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’75 Mark IV Blower Works On Max Only and ….

Hi Bill,
I’ve been reviewing your blog with enthusiasm over the last few days. In summer 2008 I resurrected my father-in-law’s 1975 Mark IV after it had sat for 17 years. I bought a few parts from you then and they did the trick nicely! We drove it as a toy for a couple summers then had to park it again due to life… until I brought it to our “new” home in 2017. Anyway this car has been a learning experience for me: first carburetored car (from the big 3’s nadir, off) and all that, but I’ve managed to sort it out by attacking one issue at a time. It looks great and runs smooth and even won a trophy at a local show.
Anyway, I’m hoping you can lend some advice for two issues. I know you are busy so I will try to be brief, but detailed… it’s a tough balance so please bear with me.
For further background: the car has 82000 miles and had a blown #8 exhaust manifold stud and gasket, which made it tend to run hot. I finally got the gasket fixed (no easy task with all the broken studs), then rebuilt and tuned the carb myself (no easy task for someone learning). The car has ATC of course and the A/C has never worked in my time – I’m sure the coolant leaked out ages ago or maybe the compressor is shot, but it’s not a priority before getting the car running right.The heat works well and is quite roasty – but the blower only works on max.
1) So the blower speed (max only) is my problem. I have the set of orange ’75 FoMoCo manuals and based on reading them and your blog, have tested / confirmed:
– Under-hood blower relay works. When I jump the harness (Or/Bk to Gr) wired to run the blower on low, nothing happens
– I checked the blower resistor pack. There are 3 resistor modules. They all bench test good with a meter, two at ~62 ohms resistance and one at ~50 ohms. The spring contact in each module works. The 3-prong module has a diode, which measures OK.
– The servo sensor was filled with a bug nest back in 2008 and I’d cleaned it carefully. The bimetal springs, etc all look clean and flex nicely.
– I checked the Servo Motor & EVM module by removing it from the plenum and observing its operation. I opened its electrical switchboard module and discovered its contact board was fairly well blackened, so cleaned it up. If I start the car I can watch the vacuum modules work – granted, I realize they may not be in spec but at this point I’m trying to get the blower to work electrically*… by ensuring the electrical sled from the servo motor was moving. Unfortunately, the blower still does not activate with the control set in the auto position. I had the system (vacuum) set to Vent and 75 deg, but I manually moved the switchboard sled and could not trigger any response from the blower – HOWEVER I could hear the EVM module pulse and/or click as I moved the swtichboard contacts, so it seems my cleaning did some good.
*I don’t have a vacuum tool handy. However, by observing the parts loosened from their mounts but connected to their hoses, I could observe their relative function which “seemed” OK. (if I manually push on the sniffer sensor, I can observe the servo respond, etc). No audible vaccum leaks and if I disconnect hoses, the engine stumbles badly.
Anyway… that’s a bit of a ramble gut given how this ATC system tries to read ambient conditions and engine status… surely I don’t need the A/C part(s) fixed for this to work, do I? Or did I overlook something stupid? I plan to get a vacuum tool shortly but meanwhile I was doing the best exploration I could with the tools I have.
Also I should mention, while the blower does work on Max, it makes an awful screeching noise for the first 15 seconds or so (sometimes longer). I’m assuming it needs replacing soon anyway, but that doesn’t explain the lack of low speeds? (Hopefully you can appreciate I am trying to identify “anything/everything else that might need replacing while I’m in there).
2) Back to that “wow but the heat is roasty”: As nice as this car is for its time, I absolutely hate how FoMoCo cheaped out with a single idiot light for temp/amp/vacuum. And knowing it was running hot while it had the exhaust leak… call me paranoid but I’d like to add a triple-gauge set for Temp, Amps, and Oil pressure. Of course I now understand the single stock temp sending unit is critically integrated to the ATC and other functions, so I can’t just plug a different one in its place. Surely I can’t be the only one who’s attempted this, but I don’t see any resources online. Can you recommend a location to tap for temperature? I’m thinking of tapping into the cabin heater inlet hose, as that would be easy and reversible, and temps should not be much different from the thermostat / radiator. I didn’t see obvious available locations in the manifold (which might still present other clearance challenges).
3) I’m sure this goes back to me “learning” the carb but this car is very hard to start after it sits for a few days. Takes 30 seconds or more of cranking (I use 6-second cycles to preserve components). I’ve read and followed the rebuild and tune guide over and over and can’t seem to get anywhere. It runs OK, and starts fine when warm. I replaced the fuel pump in 2008.
