footwell blowers

[85911]

I have an '85 911 and the 2 footwell blowers stopped working after several years of running fine. My main heating blower in the engine compartment runs when either of the heating levers are pulled up. When I turn the center dial which has settings at 1,2, and 3, the footwell blowers do not come on as they have in the past.

My Bentley manual addresses only the automatic heating system for '86 and up. I have a manual heating system.


I seem to recall reading that there is a relay in the engine compartment that is made when the main fan is turned on which allows the footwell blowers to engage when the center dial is turned to one of the 3 positions.

I also found the footwell blower resistor in the luggage compartment on the right side. I tried to take off the 3 contacts to put an ohm meter on them, but quit when they resisted. The resistor looks original and I didn't want to tear it up.

I would appreciate any advice on how to diagnose this issue.

Thanks.

[redridge]

there is a limit switch at the right side heat lever,
that switch will energize the relay, once the rear blower turns on. Once the blower is up to speed, the relay detects an amp draw from the blower... that is the permissive to turn on the footwell blowers.

[dw1]

As with the manual heat in my '87, there are 5 things to check:

The heater switch in the center console.
The engine compartment relay.
The fuse (plug-in type) on the blower motor wiring
The blower motor itself
The resistor (very unlikely the problem but what the heck)

The problems with mine turned out to be mainly, but not entirely, the relay.

Because a replacement relay is almost impossible to get I opened it up and rewired it to eliminate the time delay portion of the circuit. The blower motors also needed a bit of lube. Since then the heating system been working great.

The air con in the cars may be marginal but a fully functioning heater will bake bread.

[proporsche]

i got the relay in case of need

[85911]

redridge: If I interpret your statement correctly, the limit switch is good since the main heater blower runs when a lever is pulled up.

dw1:
1. The main blower heater switch must be good since the blower turns on when a lever is pulled.
2. Is there a procedure to check the engine compartment relay? I'm assuming my picture shows the relay. Do you know which of the 2 relays is the heater blower relay?
3. Is the fuse on the main blower, or the footwell blowers? If on the main blower, it must be good since the main blower works. If each footwell blower has a fuse, I would think it unlikely, but possible, that they both failed at the same time. If there is one fuse for both footwell blowers, do you know where it's located?
4. I disassembled all the blowers on the car several years ago and lubricated and cleaned them. They haven't been used much, so I suspect the issue is elsewhere. But, I'll check the blowers.
5. Do you know what the resistance should be on the resistor terminals?

Thanks for your replies.

[aj88cab]

There is a fuse for the footwell blowers at the engine compartment fuse block for speeds 0,1,2.
Yes, the FW blowers run on the "0" setting.

At that same engine compartment fuse block is a separate fuse for the engine blower.
This one is OK since the engine blower is operating.

Speed 3 for the FW blowers runs thru a 25A fuse & round type relay at the front trunk fuse/relay panel.
If FW Blower speeds 0, 1,2 work, but 3 does not, the problem is here (the fuse or relay).
Check those fuses and the round relay (you can switch it with another on the panel)

At some point there were in-line fuses added to the FW Blowers, I don't recall the year, but think it might be 1987
Check those fuses if installed. Check the FW Blowers with direct 12v. I don't think you need to take them out to do this.

If all this checks out, the problem is probably the pricy Heater Controller Relay.
There are two relays in this unit, and the one that operates the FW Blowers can fail.

As noted above, the resistors rarely fail.

[famoroso]

FWIW, my '87, '88 & '89 all had blade type fuses at the footwell blowers themselves.

I replaced the motors in my '88 blowers with the fan motor from the front AC condenser (some "assembly" required). There's a few big threads here with the procedure.

First thing I would do is pull the footwell blowers to ensure they're not seized. Then I'd clean, lube and reinstall.

I gutted the blowers in my '87, so the motor and fan cages are out, but the blower housings still route air as intended.

My '89 is a no-heat hot rod so blowers have been removed.

[dw1]

To see if the engine compartment relay was working I accessed its socket pins, and also checked if the applicable front trunk relay was getting power.

The plug-in fuses for the footwell blowers are adjacent to the blowers themselves (I believe the Bentley manual shows this). As mentioned above, this may not be applicable to earlier 3.2 cars, but I also know '84-'86 cars where thes fuses were added (maybe via dealer service/recall?)

I also believe the Bentley manual shows which relay is which, and the pics posted above should help.

The main thing is to study the wiring diagram (remembering that your console switch is manual and not automatic) and follow the voltage path via DVM tests.

[mysocal911]

Originally Posted by 85911

I have an '85 911 and the 2 footwell blowers stopped working after several years of running fine. My main heating blower in the engine compartment runs when either of the heating levers are pulled up. When I turn the center dial which has settings at 1,2, and 3, the footwell blowers do not come on as they have in the past.

My Bentley manual addresses only the automatic heating system for '86 and up. I have a manual heating system.


I seem to recall reading that there is a relay in the engine compartment that is made when the main fan is turned on which allows the footwell blowers to engage when the center dial is turned to one of the 3 positions.

I also found the footwell blower resistor in the luggage compartment on the right side. I tried to take off the 3 contacts to put an ohm meter on them, but quit when they resisted. The resistor looks original and I didn't want to tear it up.

I would appreciate any advice on how to diagnose this issue.

Thanks.

Yes, to both highlighted sentences. Typically, a reed-relay or a coil relay fails in the engine compartment blower relay (911 618 153.00).
Most likely your blower relay is bad. This usually occurs from an overcharging alternator, or a battery charger.

