilly in MI. I have bought a 'working' rear blower from a Pelican fellow few months ago.

Yesterday evening I tried to get the right power supply wires in the engine compartment in order to have my blower working. I have a bunch of wires available: total 8. None of them have the color of the wiring diagram...

How / when the rear blower is supposed to work? I have tried these following combinations to get 12V:

1- Ignition ON + engine not running + black lever by the hand lever pulled up: 0V
2- Ignition ON + engine not running + black lever by the hand lever pulled down : 0V
3- Ignition ON + engine not running, black lever by the hand lever pulled up, heater levers on dashboard 'ON': 0V
4- Ignition ON + engine not running, black lever by the hand lever pulled down, heater levers on dashboard 'ON': 0V

What should be the right combination(s)? Maybe I have forgotten a lever / button or the engine needs to run to have the rear blower working?
I have checked the fuses and seem to be all good.

Thank you for your help.

Vincent

[2ndTARGA]

On my '77 the blower runs without the engine running. Key on and either handle up and it blows.

The blower wires come off of the engine wiring harness near the left rear of the fan housing and are in a black plastic tube/sleeve. One side (GND) is made up of two brown wires crimped into the connector. The other is a single yellow/black wire.

At least on my car, the power is fed via the middle of the three fuses on the engine relay panel. Check to see if you're getting operating voltage there and you can probably run a wire directly to the fan motor.

Ron

[Vincent 911]

Originally Posted by 2ndTARGA

The blower wires come off of the engine wiring harness near the left rear of the fan housing and are in a black plastic tube/sleeve. One side (GND) is made up of two brown wires crimped into the connector. The other is a single yellow/black wire.

Ron, I do have these 2 wires: brown + black & yellow: 0V.

I will double check the 3 fuses in the engine compartment. Where is this relay located?

[2ndTARGA]

Vincent:

On mine, it's right on top of engine relay panel. Just above the 14 pin connector on the right. In fact, it's the only round black relay my car has in that panel. There are three other holes punched in the panel but no other sockets. I'd guess these are for other accessories I don't have such as rear wiper, etc.


You can probably troubleshoot by swapping it out for one from the front that you know to be good.

Ron

[3.2 CAB]

What the heck is OV?????? Maybe I just don't understand your terminology. Your dash controls have no effect on the engine compartment blower. If you have the two correct wires going to it, it should work, unless your heat lever switch, or the motor itself is bad. Have you tested for voltage at the fuses in the engine bay? There is a small board that holds 3 fuses, one of them is for the rear blower motor. Have you tested the motor with any 12volt power source to see if it was working or not?

[Vincent 911]

OK, I just come back from the garage. Here is what I found:

1- I found the blower relay, it is the only black round one in the engine compartment like 2ndTarga wrote

2- I have jumped the relay: +12V on PIN 30 and the blower ran right away. Good.

3- I found out I have no power on the yellow wire of the heater lever by the E-brake. According to the wiring diagram I am supposed to have +12V on this wire. Bad.

I will continue tomorrow: why I don't have power on this yellow wire? If I don't find I will add a wire from the dash board (+12 V ignition ON).

Thank you,

Vincent

[2ndTARGA]

Vincent - HOLD ON!

I think you're on the right track with tracing the problem, but hold on a minute about putting +12v to the floor lever switch. Looking at the factory diagram for US 78 models, I think the switching and relay operation is a bit more complex.

Look back at your relay diagram above. The yellow wire you are talking about from the switches is at the top labeled 85. That is one side of the relay coil. The other side is 86 and this is the HOT side, coming right from the fuse. So in fact, feeding +12V to 85 via the lever switches is not going to fire the relay.

What is supposed to happen is that the floor switches actually connect pin 85 to Ground, thus turning the fan on. It's true that the diagram shows the yellow wire from the switches going to the ignition switch, not to ground, but it goes to pin 50 of the ignitions switch, which is the momentary Start position.

So here's how it works:

With ignition on, and the lever off, pin 86 is hot and pin 85 is floating - no fan operation.
When you operate the floor switch, pin 85 gets connected via the yellow wires to a ground (looks like it may be through the starter solenoid coil.) This fires the relay and the fan comes on.
In order to save maximum cranking power for the starter, when you turn the key to start, 12v is connected to the yellow wires and now both 85 and 86 are at +12. So the relay disconects. This temporarily shuts down the fan while the starter is working.

