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1988 Cadillac Deville Sedan
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Discussion Starter · #1 ·
My recently acquired 88 Deville Sedan, is a good car overall, but I have no fuel/cc display (both displays are digital). When I got the car, it had an aftermarket stereo system installed by a previous owner, and needless to say, whoever installed it did a [email protected]$$3D job of doing it. Anyway, since I've owned it, I've disconnected the amp in the trunk (that was powering the subs), I've replaced most of the fuses, and got some of the basic components working again. A majority of the fuses previously installed had amp ratings that were too high, beyond owner's manual specifications. However, I've had no success, getting either display to work, by replacing the old fuses. I have a fuse/circuit tester, and while many of the circuits are good, I still get some bad readings in different fuses, even after replacing them with brand new ones. Once in a while, if I turn on my headlights at night, starting my engine thereafter, sometimes I will get a rather subtle flicker from both display units, but nothing more than that. All my other lights work perfectly fine.

Could these units be fried, and if so, how would I know?

What else should I do?

Thanks in advance for any, and all information/assistance you provide.
 

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85 Coupe Deville 4.1 V8 & 4.3 V6 Deisel & 1970 Eldorado
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773 Posts
Does the climate control still function even though the display is dead? Does the fan work and do you get heat and air out of it? If so then it is probably a problem with the display voltage.

The digital displays are driven by a 16 volt source generated in the Body Computer (BCM) along with the normal 12 volt B+ from the battery. If both displays are out, Then i would check to see if the 5 volt, 12 volt, and especially the 16 Volt supply is present.

If these voltages are missing, Then check for shorts in the wiring between the display and the Body Computer.
If there are no shorts, Then i would suspect the BCM needs to be replaced. These are known to fail.

If you can not find a BCM from the same year, The computers from i believe 1985 - 1988 are the same. I am not sure about later years. A check of the numbers on the computer (Not just the part number) will help guide you. You just may need to exchange the prom for the one in your current BCM.



 

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1988 Cadillac Deville Sedan
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16 Posts
Discussion Starter · #3 ·
Re: 88 Deville Sedan - Fuel Data Center/Climate Control Displays NOT WORKING

Thanks for the info man... I'll see what I can do, my climate control does function properly, but without display of course.

Also, how would I replace the PROM to match with a new BCM, if it should become necessary?
 

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85 Coupe Deville 4.1 V8 & 4.3 V6 Deisel & 1970 Eldorado
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773 Posts
The prom is under an access plate on the BCM. Just open the door (one fastener i believe) and the prom will unplug. Be careful though as the pins are fragile. Prom replacement will probably not be necessary though.

If the climate control works, Suspect a missing 16 volts or a bad BCM.

You can also try to unplug one of the display units to see if it is shorting the 16 volt line. (Gas or Climate Control)
 

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1988 Cadillac Deville Sedan
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16 Posts
Discussion Starter · #5 ·
ELECTRICAL NIGHTMARE


The prom is under an access plate on the BCM. Just open the door (one fastener i believe) and the prom will unplug. Be careful though as the pins are fragile. Prom replacement will probably not be necessary though.

If the climate control works, Suspect a missing 16 volts or a bad BCM.

You can also try to unplug one of the display units to see if it is shorting the 16 volt line. (Gas or Climate Control)

So far, here's what I've done:

  • Checked for shorting (results were the same)
  • Checked the 5, 12, and 16v (Everything is normal)
  • Replaced the BCM, Fuel Data, and Climate Control Units (No Luck)
  • Swapped Proms and plugged in each BCM once (No Luck)
  • Checked driver's side fuses (All are good)
  • Checked passenger's side fuses; behind glove box (All are good)
  • Removed ELC fuse in left fuse panel (Replacing ELC Fuse had no effect)
  • Replaced previous Alternator w/good one

So far I'm running out of ideas, what else should I do? Is there something I'm missing?

As mentioned before, the Climate Control still functions and responds when buttons are pressed, but neither unit will display. The text will illuminate when I pull the light switch, but that's it. The screens themselves remain completely blank.

Maybe my next best bet would be to check all affected wiring...? However, I'm unsure which wires to check, as there are so many of them. The previous owner disabled the OEM Factory Car Alarm that came with it, now every time I reconnect the theft module, it goes off loud as hell. Would this have any effect on my display Fuel/Climate units...? If anyone has a service manual let me know, you'd be doing me a great favor if you could inform me as to what I should check, and what I could try next. Photo uploads from such a manual would be greatly appreciated, if possible.

I'd really hate to take it to the Dealership.

You guys have helped me out alot.

Thanks in advance!
 

