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Thread: No Heat in 1990 GM - BDA, Heater Core, or Temp Sensor?

  1. #21
    Beater gonna beat sly's Avatar
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    one nut high and one nut low. I know on the 88 MGM I had to remove those from the engine bay side. Reach one from the top and the other from under the car. Also, Seems every year was a little different. So there may only be one of those or both.

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  2. #22
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    I looked back at the guide to refresh my memory.
    For '91, and I assume '90 is the same, there is only one nut to worry about in the engine bay. It sounds like you got it. That's the only mount for the top of the box, but it may be wedeged in its hole in the fire wall.

    Since the bottom is loose, it sounds like you got the only other two mounts for the heater box. Bottom of the box near the passenger feet area.

    So it sounds like the only thing you have left is to pull the box away from the firewall. Either the stud or core lines are caught into the firewall or something acting as an adhesive. Nothing else that I'm aware of based on my experience with a '91.
    Vic

    ~ 1998 Mercury Grand Marquis LS - new DD
    ~ 1991 Mercury Grand Marquis LS "The Scab" - plenty of rot, summer DD
    ~ 1997 GMC Yukon - wannabe winter DD - returning summer 2020, I finally have an engine
    ~ 1991 Mercury Grand Marquis GS "The Ice Car" - Rotting Retired Winter DD
    ~ 1989 Mercury Grand Marquis GS - Rotting Retired DD
    Gone but not forgotten:
    ~ 1988 Country Squire ~ 1987 Ford Crown Vic

  3. #23

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    Thanks for the tips, everyone.

    I have the 1990 car shop manual, which has been useful for some things, but its directions for removing the heater box were a bit of a head-scratcher. Your helpful guide for the 1991, VicCrownVic, was much easier for me to follow than the manual. Now that I've done (most) of the work, the directions in the 1990's manual are making a bit more sense. But they do leave me wondering if there is indeed an extra nut (or nuts) somewhere on the 1990 that isn't present on the 1991.

    Here are some pictures in case this is of use to anyone who comes across this topic while working on their 90 in the future. Step 4 in the manual refers to three nuts below the windshield wiper motor. I don't think I ever removed anything like that, and I'm not even sure I see them in the engine bay based on a quick look. But that could be one possibility for what's holding in the heater box at the moment. Perhaps these are those lower nuts that they're referring to as items 13 and 15 in Figure 23? It seems like I may need to get under the car to access these. Step 16 refers to "two nuts from the two studs along the lower flange of the plenum" -- pretty sure those are the ones I removed under the glove compartment. The manual also seems to suggest (Steps 14 and 15) that there are some extra screws and a push pin attaching the far end of the heater box to the floor air distribution duct. Not sure whether removing those will make things easier.

    In any case, I'll do some experimentation tomorrow to see if I can find what, if anything, is holding in the heater box.

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  4. #24
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    Below the windshield wiper motor makes no sense to me whatsoever. The windshield wiper motor is way over on the driver side.
    The three nuts that it is referring to I believe are three 15s in figure 23. Since you removed the two lower nuts from the inside there should be no need to remove those two lower 15 nuts on the engine bay side. Now nut 13 seems to be sandwiched between one of the 15 nuts, the the evaporator housing and the firewall (engine bay side). I'm not sure how easy that would be to get at, but again, if you have removed the interior nuts at the bottom of the heater box, I don't see any reason the engine bay side lower nuts (both the lower 15s and 13 in figure 23) would need to be removed. If I'm understanding you correctly,
    Quote Originally Posted by SomeoneWeird View Post
    It's quite loose at the left side, the right side, and the bottom, but it's as if there's something still securing it to the firewall in the upper-middle part.
    the bottom isn't a concern anyway so I don't think you need to worry about the lower 15s or 13 (fig 23).


    Quote Originally Posted by SomeoneWeird View Post
    I pulled the heater box back maybe one-half an inch and it won't budge beyond that, nor can I push it back to its initial position. It's almost like the stud the nut was screwed onto caught on something.
    If you have the bottom of the box pulled away the top stud threads are probably catching on the hole in the firewall that the stud passes through. With the bottom pulled out the top stud would end up at a slight angle if the box isn't pulled straight back toward the seat. Throw the nut back on the stud in the engine bay (face of the nut flush with the end of the stud, so the stud threads don't get damaged) and (if there is room) see if you can tap that stud through the firewall (careful not to break the heater box). Other than that look very carefully with a bright flashlight along the top of the box on to see if there is some other screw or something holding it. Peal up any insulation if there might be anything hiding.
    Last edited by VicCrownVic; 11-29-2020 at 01:18 AM.
    Vic

    ~ 1998 Mercury Grand Marquis LS - new DD
    ~ 1991 Mercury Grand Marquis LS "The Scab" - plenty of rot, summer DD
    ~ 1997 GMC Yukon - wannabe winter DD - returning summer 2020, I finally have an engine
    ~ 1991 Mercury Grand Marquis GS "The Ice Car" - Rotting Retired Winter DD
    ~ 1989 Mercury Grand Marquis GS - Rotting Retired DD
    Gone but not forgotten:
    ~ 1988 Country Squire ~ 1987 Ford Crown Vic

  5. #25

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    Yeah, I suspect you're right. The heater box does seem to be more or less loose depending on how I position the driver's side of the dash, which is part of what made me think I'm missing something over there, but I suppose it wouldn't be a nut that attaches to the firewall which is responsible for that. Anyway, thanks again -- I'll do as you suggest and I'll post an update once I've managed to get this darn thing out.

