@Road Trip
Sir you are a genius! I owe you a case of beer.
it was the temp sensor! I tested it out and replaced it and the truck runs like a top.
I appreciate everyone that had suggestions, and watched the videos to try to help. We did it guys!
Video is still processing, but wanted to post it here for future users.
xc_hide_links_from_guests_guests_error_hide_media
Greetings Beason,
Congrats on persevering with the repair effort & scoring a successful
fix instead of throwing in the towel and taking it to a shop. And good
on you for documenting for others what it's like to troubleshoot the
TBI system.
As for your generous offer for the case of beer, that's very cool. But in order
to make this thread even more valuable to people researching similar symptoms
on their TBI GMT400s, how about this instead? If I PM you my address, would
you be willing to throw that bad ECT sensor into a padded envelope and send
it to me?
What I would like to do is to put that sensor on my Fluke and see if I can get
a steady reading out of it? Heck, for comparison purposes I'd even be willing to get
a new sensor and measure that also, and see how close we can get to the
readings on that temp vs. resistance chart?
Why? Well, it looked like your meters were autoranging and just messing with
your ability to get a solid reading. I'd like to see if A) the Fluke will be able to
deliver on the promise, and B) I don't have a TBI GMT400, so I can't attempt
to duplicate this measurement on my own truck?
In English, if we can validate this chart & prove that it is still relevant in 2024
then we can recommend it to other TBI owners with a higher degree of confidence.
****
Speaking of which, we lucked out that measuring the sensor gave us actionable
info, for in reality using an ohmmeter on the sensor is just a quick go/no-go
check, and not nearly as definitive as monitoring the live data/what the
computer is actually seeing. (Q: Isn't the forum scan tool backwards
compatible with pre-'96 OBD1 vehicles?)
In English, if the problem is the sensor, then a good ohmmeter will give you
a ballpark idea of where you stand. (Note: The shop manual wants the ECT's ambient
temperature reporting to be within > 5° < of reality, which is pretty tight all things
considered.)
But if the problem stems from a marginal/intermittent signal path, loose connection or
smashed pin, and/or the ECM and the engine no longer share a common ground plane...
or even something as exotic as a bad A/D converter or other internal ECM failure, then
we aren't going to find that with an ohmmeter.
But in the calculus of picking the best available troubleshooting path for normal folks,
I think that trying an ohmmeter check first, (with the mindset that I'm just trying to build a case
that we really need to jump through the hoops of laying hands on an ODB1-compatible
live data scan tool) can be valid. (And I'm being consistent with all this, for back in
reply #55 I did qualify that 'live data' is best.)
Which leads me back to the bad sensor. If you haven't already thrown it away (or subjected
it to a a stress-relieving victory hammering) ...then please send it my way and I'll do my
best to give it a little failure analysis.
Tip of the hat to everyone who contributed to this thread. And kudos to Beason for
coming back and sharing what ended up fixing the issue. I wish more folks would
close the troubleshooting loop like this.
Cheers --
