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Cadillac Repair/4.5L 1990 fleetwood BCM


I'm going to the auto wreaked tomorrow to get a new BCM  I'm a novice and will have no help I need stepby step istructions on what to look for and how to remove and install this in my car diagrams and step by step instructions would be greatly appreciated its a 1990 Fleetwood Cadillac 4.5L FWD fuses are all good and car has an A/c bypass fuel data center and climate controls center display "d" with service engine soon light on heat works starts fine please help

Hello Kat,

Thanks for the rating. I have owned over 20 Cadillacs. In fact, had a new 90 Deville, 91 Fleetwood and 93 Deville. I worked for GM dealers for 11 years and our shop repaired over 10,000 theft recoveries for 17 year with a majority being GMs.
As I said, my memory on exact component locations from a generation ago is getting foggy.

Check around for the BCM. They vary in price, but you can usually get one rebuilt for about $100 plus exchange at OReily's, Autozone and stores like that and they come with a decent warranty. Pretty easy to swap out.
Check out as well.
As I stated in my firs response though is that these old systems were hard to diagnose properly. Unless you had a code in he 50s, it was not real common to have a bad ECM or BCM. Yes, it happened, but more commonly a sensor.
When there is a computer system problem, sensor or computer, the engine is running in what is known as open loop. For maximum efficiency, the system needs to be in closed loop.

Before you start throwing money away on guesses (We call this chasing ghosts) I would highly recommend locating nothing but a Cadillac DeVille/fleetwood manual from Ebay or Craigs. Will save you tons of money in future repairs as well. It is without a doubt the best tool you can ever have. That is how we repaired them at the dealer.
There is a diagnostic flow chart to track down the exact problem.
This car can have a bad computer, but what if you change it, and that is not the problem. Junk yards and parts sores don't take returns on electronic if misdiagnosed.

Unless your mechanic is my age (63) or has a manual, he could have misdiagnosed as well.
I try to help my questioners as best as possible and I don't want them to waste money!

FYI-there are also inherent problems with these cars to watch for. No big deal if you know about them, but if you don't can be catastrophic!

Aluminum engine that will melt if temp gauge goes above 225. Engine needs to be shut off immediately no matter what if it gets that hot!

Heater blower circuit can fail and will run continuously til battery is dead.

VATS (black resistor chip in key). Notorious for not preventing theft, but stranding owners, where engine starts a times. Dealer rip offs will hit you $850 to replace defective ignition lock/harness. Harness has real thin wires that break inside steering column.

Best repair, go to alarm store and have them bypass VATS for about $60.00. Dealer won't do it!

These are a very nice car, and all Cadillacs have these quirks.

I have been a Caddy man for most of my life, but changed last year when wife bought a Mercedes 500 s. Well, guess I am now a MB man--lol

If you run across the any of those problems I told you about, you know where to go.

Good luck!


A junk yard does not stock new parts and you may be best off checking with a parts store for a rebuild.
As for diagrams, I discarded all of this 10 years ago. You could go to the library and see if they have a service manual for this car. Or you could check with an internet search on how to replace the BCM in a 1990 Fleetwood.

Please remember, for you this may be a new car. To me it is 25 years old, thousands of vehicles ago for me. I haven't replaced a dash on one of these or done interior work on one for 15 years and most likely a current Cadillac tech hasn't either.

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Rob Painter


Alarm system questions cannot be answered on this forum. These systems are not what I can answer. Without being physically at the vehicle and not knowing what kind of electrical service has been done on the vehicle, there is no possible way to give an accurate answer over the internet. My expertise is in Ignition/key based anti-theft systems. These issues include GM VATS (resistor chip in key blade) PASSLOCK (MRD)-ignition lock rotation based, no special ignition key and the PKIII Transponder (computer chip in key) systems. These systems are not alarm based and are integral with the starting of the engine. This is why I cannot diagnose alarm problems without physically looking at the vehicle: Alarm systems are a completely different annimal than ignition key/lock based anti-theft system. Many alarm questions come from vehicles 10 years old, and since older, many hands that had been involved over the years.I am an expert in all GM factory (ignition/key based)systems. Alarm system questions pose to many situations beyond my knowledge as to what has been done to the vehicle over the years. Some guy may have actually wired the stereo into the alarm system. Who knows? Over my past 30 years in vehicle wiring repair, I have seen unbelievable wiring disaters done by guys that consider themselves "mechanics." I have seen stereos and alarms intalled using surgical tape. I have seen modules burn up, un-fused circuits, wiring jambed between the doors and even lamp cord used for a starter kill. To answer alarm questions over the internet without examining the vehicle is like asking; What does it take to remove a dent?


Education/Credentials-ASE certified. 11 years with a GM dealer and 17 years with a repair facility dealing with only the repair of theft recovered vehicles.

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