    On a related note, I did at least manage to get the blower resistor out. Gadget73 was right that the only thing (visibly) wrong with it is that's it pretty crusty (see picture). The female connector it attaches to is even worse. Is there a preferred way of cleaning these out which won't damage the hardware?

    I thought about buying a replacement just in case something non-visible is wrong beyond the crust. But it looks like aftermarket versions of this part for cars with ATC aren't even made. RockAuto lists some aftermarket parts for the 1990 model year, but it seems it may be erroneous. Some other websites only list the aftermarket part for 1991 onward. And pictures reveal that the contacts on the aftermarket parts are a totally different pattern than the contacts on the OEM part in my picture: https://www.rockauto.com/info/577/1712211-1__ra_p.jpg (Is this an actual difference between the 1990 and 1991 models, I wonder?)

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  6. #26
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    I do happen to know that the blower motor speed controller (BMSC) is functionally the same between a 1991 MGM and 1997 LTC. When I swapped the ATC control head from a '97 LTC in, I figured this out based on electrical schematics that seemed to indicate this including wire colors being the same. I have never seen the BMSC connector, but would imagine it is the same for '90+'91. The replacement comes with a new connector that is spliced to the original harness. Some of the same replacements are listed for '90, '91, and '98 MGM. I have no idea why all the ones that come with a new connector are not listed for all those years. WVE 4P1593 listed under '91 MGM but not '90 seems to be the cheapest option. since other parts are listed for all three of those years, WVE 4P1593 should work for '90. Other parts listed under '98 without the new connector use the same connector which tells me the service procedure for '90+'91 must have changed to use the new BMSC with new connector and the old '90+'91 BMSC must have been discontinued.
    In other words, junkyard or NOS is the only way to get a replacement BMSC that does not require splicing a new connector in.
    Vic

    ~ 1998 Mercury Grand Marquis LS - new DD
    ~ 1991 Mercury Grand Marquis LS "The Scab" - plenty of rot, summer DD
    ~ 1997 GMC Yukon - wannabe winter DD - returning summer 2020, I finally have an engine
    ~ 1991 Mercury Grand Marquis GS "The Ice Car" - Rotting Retired Winter DD
    ~ 1989 Mercury Grand Marquis GS - Rotting Retired DD
    Gone but not forgotten:
    ~ 1988 Country Squire ~ 1987 Ford Crown Vic

  7. #27
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    The more I thought about it, the more this scenario sounded familiar. http://www.grandmarq.net/vb/showthre...l=1#post738768
    What might be happening is another nut on the engine bay side on the same stud you already removed the nut from but behind the plastic. Number 13 in fig 23 (although that figure seems to indicate 13 is only on one of the lower studs.) I would not bother with it since posts on the last page of the writeup seems to indicate that there is a nut on the top stud in the cab that can be removed. The stud would then remain in the firewall when the heater box is removed (might want to put the nut in the engine bay back on.)
    Vic

    ~ 1998 Mercury Grand Marquis LS - new DD
    ~ 1991 Mercury Grand Marquis LS "The Scab" - plenty of rot, summer DD
    ~ 1997 GMC Yukon - wannabe winter DD - returning summer 2020, I finally have an engine
    ~ 1991 Mercury Grand Marquis GS "The Ice Car" - Rotting Retired Winter DD
    ~ 1989 Mercury Grand Marquis GS - Rotting Retired DD
    Gone but not forgotten:
    ~ 1988 Country Squire ~ 1987 Ford Crown Vic

  8. #28
    I'm an air-conditioned gypsy gadget73's Avatar
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    What usually fails on the BMRC is a solder joint inside the unit, or at least thats how later ones die. Pull it open and have a look.

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  9. #29

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    Quote Originally Posted by VicCrownVic View Post
    The more I thought about it, the more this scenario sounded familiar. http://www.grandmarq.net/vb/showthre...l=1#post738768
    What might be happening is another nut on the engine bay side on the same stud you already removed the nut from but behind the plastic. Number 13 in fig 23 (although that figure seems to indicate 13 is only on one of the lower studs.) I would not bother with it since posts on the last page of the writeup seems to indicate that there is a nut on the top stud in the cab that can be removed. The stud would then remain in the firewall when the heater box is removed (might want to put the nut in the engine bay back on.)
    Oof, yeah, that does seem to be exactly what's going on with me. Thanks for the link -- you may well have just saved me from breaking my heater box!

    Sounds like what they're saying in the posts right above and shortly after the one you linked is that I need to peel back the insulation to access a hidden nut. I should hopefully have some time to attempt that and also pop open the VSBC and look for any solder joints that have failed. (I'll also try to use some 99% rubbing alcohol to clean off that crustiness as best I can.)

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