[Walt Fricke]

The '84 footwell blowers were not fused. Those blower motors were not durable. Bearings would fail, plastic moving parts would rub on other plastic parts, and eventually the motor would freeze. With no fuse, heat would almost set them on fire - acrid electrical heat/fire smell in the cockpit.

Amazingly stupid design - fuses were hardly unknown.

I know this because around 1993 or 4 I had to coach by phone my panicked wife about how to remove the battery cable on her '84 so the smoking would stop. She enlisted a neighbor. I replaced the blowers, but eventually decided they really weren't all that helpful, and disconnected them. I used my 2.7 as my ski car, and never wished I had footwell blowers.

I don't recall if by '85 Porsche had fixed this with a fuse. Don't recall if Porsche ever had Bosch make better blowers.

Obviously, they then over engineered the system, with a time delay, etc.

[proporsche]

Walt... i think only later models like 87-89 have the inline fuse to the f.blowers.My 85 also does not have them

Ivan

[Ironman88]

I've been spending time trying to get at the source cause(s) of the failure of these heating components.

Seems like the failure of the rear heater blower relay is a core issue - and the cause of failure as I've noted is either use of an improper battery charging means or method, or an "overcharging by the alternator".

The over-charging by the alternator has me concerned - because it seems like that condition would likely cause a replacement blower relay to again fail. (And has been noted, those parts aren't cheap - I just paid about $430 for one.)

Question - does anyone know why the alternator does this, or, even better, what can be done with / to the alternator to avoid having the problem with the relay reoccur??

[HaroldMHedge]

Your right Ivan,

Porsche started adding the blower fuse on April 23, 1986.

Anyone without the fuse should add one.

Originally Posted by HaroldMHedge

Here's the requested TSB and an additional one on repairing the fans.

TSB 8704 Fuses for Additional Heater Fans



TSB 8706 Repairing Additional Fans

From Blower Motors Photos Heaters / AC

[Walt Fricke]

Ironman - over voltage occurs when the voltage regulator fails in an overvoltage mode. That is perhaps more insidious than failure in the under/no voltage mode, where your battery doesn't recharge and you catch on because the engine stops at night. Overvoltage sometimes can be detected by the smell the battery creates when it off gasses - sulfuric acid?

It appears the only fix is a new VR. And sometimes even new VRs are faulty - I had one last only a couple of months. It doesn't appear that overvoltage problems can arise from the alternator itself. However, under voltage can occur due to failure of one of the several diodes, both high and low curent, in the alternator.

Voltmeter, voltmeter, voltmeter!

[Ironman88]

Originally Posted by Walt Fricke

Ironman - over voltage occurs when the voltage regulator fails in an overvoltage mode. That is perhaps more insidious than failure in the under/no voltage mode, where your battery doesn't recharge and you catch on because the engine stops at night. Overvoltage sometimes can be detected by the smell the battery creates when it off gasses - sulfuric acid?

It appears the only fix is a new VR. And sometimes even new VRs are faulty - I had one last only a couple of months. It doesn't appear that overvoltage problems can arise from the alternator itself. However, under voltage can occur due to failure of one of the several diodes, both high and low curent, in the alternator.

Voltmeter, voltmeter, voltmeter!

Good information and advice. Thanks.

[aj88cab]

Originally Posted by Ironman88

I've been spending time trying to get at the source cause(s) of the failure of these heating components.

Seems like the failure of the rear heater blower relay is a core issue - and the cause of failure as I've noted is either use of an improper battery charging means or method, or an "overcharging by the alternator".

The over-charging by the alternator has me concerned - because it seems like that condition would likely cause a replacement blower relay to again fail. (And has been noted, those parts aren't cheap - I just paid about $430 for one.)

Question - does anyone know why the alternator does this, or, even better, what can be done with / to the alternator to avoid having the problem with the relay reoccur??

Ironman88, could you possibly site a source for this information. I am not questioning, nor trying to debate, I am just curious about the details of this...Thx

[Ironman88]

Originally Posted by aj88cab

Ironman88, could you possibly site a source for this information. I am not questioning, nor trying to debate, I am just curious about the details of this...Thx

Check out Post #2 in the thread linked below:

https://rennlist.com/forums/911-forum/1280140-automatic-heating-control-not-functioning.html

[Ironman88]

Originally Posted by aj88cab

Ironman88, could you possibly site a source for this information. I am not questioning, nor trying to debate, I am just curious about the details of this...Thx

Also reference:

Porsche 911 Performance Handbook 1963 - 1998 3rd Edition - Bruce Anderson

Page 62 - Carrera 3.2 Checklist provided by Tony Callas

Electrical Issues
Alternator Overcharging

[85911]

socal911: I have used a trickle charger on my '85 as I don't drive it often in the winter here where they use salt on the roads when it sleets or snows. Should I disconnect the battery when I install the trickle charger to protect the footwell blower relay?

I checked one blower and it runs well. I suspect the other does too. All 3 fuses are good at the engine compartment relays.

I do not have fuses at the footwell blowers nor does a large fuse in the front truck fuse block control these blowers per the manual.

So, my next step is checking the relay. My Bentley manual is not much help. It does not identify pin no's on the relay nor outputs to the footwell blowers. I assume I'm looking for a 12V input coming from the levers in between the seats (I know they work because the engine compartment blower works), and a 12 V output to the footwell blowers. Again, I don't see this per the wiring diagrams in the manual.

Does anyone have a procure for checking out this relay?

Thanks.

[aj88cab]

This diagram might help. The red line is a correction to the original diagram that has circulated for years.

You will have to open up the unit to examine & test the relay for the footwell blowers since it operates with engine blower current flow thru the reed switch. There is no way to operate it externally without the engine blower connected and running.