Rear Defrost

[cabriobrad]

Has anyone tried to add in the electrical type rear defrost to a car that had the "forced air" type of rear defrost?

I have a 1968 911L with what used to be the tubes that come up to supply air to the rear window...but they are not on the car. Why you ask...PO has covered them over, and I think I would rather go with the electrical type anyway. That, and my rear window is heavily scratched and needs replacing.

Other than the obvious electrical changes I will have to make which really is no problem for me, should the rear glass just "fit" right in there?

Thanks.

[ossiblue]

Since you are aware of the electrical additions, you are aware of the biggest problem. The size of the rear windshield is the same and the replacement with the heating grid should fit right in.

[Grady Clay]

The single stage and the 3-element, two-stage heated windows will directly interchange with your original unheated window. You must use the window gasket for the heated window and provide a hole in the frame for the wire access. Be sure to carefully paint the new hole for rust prevention.

You will want to use the Factory wiring harness (911.612.037.01) between the window and the engine compartment as it nicely fits the gasket. With the 2-stage 100/250 W window (most commonly available), you need to use the Hella controller. Running the window on all three elements can lead to overheating.

The Hella controller (911.615.115.00) is commonly available ‘good used’. It has an end-flange that can mount to your electrical panel. There is a small wire harness (911.612.032.01 and other) commonly found on salvage electrical panels that has the mating connector for the widow wiring harness, the 6-pin receiving socket for the controller and four connections (12V power, ground and the high/low control pair). You can make a small sub-harness between the electrical panel and the dash for an Off/Low/High switch with indicator light. Two wires in this harness should be the same as the control wires (white with black tracer and black) and black heat-shrink makes it appear original. I would add a red/something wire that is switched and fused with the window and controller for the indicator light in the switch. The 12V power should be switched and fused. A relay (like the SWB starting relay) and the 3-fuse block from a ’70-> electrical panel allow connection to the B+ terminal on the electrical panel and controlled from the same circuit as the ignition and fuel pump.

I intend to do this on my ’68 coupe.


There is also a ‘fine-wire’ heated windshield.


Best,
Grady

[gulf908]

A very helpful chapter and verse from Grady as always !
Something to take into account was the upgrading of the whole electrical system from the '68 MY to the '69.
If my memory serves me correctly ,the alternator and regulator(?) were upgraded and the two battery system was introduced in late '69.
This was to cope with the ever increasing popularity of electrical power draining features such as air conditioning,power windows and the heated rear window,whether the half or full screen version.
Grady should be able to advise if you need to change your electrics for this one addition to your electrical load.

HTH,
Dennis.

[Grady Clay]

Dennis brings up a good point.

The 911s ’64-’68 all have a Bosch alternator nominally rated at 550 W. The specs are: 490-560 W, 35-40 A @ 14 V. Not much excess capacity.

Starting with the ’69 model, the capacity was increased to 770 W, 55 A @ 14 V. The ratings were the same for both the Bosch or Motorola. The part numbers changed in ’70 but the specs remained the same. Somewhere about ’71-’72 Motorola became S.E.V. Marchal.

Starting with the 1975 models, the capacity was increased to 980 W, 70 A @ 14 V. This is continued through the 911SC models.

Starting with the 911 Carrera in ’84, the capacity was again increased to 1260 W, 90 A @ 14 V. This is when Porsche felt the need to add alternator cooling slots to the fan.

The 964 was 1610 W, 115 A @ 14 V.


During this progression, the fan housings changed to accommodate a ‘deeper’ (longer axially) alternator. The outside diameter remains the same. I think (but not absolutely sure), for practical purposes all the fan housings interchange from ’69 through ’88 and possibly later. The big exception is the fan housing for the smaller (226 mm) fan in ’78-’79. It will interchange but you don't want the small fan.

The issue with the SWB cars is the ’69 and later fan housings all have an integral ‘coil’ mount. This does not fit the ’68 and earlier fan shroud without ‘butcher’ appearing modification. The ’69 and later fan shroud can be added along with the fan housing but is not period correct. The later fan shrouds do have improved air flow to the oil cooler. The CIS fan shrouds have slightly less air flow to the ‘front’ (#3 & #6) cylinders and heads.

I don’t know how much a ’68 magnesium fan housing can be modified to accept a larger capacity alternator. I don’t recall very much material there. It may be that the ’69-’74 770 W version may fit without modification. It is time to measure.

Best,
Grady