1974 Mark IV Climate Control Questions

Hello Bill –

I am having trouble with my 1974 Mark IV climate control. I purchased a blower resistor module from your shop some time back and now I managed to get the blower working. I also filled the system with gas, and now I got cold air blowing. But there is a strange problem. It blows cold only in some temperature settings. If I try to go max cold, the blower motor just shuts off completely. The dead zone usually seems to be 65-70 degrees. If a go a bit warmer, it starts blowing cold air. The zone with no blower movement seems to vary a bit from time to time and it seems to me that it is affected by the ambient temperature. The warmer weather outside, the bigger the dead-zone. It could be that I’m missing the max speed setting. At least that would be the logical explanation. At first, I suspected that the slider is defective in one end. Now I have disassembled it and it measures out just fine. At 65F 15Ω and 85F about 550Ω. Also no dead zones on the rheostat. And now I’m running out of ideas, what could it be. I checked the 2 relays under the hood and the A.T.C range relay (C-2206) and the High Blower relay (C-2207) and they seem to work just fine when tested on my workbench. Do you have a suggestion what should I check next or where could I find the problem.

Hannes

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Hello Hannes –

Your description of the issue sounds exactly like the very popular High Blower relay problem. I’m not sure how you tested that relay but the best way to test it on the bench is electrically as per the wiring diagram. It does sound like you have the correct necessary wiring diagram. If you only hear it clicking, it could still have a poor contact at the points internally. With the relay apart it can be activated and the power path tested with a 12v test light. The power plug at the relay can also be momentarily jumped from its power wire directly to the blower motor wire. This will bypass the relay and prove that the circuit is operative. A bad contact at the contact points can easily be seen and if the relay proves faulty and cannot be repaired, it can possibly be tested and repaired here at Lincoln Land or replaced with one of our replacement relays. Of course, the relay activation wire from the ATC box to the relay plug must be live with the system operating in the High blower speed for testing. Good luck to you with a speedy diagnosis. Let us know how the tests go.

Sincerely,

Bill

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Hi Bill –

Thanks for the reply. I really hope I have the right wiring diagram and I really hope that I’m reading and understanding that correctly. I had already in my head moved away from the relays, but your reply made me double-check the relays and I discovered something new that I missed earlier.

Please see the attachment. If I’m understanding the diagram correctly, the High Blower relay should provide power from the yellow wire (which comes from the battery) to the red wire (that goes to the blower) if it gets an activation signal from the orange wire. I hope I’m on the right track this far and understood the diagram correctly?

The High Blower relay itself seems to be just fine and when activated it closes the contacts between the yellow and red wire, but I have no power coming from the 37Y (yellow)! I see that there is a fuse link marked on the diagram and that would be my next suspect. And there are points marked S-2305 and ..06, but I have no idea where they could be. If you have some hints for me, it would be greatly appreciated.

BR,

Hannes

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Hi Hannes –

Your wiring diagram looks correct but is incomplete. The yellow wire from the Hi Blower relay must have power at “ALL” times. The Fuse Link wire between C2306 and C2305 can be found taped and buried in the wire loom and is located fairly close to the relay. The yellow power wire originates at a junction with other wiring near the battery. Your Fuse Link wire could be burnt out due to an excessive power draw from the blower motor or the yellow wire is not connected at the battery junction etc. If the fuse link circuit is found burnt open the blower motor amp draw must be tested and if it is too high a draw ( above approximately 27 amps ) , the blower motor must be replaced. The burnt Fuse Link must be replaced or an easily replaceable fuse and wire can be spliced in its place. The complete wiring diagram would help you if you have further problems locating any wiring.

Sincerely,

Bill

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Hi Bill –

I just wanted to say thank you! I found the fuse link and it was physically damaged, not burned. I replaced it with a regular 30A fuse and it all seems good now. I measured 20A draw at the blower, but it could be that the blower wasn’t at its max at that moment. I’m very happy that I was able to finally resolve that issue.

BR,

Hannes

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Hi Hannes –

Great! and thank you for reporting your successful diagnosis and repair. Your success shows that “proper” diagnosis and repair is extremely important and may help others with a similar type of situation.

Sincerely,

Bill & The Staff at Lincoln Land