Heater Woes

tkosier · 11-11-2007, 06:49 AM

Finally got below 60 here in south Florida (only in the morning - still hitting 80 in the afternoon). Of course, my heater didn't work. After checking out the many good posts on heater woes here, I was able to chase it down to a malfunctioning heater control module. Knowing my wife doesn't like to have cold feet, I went ahead and ordered the module from our host. Great, heater works - for about a week.

In essence, the heater blower motor in the engine compartment will not come on when I move the heat levers in the cockpit. The heater blower motor and wiring to the blower motor are fine as I can force them too run by applying twelve volts to the connections corresponding to pins 5 and 6 of the heater control control module( after removing the module of course). With the module removed, I get 12.7 volts at connector positions 5,7 and 8, (hot at all times) and I get 11.7 volts at connector position 12 (hot on run). When I raise the heater handles (manual heat) I get a closed connection at positions 9 and 11 (switch is working fine)- all as it should be.

I'm concluding that the new heater control module is bad and need to send it back for a replacement.

Am I missing anything?

Thanks, Tom

1988 Carrera with Non-functioning "manual heat".

jimmcc · 11-11-2007, 09:04 AM

Did you order the entire module or just the control card? I couldn't find either in the part listings but am sure I missed it. Pass on the Pelican part number you ordered.

The module itself controls the movement of the lever based on heating requirements. The engine fan is controled by the contracts on the module you referred to. Let me look at the wiring diagram as I believe there is a relay in the engine compartment that controls the fan and that is what you are testing. Could be the fan is shot as it is a common failure.

jimmcc · 11-11-2007, 09:55 AM

Disregard my last post. I forgot about the changes in 1986 and failed to see where you indicated the fan motor worked properly. My last wiring diagram is for a '86 which is probably close to correct but it doesn't show connector numbers as high as you stated. If I can locate an '88 diagram I'll take another look.

Sorry!

tkosier · 11-11-2007, 12:15 PM

jinmcc, thanks for taking a look. I'm using Bentley manual wiring diagrams. The '86 looks very similar. The part I seem to be chasing is called the "blower control unit" on the wiring diagrams and it is physically located in the engine compartment on the left side. It is a twelve pin unit sitting next to what I think is the rear window defroster relay.

I took the cover off of the old unit and there are two relays inside with a bunch of other components. With the unit plugged into its connector, I can manually close the contacts of the larger relay looking thing and the engine compartment fan will run. What ever is controlling that relay seems to be the culprit.

jimmcc · 11-11-2007, 05:19 PM

Look on your wiring diagram and locate the Blower Temperature Switch which comes into the Blower Control Unit on pin 10. The diagram shows it as normally open which I'll assume to be "cold". Check to see if the switch is open or closed when the car is cold. If it is closed when cold unplug the switch and then see if the fan will run.

I have to confess to not being familiar with the late 80's systems. I repaired many of the older heating control units but these don't die as often.

This looks similiar to the heater fan thermal switch that is located on the air plenum for the engine compartment fan on the 993 but I can't recall if it on the intake or pressure side of the fan. They have a bad habit of failing. It pops out of the plenum and you can clean up the contacts and get it going or else replace it.

I don't have the part locator section of the '88 wiring diagram.

I'm not sure what is the function of this switch in the over all scheme.

Let me kno whow it goes.

tkosier · 11-12-2007, 04:59 AM

Thanks again for your interest. I checked pin 10 and it shows open when car is cold. I went over everything again and it is as I described above. With the cover off of the unit I can manually close the large relay and the engine compartment blower will run just fine. If I manually close the smaller relay, the foot-well blowers will run just fine.

I am concluding that the new controller is bad and I need to return it to Pelican for a replacement. I guess that happens, but every new part I have ever installed has worked fine.

There is alot of info on the later model controller on this thread:

Help! Does anyone understand the footwell blower circuit?

but these guys were dealing with foot-well blowers that didn't work - I can't get the main blower to work correctly.

Anyway, thanks again for your replies. If anything jumps out at you , let me know.

