I had the same symptoms happen to me once, but my problem was that the back came off of the switch. Inside of the switch there is several wafer boards inside of the switch. It's been 20 years so I don't recall what the wafer boards are made from. I wonder if the ones for heat and def are worn and not fully actuating things internally.
I was hoping it would be another cause. But a bad switch seems the most logical thing at this point.
I will first try to fix mine before I buy a new switch.
These switches have small vacuum leaks, and that's okay because of the constant source of vacuum from the engine (and the reservoir). Your trouble is electrical, not from a vacuum leak. How so? Turn your key to Accessory (engine off) and the fan will blow without any vacuum (in a properly functioning switch).
Seems to me that somehow the contact inside the switch for Heat and Defrost modes (it's the same power to the fan) isn't making contact, or the Heat connector/wire isn't making a proper contact somewhere. If you can prove to yourself that the wiring from Heat through the Resistor and to the Blower Switch are good, I'd try to 1) spray contact cleaner into the electrical part and work the buttons about 50 times or 2) take the switch apart (if possible, I don't know) and examine/clean the contacts. The red eraser on the end of a yellow pencil is a great tool to clean a copper contact.
In this pic from the FSM, you can see there are two power circuits into the blower switch, one from the AC and the other from the Heat. You can see that the Heat circuit goes through the Resistor, so you might want to consider that as a failure point, too.
View attachment 462671
Resistor and blower switch were disassembled in the past weeks, cleaned and checked out fine.
I've also cleaned and checked all other connections and I've reworked some of the wiring back to stock.
Thanks for your picture!
This one makes a lot more sense than the picture from my '68 chrysler FSM...
My FSM doesn't note which line one the back of the red switch is HEAT and which is AC.
Don't mind the Yellow line.
The 16LGN wire isn't there and never has been, hence the red stripe...
Check the red plug going into the switch, these plugs were famous for over heating and going "open" due to corrosion or melted plastic; check the terminals the red plug hooked to with a voltmeter to see if the circuit is complete in the heat/defrost position. If you have an open circuit due to bad contacts in the switch the blower for heat or defrost will not work.
Dave
The red plug is in excellent condition.
I've cleaned all contacts and know that the rest of the wiring is okay since the Fan is working on A/C modes on LO/MID/HI.
Seems like my switch has a bad connection in HEAT/DEF modes...
If you find that the trouble is inside the switch, you can take it apart and try to repair it. You can heat up the melted pins with a soldering iron or grind them with a Dremel to get the case apart. There are a lot of parts inside the switch, so you need to be careful to keep it all in order. It's a clever, but odd design.
Jeff
Thanks. That is what I'm going to try.
I was thinking of cutting the plastic pins on the back of with a sharp xacto knife to open it up.
And drill small holes in the pin towers to screw it back together after opening and cleaning it.
But this depends on what I can find inside.
Any tips on resealing the housing while reassembling it? A very very small bead of silicon or other sealant around? (making sure it doesn't get on the inside, preventing the workings of the switch)
In case that my switch is unrepeatable, any tips on which specific ebay/amazon/aftermarket switch to get?
I've noticed that there are a lot of different variants with different colors and pricings....
