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

  1. #1

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

    Long-time lurker, first-time poster. I've found the guides on here helpful in the past when I needed to replace my odometer gears and rebuild my front power window motors.

    At the moment, I'm a bit puzzled about what exactly is wrong with the climate control system in my 1990 Grand Marquis. For several years now, the car has had AC which works perfectly fine, but no heat. I can move the ATC temperature selector on the dash all the way up to 85, and it stays just as cold as at 65. I used to hear a faint hiss and then a click when I moved the selector from one extreme to the other -- which I thought was the sound of the blend door actuator moving the blend door. I no longer hear that when I move the selector.

    So, I thought this was an open-and-shut case of the blend door actuator malfunctioning. Everything I read up on suggested the small gear inside the BDA unit is what almost always fails, so I bought the replacement gear, followed one of the helpful guides on here to take apart the dash for a 90-91 model, removed the BDA unit, opened it up, and... all the gears look perfect. I moved the metal gear which extends from the actuator itself with my finger to ensure that the plastic gears and the actuator arm would turn all the way from one side to the other and then back. They did.

    So I guess this means there are a few remaining possibilities:
    1. The actuator itself has somehow malfunctioned and no longer spins, in which case I suppose I should buy an all-new BDA unit.
    2. The heater core is not working properly.
    3. The in-car temperature sensor is not working properly and constantly thinks it's like 100+ degrees in the car at all times, so the heat is never activated.
    (Let me know if there's another one I'm overlooking.)

    Any thoughts on which of these is most plausible? Or any suggestions on how I should diagnose which of these is the culprit (without having to reassemble everything first)?

    Thanks for any comments!

    Edit: I should mention there was a period of a couple years when the heater "sort of" worked. It seemed like, on some occasions, when I switched the selector from 65 to 85, I could get it to switch into heat and, on other occasions, I couldn't. But once I got it to switch, it stayed on heat. This situation was also what initially caused me to believe it was something with the BDA; these symptoms seemed (to me) more consistent with a gear deteriorating than something about the temperature sensor or the heater core.
    Last edited by SomeoneWeird; 11-22-2020 at 12:59 AM. Reason: added details

  2. #2
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    Take a look under the hood. Find the 2 heater hoses and see if they are linked together and not going into the heater core itself (a.k.a heater core bypass). If the core went bad before you got it maybe someone just bypassed it since it's a PITA to replace the cores in these cars.
    These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.

  3. #3
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    If the BDA is Good, I've heard of the blend door itself breaking. Although that may have been on later models.
    The vacuum hiss and door that you may have heard before would be mode doors or recirculation door since those are vacuum controlled while the BDA is electronic. So the absence of that hiss/click may be unrelated to the no heat issue.

    Something I've had happen is a restricted but not totally clogged heater core. The old trick of feeling if both hoses to the heater core are hot did not work in my case. They were both hot but my heat was non-existent. I found that if I ran around with the heat off for a few miles then turned the heat on, I could feel heat then it would go back to cold. The blower motor off was not removing heat from the heater core allowing it to slowly build up despite the heater core restriction. Once the blower motor was on, the heat was removed faster than the restriction allowed the core to heat up.
    Flushing the heater core would be worth a try. I have 2 old heater core hoses that I modified to connect to garden hoses. One garden hose to the water supply, the other to a large tote (sitting beside the car) to easily catch the discharge. On my '89 one flush worked for years. On my '91, it worked for maybe a month before I had to flush the core again, and then again another month later, repeating until either Winter was over or I replaced the heater core.
    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

  4. #4
    Wagon Addicted Tiggie's Avatar
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    Move the temperature slider and be sure it’s actually moving someone under the dash. Not intimately familiar with the 90+ ATC, but I know the sliders tend to suck, and if it’s like my 88, it still moves a cable for the temperature.

  5. #5
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    Electronic BDA for '90+ ATC equipped cars. Even the manual controls went to electronic BDA in (I think) '95, though I could be wrong on the year.
    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

  6. #6

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    Thanks for the suggestions, everyone. Both of the hoses were indeed still connected to the core. But I (foolishly) didn't try the trick of feeling their temperature before dropping the steering column, removing the dash, etc. since I was overly-sure it was the BDA.

    Quote Originally Posted by VicCrownVic View Post
    If the BDA is Good, I've heard of the blend door itself breaking. Although that may have been on later models.
    The vacuum hiss and door that you may have heard before would be mode doors or recirculation door since those are vacuum controlled while the BDA is electronic. So the absence of that hiss/click may be unrelated to the no heat issue.