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1993 Coupe Deville, 2009 XLR-V 1 of 6 Crystal Red
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2,484 Posts
ELECTRICAL NIGHTMARE





So far, here's what I've done:

  • Checked for shorting (results were the same)
  • Checked the 5, 12, and 16v (Everything is normal)
  • Replaced the BCM, Fuel Data, and Climate Control Units (No Luck)
  • Swapped Proms and plugged in each BCM once (No Luck)
  • Checked driver's side fuses (All are good)
  • Checked passenger's side fuses; behind glove box (All are good)
  • Removed ELC fuse in left fuse panel (Replacing ELC Fuse had no effect)
  • Replaced previous Alternator w/good one

So far I'm running out of ideas, what else should I do? Photo uploads from such a manual would be greatly appreciated, if possible.

I'd really hate to take it to the Dealership.

Thanks in advance!
I'd hate to see you spend much more than this right now....
http://cgi.ebay.com/ebaymotors/1988...Motors_Manuals_Literature&hash=item53e84e7552
 

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1988 Cadillac Deville Sedan
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16 Posts
Discussion Starter · #7 ·

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1993 Coupe Deville, 2009 XLR-V 1 of 6 Crystal Red
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2,484 Posts
Good find, dude... I should have just looked for one of those in the first place. Looks like shipping will be taking a few weeks. I'll see what I can do, and I'll keep ya guys posted.

Thanks for any additional assistance.
Three weeks!!?? Where the hell you live, Timbucktoo??
 

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88 sedan deville
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hi cadillac devil, could you tell me if you had ever resolved the problem you had with your deville regarding the climate control. i am going through the same problem at this point in time. i replaced the speedo head, climate control, along with a bcm and still no luck. my primary car was repossessed and i have to put the caddy back on the road. finances are really tough at this point and any bit of help will really be appreciated. i have no digital dash display at all. no gas guage, data center, climate control or speedometer. i checked all fuses and relays. all seem ok.when it went out it faded and then went out completely.hoe you can guide me in the right direction. thank you so much. jeff [email protected]
 

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1993 Coupe Deville, 2009 XLR-V 1 of 6 Crystal Red
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Your BCM turns on all the readings for the display on all of your FDC, ELC, Speedo, etc. Your headlight switch dims them, but you should at least have something glowing. You've thrown a lot of parts at it and if I were you I'd get a FSM before spending any more money on parts. Ebay is your best friend here. Check your fuses, might be on your instrument panel fuse block cover. What you have is a display dim enable wire from your BCM to the head light switch light switch that you need to get voltage from one of your fuses. If you have headlights, then it might just be a 10 amp fuse that is blown.
 

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1986 Cadillac Sedan De Ville
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2 Posts
I have the same problem with a 1986 and all the fuses are good, can't find the relays. Someone please help, heater and blower worked for about a day now have stopped winter is coming and this is terrible. This all happened after installing a cd player and forgetting to unplug the battery before installing. No harness wires were cut, tried putting the new radio back in and nothing. I obviously messed something up. :/

EDIT: found relays and fuses nothing wrong with them, also lights on cc and fuel data center flicker sometimes when key is turned either way.. faulty ground or something?
 

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1993 Coupe Deville, 2009 XLR-V 1 of 6 Crystal Red
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2,484 Posts
What happens when you disconnect all of the connections to the new radio. Your dash lights aren't common, can't connect to the wrong circuit for the radio.
 

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1986 Cadillac Sedan De Ville
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I tried everything from taking the new radio out and putting the old one in, and tried it without one at all. Any help on this is greatly appreciated. :)
 

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Former owner 2008 CTS
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I had an 87 about 10 years ago that I installed a radio in. I plugged the smaller connector in upside-down (forced it not even thinking gee it may be upside-down...idiot move) and lost the displays and the climate control itself. Had to replace the BCM. That brought back the displays, and the climate control worked but was on high all the time. Ended up having to replace an in-line fuse behind the glovebox in a wire to get variable speed back. Traced it using a wiring diagram (I worked at a repair shop at the time). The wiring diagrams were the best thing I found to try to trace the problems...good luck!
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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Did the same thing on my 88 deVille. I lost all the display after an aftermarket radio replacement. Turned out the illumination wire to the radio shorted out the BCM. If I recall it was the 2 wire plug pigtail separate from the main radio plugs.( orange and brown wires?) I disconnected that pigtail and replaced the BCM from a same year wreck.
 

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85 Coupe Deville 4.1 V8 & 4.3 V6 Deisel & 1970 Eldorado
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773 Posts
The displays need a few things to work properly. The main being TX data RX data and 16V and 12V and a good ground. I suspect that tour RX data line has a problem. Either something is shorting it our or you have an open circuit. (assuming the BCM is sending the RX data) There are troubleshooting flow charts that will guide you through the proper diagnostic procedure in the Factory Manual. It will tell you what circuit numbers and how to test them. It looks like you have already replaced all of the components so this is the way to proceed.

Make sure that you follow the flow charts from the beginning even though you may have already checked a particular item. The order in which the testing is done is important. I am confident that you will find that the trouble is simple and easy to overlook. You will also find the Manual to be the best investment you have made in your car. I guarantee it will save you LOTS of money the first time you need it.
 
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