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1979 Mark Engine Fire - AC/Heat Related Issues

Hello Bill,

My '79 Mark V had an engine fire. I got the fire out with no damage to the original paint. It burned a lot of the vacuum hoses and wiring. I have '79 parts car that I was able to get the wiring harness out of and replace the burned harness. I replaced all the burned vacuum hoses, a very big job. Now after the engine is warmed up and the heater levers in the heat mode, the heater fan will not run when turned on. Also with all the heater control levers off the defroster is on all the time with no fan. This heater system is a very complex system to work on. I have the Ford factory shop manual and it's not any help.



Greetings Lyle -

Some of these Automatic Temperature Controls sure can be tough to diagnose and repair if you have little experience. Sometimes the experience does not help either especially if a fire has occurred.

You do not mention if these malfunctions occurred before or because of the fire. In any case we can offer the following suggestions. The factory service manual is excellent in all respects for troubleshooting and understanding how the system works. I don't know which section you are reading but in Volume Three the 1979 Mark V ATC section begins on page 36-74-1.

An under the hood fire that damaged rubber vacuum hoses as well as electrical parts can only repaired by someone there on scene with the correct manuals and so on because I cannot see the extent of the damage at all from here. It would appear from your description however that you still have an existing vacuum problem or two. The shop manual shows the complete vacuum diagram along with the colored tracers on the hoses as well as a chart that shows what vacuum is sent out to the vacuum actuators with the various positions of the control switch.

No blower in the HEAT position is a common issue and is caused almost always by a faulty EVR (Electro Vacuum Relay) unit that is located under the dash above and to the rt. of the transmission hump. Its operation and installation drawing is well explained in that same shop manual. Be advised also that wiring for the EVR operation runs to the top of the engine and to the Temperature Sending Unit at the rt. front of the 400 engine. This relay functions to hold the blower motor inoperative in the HEAT position when the engine coolant is cold and the ATC is calling for heat. A fire could possibly have shorted this wire to ground so that EVR is in the cold engine mode with the control in the HEAT position. If you suspect but can't test the EVR yourself, we can offer test it for you at Lincoln Land.




Hi Bill,

I have looked at the section 36-74-1 in the manual. The blower is working OK now. I guess the engine wasn't warmed enough before.

Everything points to a vacuum leak. The manual says if there is a vacuum leak the defroster will go to the default on mode but I sure can't find it. There's no hissing noise anywhere that I can hear.

My son is a master auto mechanic so I'll ask him to help me.

Thanks for the help.



Hi Lyle -

Great! You are indeed lucky. Sounds to me like a good idea. Your son will understand the manual and the diagrams. Keep in mind that all vacuum leaks cannot be heard easily and some components need to be tested with a hand held vacuum pump that has a built in gauge. We have most parts for you already in stock to repair this system. We wish you a speedy repair.