    Something I've had happen is a restricted but not totally clogged heater core. The old trick of feeling if both hoses to the heater core are hot did not work in my case. They were both hot but my heat was non-existent. I found that if I ran around with the heat off for a few miles then turned the heat on, I could feel heat then it would go back to cold. The blower motor off was not removing heat from the heater core allowing it to slowly build up despite the heater core restriction. Once the blower motor was on, the heat was removed faster than the restriction allowed the core to heat up.
    Flushing the heater core would be worth a try. I have 2 old heater core hoses that I modified to connect to garden hoses. One garden hose to the water supply, the other to a large tote (sitting beside the car) to easily catch the discharge. On my '89 one flush worked for years. On my '91, it worked for maybe a month before I had to flush the core again, and then again another month later, repeating until either Winter was over or I replaced the heater core.
    Interesting. When you were having those issues, were you having any other related issues? My (potentially incorrect) understanding had been that when heater cores start to malfunction, it typically causes other issues, too: coolant leaks, windows continuously fogging up, engine overheating issues, etc. I've had no problems like that, but if you didn't either, maybe it is my heater core after all.

    One thing I was just thinking about. You guys can tell me whether this seems reasonable or totally absurd. When I removed the BDA unit, I noticed the little metal arm guided by the BDA which moves the blend door. I grabbed it with my hand and moved it back and forth, and I could tell that it was moving the blend door since it had some weight to it and made a soft thud when I moved it all the way to one side or the other. There have been a grand total of zero days in my entire life when I've wanted to use heat and AC on the same day. I switch to heat, then six months later to AC, then six months later to heat, etc. If it's an issue with either the actuator or the sensor, couldn't I just use some duct-tape (or something with superior heat-resistance if necessary) to affix the blend door into the heat position? I noticed before removing the dash that, if I dropped the glove box, I could see the side of the BDA unit from within there -- the side that I now know makes contact with that little metal arm. So maybe I'd even be able to change its position every six months by reaching in there through the glove box and adjusting it.

    This would totally bypass both the BDA and the temp sensor, so if it's a problem with either of those, problem solved. And if the heat still doesn't work after I do this, I'll know it's the heater core for sure.

  7. #7
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    No other problems in my case. I got tired of flushing the heater core so decided to replace it.

    Oh, I almost forgot, make sure your engine coolant is full. Low coolant can cause no heat. (Sorry if you already mentioned doing this.)

    Fogging and coolant loss are symptoms if a leaky core. A clogged core would not have those issues.
    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. #8
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    As for the blend door, it sounds like it probably is not broken. The idea of fixing it in one position could work. Varying the temperature will not be as simple/possible but if you gain heat by doing that it could be worth the trade off until you can fix it.
    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

  9. #9

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    Makes sense -- thanks again. I'll start by fixing the blend door in the heat position (hopefully I can figure out which one that is without having to try both) to see whether I get heat when I do so. If that doesn't work, I'll plan on replacing the heater core -- might as well while the dash is off.

    Related question: Is there some trick to removing those heater core hoses? I completely unscrewed the hose clamps to the point of taking the screws fully out, but even after pulling quite hard on the hoses, they wouldn't come off. I guess I could have pulled even harder, but I was worried about damaging the hoses. I know more recent cars have a clicking mechanism for taking them off -- for the box GM, is it just a question of pulling really hard on the hoses? (I wonder if they've never been touched in 30 years and are basically just stuck at this point.) I was trying to get them off to make it easier to remove the BDA unit, but I eventually gave up and used the world's tiniest wrench in a contorted angle to do so. If I'll be swapping out the heater core, though, then I'll definitely need to remove those hoses.

  10. #10
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    Those hoses are a pain.
    They basically over time mold/glue/weld/bond (not sure what the correct tem is) themselves to the core inlet and outlet. The easiest way to remove them that I've found is to cut them, but if you don't have replacements on hand that might not be a reasonable suggestion. I cut starting at the end of the hose straight forward toward the front of the car. Without cutting you need to wedge something between the inlet or outlet tube and the hose. There isn't room to really do this but a right angle pick might work if a flathead screwdriver is too long. I did it this way once and it really sucked and took forever, but eventually I got the hoses to stop bonding with the tubes.

    When putting new hoses on I put some dielectric grease on the inlet and outlet tubes. I'm not sure if that grease is ok to use with the rubber hose, but I haven't had any issues with it.
    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

  11. #11
    The Brown Blob 87gtVIC's Avatar
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    They become one over time is right. You really have to be gentle on the core as well. Its old and can become disturbed easily by wrestling with those hoses. As said above it is best to cut the hose open when removing the core. Replace with new.
    ~David~

    My 1987 Crown Victoria Coupe: The Brown Blob
    My 2004 Mercedes Benz E320:The Benz

    Quote Originally Posted by DerekTheGreat View Post
    But, that's just coming from me, this site's biggest pessimist. Best of luck

  12. #12
    I'm an air-conditioned gypsy gadget73's Avatar
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    silicone grease (dielectric grease is silicone) works fine with rubber. Thats what the stuff specified for O-ring lube actually is. It very slightly swells the rubber too, makes for a better seal, but as a bonus it makes things let go.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

  13. #13

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    Well, unfortunately for me, it seems that I'm not getting heat even with the blend door fixed in the heat position, bypassing the BDA and the in-car temperature sensor. So it looks like it is indeed the heater core. I'm going to go ahead and buy a new one and do the replacement instead of a flush since I've taken off the whole dash already.

