1988 TC - Erratic blower speed.

[sly]

no... we're talking about the part actually ON the blend door control arm.

[gadget73]

yeah, a different goofy ATC related part

[lincolnlarry]

I'm not thinking it's the resistor but I thought there were separate resistors for auto and manual. The setup in my 89 has 4 resistors in there.

[gadget73]

Its one resistor with 3 sections. Same unit does the speed control on both modes, the "auto" position changes which switch makes the connection though.

[lincolnlarry]

Hey Derek,

Sorry to say mine is getting flakey also. I haven't run the auto part of mine much so didn't notice it. It is easy to check with a piece of hose. Open the glove box and over the left corner you will see the gold can for the vacuum switch/blend door. Take off the hose and attach your own to the can to pull vacuum through. Get the blower running. Doesn't matter what setting as long as the fan switch is on auto. With no vacuum it should blow high and full vacuum should be low. When I released vacuum on mine it sometimes would not blow high. Had to apply and release vacuum a few times to get it to make contact. I don't have any fancy bits to pull vacuum so I provided my own vacuum source. That method got me a mouth full of gasoline in the past.

I thought something odd was going on during this but I was not able to hear well enough with the fuel pump whine and stuff. Pulled out a little power supply so I was able to power just the fan. I haven't seen it mentioned anywhere but there are 4 speeds in automatic. Had to verify this with a volt meter. The power supply was enough to run the fan but there was a voltage drop because high should be 12v. But got steady readings with fan all speeds. High= 7.5v, med2= 6.4v, med1= 5v and low= 3.4v. The schematic always confused me because it only shows 3 positions high, M2 and M1 but there is a 4th where it makes contact to nothing for low. No matter if in auto or manual the blower is always connected to ground through the resistor winding for low speed. That always bothered me because without the vacuum controlled switch having an open position there would always be something connected in parallel to the low resistor. Making the auto mode never run as slow as manual low.

[lincolnlarry]

EDIT:

I should not say full vacuum to the can is low speed position. Actually medium vacuum is low. While going up on the vacuum the blower will slow down to low then rise back up in response to the door going full cold or full hot.

[gadget73]

yeah, mid-range on the blend door is minimum fan speed. Full speed is at either end of travel. I'd like to figure a means of making that fully variable using a later model blower motor speed control unit. Making it variable is simple, but figuring how to mount a variable resistor to the blend door actuator so it does what I expect is the hangup. I'm thinking linear variable resistor, some connecting linkages, and probably a bit of electrical voodoo to get the signal how it needs to be. Maybe leave the manual control as fixed 3 speed since that would be easier.

[85cs2]

This is an old thread - but wondering if anyone ever figure it out - I have exactly the same issue in my 1985 Country Squire - everything works in the AC / temp / floor/ vents etc - there Manila positions of the fan but the auto function - it used to work and dropped suddenly. Any guidance would be appreciated