Well anyway I apologize for the dissertation. I hope it at least gave you enough information to consider some advice but if you need any more info, please ask. Thanks again for your service and expertise! I’ve attached some pics of the car… I called it the “Angstmobile” when getting it running. I let my 15yo daughter practice some 3-pt turns with it the other day, too. Also, I am led to believe the Pastel Blue / Navy combo is fairly rare, and when it came time to replace the cluster bulbs, I went with light blue LEDs to match the car and arguably “update” it a bit.
Kind regards,
Greetings Tony –
Welcome to Lincoln Land and Bill’s corner. It is good to know that you have the correct manuals for your Mark IV. They are among the best tools in your toolbox.
The blower motor on your Mark uses a completely separate circuit for the lower speeds. For max. the power is from a fuse link directly from the battery and through the High Blower relay. For the lower speeds and the other ATC controls there are two fuses that are located in the fuse box. Your information regarding the blower squealing for a period of time is important for the diagnosis as this indicates resistance. It is “possible” that because of this resistance the blower motor cannot turn with the lower voltage of the lower speeds.
You should begin by testing the circuit as shown on page 36-74-6 of your Electrical manual with a 12v test light. Start by testing for power at the blower motor with the engine off but warm and the key switch on. Turn the ATC on and select a lower blower speed. With your test light check for power at the blower motor connector. If there is power but no blower operation the motor would need replacing or lubrication. A draw test on the blower at this point would also be a good idea.
If there was no power at the motor connector, test the fuse box fuses. If they are good, you will then need to test the circuit to and from the Hi Blower relay through the resistor and all of the switches etc. with the test light. When testing watch for burnt connections and/or melted plastic. After the repair a blower motor “draw” test is very much advised.
To answer your other question, the actual refrigeration system does not need to be operating at all for the blower to work properly.
If you are installing some sort of aftermarket temperature gauge, my opinion as yours is that the hot heater hose close to the engine could be a good location to tap into.
We will have any of the ATC parts that you may need as well as a new blower motor here at Lincoln Land for you as required. We wish you the best of luck with the testing and diagnosis.
Hi Bill,
Sorry for my delayed response here, but I’d like to thank you very much for your prompt reply. What you suggest makes sense, and hopefully I’ll have some time this weekend to poke around again for follow up and let you know how it goes with the blower circuit. Thanks again!
Hi Bill,
As promised, here’s some follow-up: After thinking about your hint that “possibly” a weak motor would have too much resistance to work on lower speeds, and what I already knew about the car’s 80K historic miles sustained under smokers, farm dust, and who-knows-what else… I decided to finally dig in and remove the blower motor as I’d actually been meaning to for a while, but wasn’t looking forward to the job. Well, after finally getting it out, I guess I should be shocked and amazed that it even worked at all?! I was instantly greeted with lots of debris, rust scale, caked dust and ash, and who knows what else. The wheel is somewhat difficult to turn by hand and there is a definite drag spot somewhere in the armature. Once I get it turning, and removed the wheel, the bearings squeal even by hand. But arguably the worst telltale of all is I could not separate the harness plugs for the orange wire: the “car side” wire lead pulled out of the harness, and eventually with it removed from the vehicle, I can now see they melted / deformed together (so much for using the test light at the connector)! See attached photos….
So it looks like a new blower motor is in my future regardless of the overall ATC health. The wheel will clean up nicely, but the shaft retention spring clip launched itself to the nether as I attempted to set it down after removing it.
If you’d like to quote a replacement motor and wheel clip, and perhaps the harness connector(s) (I may yet be able to separate and salvage the originals, or use standard connectors if need be) I’d be grateful. Everything else looks OK thus far, and the fuses look good too.
Thank you,
(PS and BTW, FWIW I am no stranger to difficult DIY repairs in tight spaces (Fox-chassis heater cores; steering rag joints, removing transmissions and engines, etc etc etc) but getting the two rearmost screws out of that lower blower housing ranks up among the most sanity-checking miserable tasks I’ve endured in a while! I’m embarrassed to admit just the one of them took me almost an hour of struggle and contortion to find the magic balance of reach, fit, bite, and angle. Hopefully it goes back together more easily!)
Hi Tony –
We were happy to see your reply and that you have found a definite problem ( actually several problems ) and a new plan of attack to cure your blower speed issues. With the installation of your new blower motor that you ordered at Lincoln Land today along with the repaired electrical circuit, that part of the Automatic Climate Control should work as good as new.

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