So double check everything as follows:

Pull the relay and check to see you have +12 on pin 30. This should be hot all the time and comes through the middle (blue #23) fuse just below the relay. If you then can jumper this to pin 87, your fan should run.

Next, check for +12 at pin 86. This should be hot only when the ignition is ON. This comes through the Yellow fuse 22 which is in the top slot just below the relay. Do you have that fuse?

Finally use an ohmmeter to check between pin 85 and ground. You should find there is no connection to ground when the lever switches are off. When you pull up on the switch, you should see pin 85 go to ground.

Hope this all makes sense. It seems to me that either you are missing the top fuse, or that there is a problem with the yellow wiring somewhere. BUT, connectng +12 to the yellow wires is not going to solve the problem and could mess something else up.

PM me with your phone # if you want to discuss.

Ron

[3.2 CAB]

+1 with Ron, that heater lever switch, just completes the ground circuit. I believe it is the same principal as the handbrake lever switch, it just completes the ground circuit.

[Vincent 911]

Originally Posted by 2ndTARGA

Next, check for +12 at pin 86. This should be hot only when the ignition is ON. This comes through the Yellow fuse 22 which is in the top slot just below the relay. Do you have that fuse?

Ooops, no I don't have this fuse. I will continue to work on this issue tonight and make sure I have +12V on pin 86.

Thank you very much for these remarks. I was ready to put +12V on the yellow wire...

Vincent

[2ndTARGA]

Good news about the missing fuse - hopefully that's the ticket.

On further thought I'm pretty sure that putting +12 to the yellow wire would immediately fire your starter - not fun if you happend to be in gear!

Let us know how it goes.

Ron

[2ndTARGA]

Vincent: OOPS!

I guess you can't always believe the wiring diagram.

My wiring diagram showed the fuse that feeds pin 86 to be yellow S22 and in my car that is on the top. So I told you to check for that one being missing.

However, I just tried pulling that yellow fuse and it has nothing to do with the fan. It's the BOTTOM fuse that feeds pin 86. This is supposedly fuse S24 (Rear Window Defroster) but it ovbiously feeds pin 86 on my car. (The top fuse slot may be for sport-o-matic according to the plastic cover so it's fine if it's missing)

Sorry for the confusion, but the main thing is to see if you have +12 at pin 86 when the key is on and ground at pin 85 when the switches are pulled up. If both of these are true (and you still have power to pin 30) then you have a bad relay. If you can't get a ground on pin 85 then there is a problem with the yellow wires that go to the switches, then back to the engine comparment to be joined with the starter circuit at the 14 pin connector.

Ron

[shoguneagle]

You have been most helpful with the information in this thread. I am doing a 3.2 conversion into another type Porsche and you have simplified my understanding on what is going on with the 911 engine fan system. Thanks for the very informative and understandable information along with valid reasoning. One less hurdle out of the way to complete the conversion.

Steve Hurt

[Vincent 911]

It worked! Now the blower is working properly: ignition ON and heating lever down. I put +12V on terminal 86 and changed the relay and the blower started right away.

Thank you for the very good comments

Now it is time to fix the front blower...

BTW I replaced also a rear wheel bearing yesterday evening. The car is now back on its for wheel: time to go for a ride.

Vincent

[3.2 CAB]

Vincent, did you mean to say the heater lever UP? Because, it appears that you are now having the blower come on with the lever in the down position?? This is why I ask? Tony.

"It worked! Now the blower is working properly: ignition ON and heating lever down. I put +12V on terminal 86 and changed the relay and the blower started right away.

Thank you for the very good comments

Now it is time to fix the front blower...

BTW I replaced also a rear wheel bearing yesterday evening. The car is now back on its for wheel: time to go for a ride.

Vincent"

[Vincent 911]

Originally Posted by 3.2 CAB

Vincent, did you mean to say the heater lever UP? Because, it appears that you are now having the blower come on with the lever in the down position?? This is why I ask? Tony.

Actually I don't remember if it is up or down. I will double check tonight and let you know.

Vincent

[Vincent 911]

Originally Posted by 3.2 CAB

Vincent, did you mean to say the heater lever UP? Because, it appears that you are now having the blower come on with the lever in the down position?? This is why I ask? Tony.

Tony, I checked yesterday and the blower runs with levers UP.

Vincent