Climate control and fuel data center

Tiny
STUK69ER
  • MEMBER
  • 1989 CADILLAC DEVILLE
  • 4.5L
  • V8
  • 2WD
  • AUTOMATIC
  • 129,322 MILES
Recently purchased this car. Yesterday we ended up putting a new battery in it because the old one, which was supposedly "new-ish", only tested out at nine volts. This solved the problem of the car not starting regularly but the next problem is the big issue. When you start it up, the climate control center and fuel data center readouts only display a small "c" on them. I tried a little research on the old internet and found the diagnostic testing by holding down the "off" and "warmer" buttons on the climate control center for three to five seconds which did not work for me. However, after pushing and holding different combinations of buttons for the three to five seconds I was able to get the displays to work on both the climate control center and fuel data center. However, it did not last and the small "c" returned after the very next start-up. Here is a little more information, the blower comes on automatically at start-up and air can be felt coming out the front floor vent but there is no way to control it as the buttons do nothing when pressed and there is still no display on the climate control center, other than the small "c" that is.
After checking under the hood, we noticed there is an 8" vacuum hose with a screw stuck in it because it is not attached to anything else and there appears to be something missing behind the MAP sensor up by the firewall as there are two threaded studs with rubber spacers and nothing attached or connected there. We have looked for pictures to clue us in on what should be attached but have had no luck yet.
Also, I know the speedometer is not calculating and displaying speed properly as it reads 0 mph while driving and the trip odometer constantly reads 0.0 miles driven. The odometer does not seem to be changing either as we have driven it a few times and still reads the same.
After start-up, the engine runs but does give an indication of a slight miss and will lightly backfire if you rev the engine high enough. The guy we bought the car from said he changed the spark plugs and put on a new distributor and rotor but we went ahead and bought new plugs just to be safe. Have not changed them yet but will be doing that soon. When you take the car out on the road, it has a horrible miss and will almost not want to accelerate at lower speeds. It takes it a little while but eventually you can get up to highway speeds but it seems to have gotten worse instead of better.
I know this seems like a lot to take in but we would sure appreciate any help or insight anyone might have on these issues. My son and I are not what you would call "car guys" but we will dig into this and try to get it figured out as the guy we bought it from said it used to run really good.
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Sunday, May 27th, 2018 AT 9:03 AM

7 Replies

Tiny
ROY B
  • EXPERT
Good afternoon.

The most common failure for this is the body control module. It controls both displays and is a common failure.

Try tapping on the unit with the key on and see if the display comes back.

Roy
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Sunday, May 27th, 2018 AT 9:39 AM
Tiny
STUK69ER
  • MEMBER
No luck with tapping on the display panel, is that what you were suggesting? Or were you referring to the control panel inside the dash?
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Sunday, May 27th, 2018 AT 10:28 AM
Tiny
ROY B
  • EXPERT
I was talking about the body control module I sent you are the picture of. Not the display unit at all. They very rarely fail.
Three of the four pictures are under the hood and have nothing to do with your current issue.

Roy
Roy
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Sunday, May 27th, 2018 AT 10:37 AM
Tiny
STUK69ER
  • MEMBER
Got to the BCM behind the glove box. The pictures under the hood were meant to be included with the original problem that detailed issues with vacuum lines. We are currently chasing multiples problems. Changing out spark plugs now and will check the climate control display when we reconnect the battery.
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Sunday, May 27th, 2018 AT 11:12 AM
Tiny
ROY B
  • EXPERT
Sounds like a plan.

Roy
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Sunday, May 27th, 2018 AT 11:19 AM
Tiny
STUK69ER
  • MEMBER
Turned it to run and heard some clicks coming from the BCM and the displays still showed the small "c" so probably going to have to get a new BCM.
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Sunday, May 27th, 2018 AT 12:26 PM
Tiny
ROY B
  • EXPERT
Yep, sounds about right.

Roy

https://www.rockauto.com/en/catalog/cadillac,1989,deville,4.5l+273cid+v8,1025693,electrical,body+control+module+(bcm),2888
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Sunday, May 27th, 2018 AT 12:28 PM

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