Re: Engine swap computer & wiring question

["Shawn R. Lin" <slin01@mail.orion.org>]

Tim De Coster wrote:
> 
> Thanks for the tips.  Man, that Blazer must fly!  The reason this swap may
> get a little bit hairy is because I don't think I'll be able to use the
> entire harness out of the donor car because it will be an OBDII computer,
> and from what I understand, it's not easy to get a OBDII computer to
> function in an older car.  So do you think that my existing computer could
> control this new engine if I get a new EPROM module for it?  I already have
> one from a '91 Z24 (It had a 3.1 v6), and I am hoping that it will work.  It
> may because the 3.4 is basically the same exact engine as the 2.8, except it
> has revised heads, among a few other things.

If you don't plan to do any tuning to the engine yourself and just want
to swap it in, I think you should be able to transplant the entire
OBD-II electronics as well.  You would probably need to get an O2
simulator for the post catalytic O2 sensor and find similar ways to
bypass any other annoying sensor (like the fuel tank pressure sensor
that checks for a loose fuel cap).

> I know that the Cadillac Cimarron's digital dash does talk to the ECM,
> because I was going to put the Cimarron digital dash into my Sunbird...
> until I found that most of the info. for the gauges goes through the ECM
> before it gets to the dash.  This probably would mean that the Cimarron
> wiring idea is out of consideration, right?  The only reason I thought of it
> was because the dash is physically the same in it, and it has a 2.8 mpfi v6
> engine.

The digital dash in my Cutlass Supreme talks to the ECM via ALDL as
well.
Most gauges like fuel, oil, temp, volts, and speed are obtained by
standard senders.  The tach was the only gauge that relied on data from
the ECM.  I know next to nothing about how my digital dash communicates
with the ECM, but I'm going to guess that it gets the tach signal from
ALDL constant chatter.  If your dash gets data in a similar way, it
MIGHT work, because OBD-II PCM's usually have a legacy serial data line
labeled as the "UART" line.  I recall someone awhile back tested the
UART line and concluded that it is the same format as the ALDL and also
has some chatter on it, in possibly the same format as the old ALDL.  In
fact, even with the Class 2 serial data line, the trip computers and
such in modern GM cars still get their data from the UART line.

I think there's only one easy way to see if the data from a PCM's UART
line is compatible with your digital dash, and that's to set up a PCM on
your bench and see if the dash recognizes the data on the UART line.

Shawn
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