View Full Version : Basics for operating ECM on 84 Deville HT4100??

06-13-11, 06:56 PM
Does anyone know the basics of how to operate the ECM for a 84 Cadillac Deville with HT4100? The climate control panel is where most of the magic happens, but I'm not sure of what all needs to be done to get readings, etc.

I am able to get the diagnostic codes by holding 'off' and 'warm' at the same time. But that is pretty much the extent of what I know about operating the computer.

Can anyone provide a little more insight as to what can be done with the ECM and how?

Thanks everyone.

06-20-11, 07:45 PM
This is actually a good question that some of the 'experts' here ought to document in a sticky or something. Checking out some of the past archived responses to this subject was actually disappointing when I first started using it and trying to learn about it.

One great feature is you can measure what values are being input to the ECM, such as the throttle position sensor angle and engine rpm, etcetera. You can also manually change things through the ECM, such as the idle-speed control position.

I think the procedure for accessing these features varies a bit depending on your generation of 4.x engine. At one point Cadillac even changed the name from ECM to PCM! That means Powertrain Control Module (PCM), by the way.

This is not documented particularly well in my 1992 Factory Service Manual, but I was able to figure it out after some playing around. It is mostly accomplished by selecting the options through the climate control fan 'higher' or 'lower' buttons to what values you want to monitor or parameter you want to change.

06-20-11, 07:51 PM
There's a fairly complex option tree that isn't feasible to replicate here. As said above, you can do some interesting stuff with it (run a test of all the cabin switches, manually test the electric fans, etc), but you pretty much need to have the FSM to understand what they do and how to work them.

06-22-11, 03:16 PM
When I experiment and push 'hi' or 'low' when in diagnostic mode, it doesn't seem to do anything. All I can seem to get it to do is flash the error codes. I've tried pushing the hi or low buttons (really all buttons) when it was flashing the codes and also when it was done flashing (but still in diagnostic mode). Nothing seems to work.

I wonder if the 84 ECM doesn't actualy have these capabilities??
Were these features possibly added after 1984??

06-27-11, 03:19 PM
Someone should correct me if I'm wrong, but I believe the basic procedure (on a '92 4.9 liter) is to first push 'warmer' and 'off' at the same time, then wait until all the current and historical trouble codes are displayed. This could take some time (depending how messed up your Caddy is! :eek:)
Then, the display will read 'ECM?'

At this point, you would either push the 'fan high' button to continue down this ECM path of choices, or push 'fan low' to do other options besides the ECM tree of choices.

07-05-11, 03:40 AM
If this is a rear wheel drive DeVille the only thing you can do in the climate display is get codes to display and erase them. The 85 front wheel drive cars are equipped to display throttle angles, alternator voltages etc. The 84 has an ALDL connector under the dash.

07-05-11, 02:16 PM
This is a rear wheel drive, and it does have the ALDL connector under the dash.
I kinda thought that this car might not have the capabilities since I can't get the computer to do anything else, but its nice to have confirmation that I'm not just being a moron :hmm:

Thanks for your help.
I guess I'll have to connect the car to a external testing device to get the info....