    One last question before I attempt that, though. You guys mention cutting off the old heater hoses. And while I don't have new ones on hand, since I'll be buying a new heater core, I might as well buy a couple new hoses at the same time. Problem is I can't figure out which two of these hoses are the right ones: https://www.rockauto.com/en/catalog/...se+/+pipe,6892

    Only the Motorcraft part uses the familiar inlet/outlet terminology, and they only have the Motorcraft outlet hose listed, not the inlet hose. Any idea which off-brand ones would be the right equivalents? Judging by shape/size, the Gates 18741 and Gates 18708 look like the right two, but they both say they go to "Pipe-1".

  14. #14
    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    Gates part numbers:
    19610 goes from the heater core to the thermal blower lockout (TBL) switch.
    18708 goes from heater core to the pipe under the passenger side of the upper intake (pipe-1, first time I've seen it called that).
    18741 goes from pipe-1 to the water pump.
    19683 goes from the TBL to the lower intake (front passenger side corner where the ECT sensor is.)
    19682 can replace 19619+19683 IF you wish to eliminate the TBL.
    The only other hose I haven't mentioned (besides upper and lower radiator hoses) is the bypass hose. Really short right angle PITA to get at hose that comes off the water pump, 20662 under cooling system.

    Edit: also forgot there are 2 little hoses behind the upper intake for the EGR cooler. Even bigger PITA to get at with the upper intake on. Just a couple feet of plain 1/4" hose for that.
    Last edited by VicCrownVic; 11-23-2020 at 12:14 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

  15. #15
    I'm an air-conditioned gypsy gadget73's Avatar
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    I'm not a big fan of the TBL. Usually fails and keeps the heater blower from running and it causes vacuum leaks. If you delete it, jumper the two electrical wires together and seal the vacuum connections. Ball bearings and a wee bit of silicone works nicely for this.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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    2 decades of DDing Box Panthers, now in a Whale VicCrownVic's Avatar
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    Quote Originally Posted by VicCrownVic View Post
    Gates part numbers:
    19610 goes from the heater core to the thermal blower lockout (TBL) switch.
    18708 goes from heater core to the pipe under the passenger side of the upper intake (pipe-1, first time I've seen it called that).
    18741 goes from pipe-1 to the water pump.
    19683 goes from the TBL to the lower intake (front passenger side corner where the ECT sensor is.)
    19682 can replace 19619+19683 IF you wish to eliminate the TBL.
    The only other hose I haven't mentioned (besides upper and lower radiator hoses) is the bypass hose. Really short right angle PITA to get at hose that comes off the water pump, 20662 under cooling system.

    Edit: also forgot there are 2 little hoses behind the upper intake for the EGR cooler. Even bigger PITA to get at with the upper intake on. Just a couple feet of plain 1/4" hose for that.
    Opps, 19619 is a typo, should be 19610.

    Quote Originally Posted by VicCrownVic View Post
    19682 can replace 19610+19683 IF you wish to eliminate the TBL.
    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

  17. #17

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    Thanks for the help, guys. That list of hoses is quite useful. I'll buy the heater core and the necessary hoses today, so hopefully next weekend I'll get this taken care of and have heat once again for the first time in years.

    For now, I'll just leave the TBL as-is. Since this heater core replacement is a bigger job than I've attempted in the recent past, I'm slightly wary about doing too many things at once in case I somehow screw up and need to figure out what exactly went wrong. But I'll definitely keep that in mind about bypassing the TBL if I need to do so in the future.

    I am, however, going to replace the variable-speed blower control, because that is not working and hasn't worked in the seven years I've had the car. Never bothered me that much, but since it's right below those hoses that I'll now be removing, I don't really have an excuse for not taking care of it.

    I'll post an update after I've (hopefully) gotten things in place and working.

  18. #18
    I'm an air-conditioned gypsy gadget73's Avatar
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    half the time its just a crusty connection, but the resistors do die. I think a '90 still uses the old resistor style. if its the modern electronic one, those usually just have broken solder joints that can be fixed for no $.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

  19. #19

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    So I managed to successfully get the heater core hoses off without cutting them or otherwise damaging them. I bought and used some hose removal pliers, which worked quite readily. And then I unscrewed the retaining nut right next to the heater core hoses.

    I'm having trouble on what I guess is the very last step, though. No matter how hard I pull and which angle I come at it from, I can't get the heater box to move. 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. Like I said, I unscrewed that retaining nut. 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.

    I'm pretty sure I followed every step in the heater core write-up on here, including removing the two nuts at the bottom of the heater box. Has anyone else had issues pulling the heater box out? Is there something obvious I'm overlooking?

  20. #20
    I'm an air-conditioned gypsy gadget73's Avatar
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    I think there is one more screw in there that you may have to access from the engine bay side. I did mine twice but its been a long time and I don't quite remember but thats whats coming to mind.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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