Mysterytrain · 11-12-2007, 05:08 AM

Off the top of my head I'm thinking about the temperature sensor located on the driver's side flapper box. The sensor will not allow the fans to operate until the heated air reaches a set point. I believe the logic behind that was to prevent cold air from rushing into the car from all those fans. Anyway, If the sensor goes south it will never send the signal to the engine compartment control..the rear blower will not come on and if the rear blower doesn't work the footwells won't work. I think the Bentley has a test for the sensor.

tkosier · 11-12-2007, 08:23 AM

Ron, thanks for your post. I got pretty excited as that is they kind of thing I was looking for. Unfortunately, I have the manual control setup in the cockpit (option M533 I think) and don't have the heat exchange temperature sensor. I even took the cover off of the cockpit controller to see what wires are coming out of it and the temp sensor "harness", as shown in the Bentley manual is not present. The manual set up has a simple three speed blower setup and a micro switch under the two red levers.
Can you think of anything else?
Thanks again, Tom

jimmcc · 11-12-2007, 09:15 AM

I see the M533 option in a 1986 wiring diagram (the last factory one I have). It shows the Blower Temperature Switch and does not have an indication that it is deleted on the M533 option. And it shows the switch as open which I would assume means cold.

The blower control unit looks like a couple of relays, a heated timer switch and an unknowium electronic module that controls the relays.

It's not uncommon for new electronic parts to fail sonn after install so maybe you do have a bad board. Have you talked to the guys at Pelican to see if they have any diagonistics available.

Good luck.

aj88cab · aj88cab's Garage

Tom

Pins 5 & 6 at the connector are both for power out to the engine compartment blower. Pins 7 & 8 are power into the controller for the engine blower. The circuit runs thru a relay in the controller. Pin 12 is control power for the logic circuit in the controller. If pin 5 is hot all the time something is not right. It sounds like a bad controller. Confirm that pin 6 is hot too. Do you hear a "click" from the controller when you move the levers on and off? You can check this with ignition ON, but engine OFF. If you hear a click the relay is working.

Good luck
Andrew

Mysterytrain · 05:16 PM

Ok, well I thought they had used the duct sensor in later years even with manual heat...sorry for the brain fart.
Another thought is the functioning of the red levers. I have a 78SC so I can only speak from my hands on knowledge of that system, but the microswitches under the levers provide a ground for the engine compartment fan RELAY in my case. The controller must turn on and off the same way.
edit. see post below

tkosier · 11-12-2007, 03:58 PM

Rechecked my work and I made a mistake in my original post - with the module removed, connectors 3, 7 and 8 read 12.7 volts not 5, 7 and 8 (good catch Andrew!). I'll check the red lever and cockpit controller grounding tomorrow. I'll also send a note to pelican - good suggestion.
Thanks everyone for your help - this is one of those minor annoyances that has little real effect on the overall immense enjoyment I get from this car, but it is driving me nuts!

Mysterytrain · 11-12-2007, 05:27 PM

I broke out the 88 schematic and it is a bit more detailed then the generic controller in the link I was refering to above. So, here goes once again.
The red levers trip microswitches. The switches have a pair of yellow wires coming from them.
One yellow wire goes to pin 9 [becomes yellow/ red at the controller socket] and the other yellow wire goes to pin 11. The pin 11 connection is tied into the starter circuit. Typically what happens when you crank the starter, the solenoid relay applies 12 volts to turn off circuits that would load the battery down and interfere with cranking ie: A/C, heater, headlights etc. When you return the key to the run position the yellow wire goes low..hopefully to ground. Ok, So the connecting of pins 9 and 11 [which I believe is ground within the engine compartment controller] will bypass the microswitch and should turn on engine compartment controller, and hence the rear blower. You would need to get to the socket base to try that test. Another connection that I noticed is pin 2. That connection is tied to ground point G302. Once again I'm speculating..but that appears to be the main ground for the controller. If that is faulty, I doubt that the controller would work. I would check this first.
Bentley says that G302 is the engine/tranny ground point. If the pin 2 ground is bad you will need to figure out the wiring to that ground point..might be thru the engine harness.
Also... Pins 3, 7 and 8 are getting 12 volts from the rear panel fuses 1 and 2 but also note that Pin 12 gets 12 volts when the ignition switch is in the run position. I'm thinking this is the power connection for the controller. I doesn't appear to be fused.