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June 2, 2021

1976 Mark IV Heater Core and Blower Motor Questions

Bill -

My 1976 Mark IV seems to have hot water to the heater core at all times. This raises passenger compartment temps 10-20 degrees at all times vs ambient temps. My factory shop manuals indicate there should be a vacuum controlled water valve for one of the heater core lines on the engine side of the firewall. Mine has none, just full flowing red heater core water hoses on both lines. This is now a Summer only car. Should I install a manual or vacuum valve on one of the hoses the knock down the interior heat? Which hose line should be valved.. the one from the water pump or the one from the block? Also, do I need an amp meter or just a volt meter to check electrical output to the blower motor? Mine only works on "Max" and I have checked the fuses, relay and resistor.




Hi Greg -

Your 1976 Mary IV has the automatic climate control system from the factory. They work very well if they are maintained and kept with all of the factory designed operations intact. This system would have had a water valve installed on the 5/8" hose to the block when new and removing it tells me that the possibility exists that someone may have made some other "unknown" negative modifications as well.

The good news is that you have the FoMoCo service manuals. Volume 3 starting on page 36-74-1 has this system and most testing procedures explained in excellent detail for you or your mechanic to work with. There is therefore no need for me to repeat all of this info here. In order to diagnose and repair this control system however, some important prior knowledge of this type of operation is a great asset. If this type of control system is new to you or your mechanic, the testing and repairs could prove very time consuming.

If you wish to eliminate or control the heating portion as you suggest you could install some sort of manual water valve as you suggest or even bypass the heater core. This could be used to test your theory and would be inexpensive. We have seen many of the item on page 36-74-4 known as the Servo Vacuum Diverter Valve fail. This valve operates the Water Valve along with other features. These valves if faulty are difficult to find new or in good used condition.

The schematic for the blower motor speeds is well shown on page 36-74-6. The High Blower relay is a known good candidate for failure along with other blower control parts as shown in the diagram.

We wish you good luck with the diagnosing and repair. If any parts, further direction or advice is needed, please contact us at any time.