GTG, I just pulled this Caution out of the '89 Service, Driveability, & Emissions manual:
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If the key was on when the battery was connected backwards odds are the computer is smoked.
Even if the key was OFF, there is still 'Hot At All Times' power on a separate pin that's used to to keep
the contents of long-term fuel trim data and error codes stored in the computer's volatile memory intact.
In other words, reversing the power is going to be visited upon the computer internals, through 1 or more
power pins. Now to find out if the '89 ECM is diode protected in it's internal power supply to help protect it
against this type of human error during maintenance. :-(
No problem. Everyone in here wants to help others keep their GMT400 on the road as a
reliable traveling companion. But in order for us to help you remotely, we are literally all going
to have to be on the same page in the relevant FSM.
I've attached a couple of screen snaps showing that Section 3 of the '89 Service, Driveability,
& Emissions Manual is where we need to be in order to follow the factory troubleshooting flowcharts.
Thanks to the contributions of fellow members in this forum, you have the ability to download these
Factory Service Manuals for free. If you haven't done so already, please take advantage of this
opportunity to give yourself the best documentation available: (
'88+ GMT400 FSM download links)
So, we need to regain the ability to start the truck asap. A quick common sense check to
see if your computer is still with us is to turn the key to the ON position & see if the SES
light comes on steady. (It's supposed to, allows a bulb check for you/mechanic/inspection station.)
If the light comes on, then I would proceed to shorting Pin A to Pin B on the ALDL, and see
if the computer is healthy enough to flash error codes via the SES light. (At a mininum, 3
flashing 12 codes saying all power-up diags passed.) If there are error codes, then of course
we start off by fixing the obvious.
And if the SES light doesn't come on, then we need to verify all the power to the computer.
(Following FSM guidance & wiring diagrams.) And since you mentioned your trouble light, I know
that base is covered. But if you don't already have a trustworthy multimeter then you
need to add this to your tool kit. Some stuff will be +12V, others will be regulated +5v
for use by the sensors, etc.
More to follow. Obviously if you take this to a shop (assuming that they would even take it on)
then the price to troubleshoot all this would be a nontrivial amount of money. But if you
are up to learning all this, then get those manuals downloaded and between the all of us we
should be able to start with the recovery process.
Hope this helps. Been there, toasted that, so I have empathy for the predicament that you
find yourself in.
Best of luck! Let us know what you discover & we will go from there.
