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I have an 89 DeVille, and service engine light stays on and…

I have an 89 DeVille...
I have an 89 DeVille, and service engine light stays on and my fuel and temperature displays are not showing numbers. The ac and heat doesn't work. My battery is also being drained by something in the car. I unhooked my battery for 2 days, reconnected it, and the car started up with no problem. My research showed me that the BCM may likely be the issue for the displays. Could a bad BCM drain the battery and cause the displays not to work?
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Answered in 1 day by:
8/22/2017
Jerry Newton
Jerry Newton, Cadillac Technician
Category: Cadillac
Satisfied Customers: 4,918
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Absolutely. BCM's were notoriously faulty in these years.

If you enter diagnostics, do the fuel and HVAC displays light up? Do you know how to enter the diagnostic mode?

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Customer reply replied 5 months ago
I don't know how enter into diagnostic mode, could you tell how? Thanks!!

With the key on, engine off, press and hold the OFF and WARMER buttons on the climate control panel, until all of the displays light up, then release them. Should be a few seconds for this to happen.

I'd be most interested to see if the fuel data center and HVAC control head work at all, if they don't, then we can't get trouble codes this way, as the codes are displayed on the FDC. But just trying to enter diagnostic mode and seeing what happens will be a step in the right direction. If you do get numbers on the FDC, write them down and let me know what they are.

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Customer reply replied 5 months ago
Ok great I'll do that tonight when I get home! How will I know whether the HVAC control head works, how does that display look like?

You said AC and heat don't work, but it's not clear to me... does the temperature and whatnot display on the control head, or is it blank?

When you enter diagnostics, all of the green displays on the dash, FDC and HVAC panel should illuminate, like a bulb check.

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Customer reply replied 5 months ago
Oh ok. So yes, the fuel and temperature displays both don't display numbers to indicate my remaining fuel amount and the temperature setting of the ac/heat. They ac and heater don't work either. The ac use to work prior to this issue, but my heater stopped working about 4months prior to the displays not working.

If the display is blank, then it probably isn't working internally, either. Entering diagnostic mode would tell you if the display has the ability to light up. If this were in my shop, I'd start there, and just see what it looks like. If you don't get any display, you may be looking at a failed BCM, which has the power supply for these flourescent displays (they run at higher voltage, 55v).

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Customer reply replied 5 months ago
Ok I tried to put in diagnostic mode, but no displays lit up other than the battery light that was already on when I turned the key. I held the buttons down for at least 10secs three times, then my battery died

OK, that's a good photo! The displays work, you're getting a letter C on each display. That's good, that means they have the ability to light up. However, it also means there is a problem with data communication, which means your BCM is likely faulty.

I'd replace the BCM and try this again.

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Please take a moment to rate my answers, that's how I get compensated. Let me know if you have any further questions, I am happy to follow up!

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Customer reply replied 5 months ago
Awesome!!! Great answers! Definitely appreciate you for helping

It's my pleasure, good luck with it! Let me know if you need anything else!

Please remember to rate my answers, that's how I get compensated.

-Jerry

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Customer reply replied 5 months ago
Thanks will do!!

.

Jerry Newton
Jerry Newton, Cadillac Technician
Category: Cadillac
Satisfied Customers: 4,918
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
Verified
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