Got a 90 Cheyenne 4x4. 5.7 .
Relaced ac complete under hood. So it runs great until you use the signals or turn the fan blower on doesn't matter ac or not . engine bucks like wants to quit. Turn off blinkers or fan and it's normal again..
Hello elfbrew,
I don't want to assume anything, so I may ask more questions than
absolutely necessary.
While you are reading my questions, please refer to the 2 pages I attached to
this reply to get a general feel of how you would research the
exact answer
in the '90 Service Manual. (Disclaimer: I have the '99 books, so what I posted
is just to give you a general idea.)
Q1: Before you had the A/C repaired, were you able to run the fan blower
without impacting the way the engine ran? In other words, since there was
no A/C maybe you didn't run blower motor? (I'm trying to prove/disprove
that we had a latent fault lying in wait for you to start using this circuit
with the A/C repaired? Of course there's that nagging turn signal symptom,
I'm sure that they worked OK pre A/C repair?)
Q2: I'm reading "...
or turn the fan blower on doesn't matter ac or not".
So I could assume that the fault still occurs even though the electric A/C
compressor clutch is de-energized? You see, if we were just switching on the A/C,
we would be energizing (and simultaneously putting a load on the
respective grounds) for
both the A/C compressor clutch under the
hood + the ground supporting the blower motor under the dash.
It may seem redundant, but if you were to temporarily disconnect the
plug to the A/C clutch and
then turn on the blower, then if the problem
still shows, then we sharpen the focus to a ground under the dash.
If the problem disappeared, then the ground for the clutch under the hood
is where we would go next. (G105 on p.8-381 for my '99.)
Q3: Here I'm going to assume that when you disconnected the A/C compressor
clutch the symptom remained. And further, that you can stimulate the fault
just with the blower motor, whether A/C, vent, heat, etc are selected.
In English, fan powered up = disruption of how the engine runs.
So on the first attachment you can see that the blower motor is grounded
at G202. On the 2nd attachment, the book claims that G202 is located
"on the right side of the instrument panel, mounted to the HVAC plenum
bracket." From my (admittedly distant) vantage point in upstate NY, I'd
say that during the course of replacing your A/C that the mechanic could
have been in this neighborhood & possibly either disturbed or disconnected
this ground? And now it's marginal, so that when the blower motor starts
to sink current to this ground, it raises the voltage on this gimpy ground
point just enough that it's affecting other circuits trying to use this same ground?
****
Here's the good news -- you have figured out how to stimulate the fault at
will. With the divide & conquer power that the fuse panel gives us, if the
above didn't narrow down the problem, you can pull fuses one at a time
and see if some logically unrelated circuit is contributing to the failure
thanks to unseen wires getting smashed together, creating a new
& wonderous undocumented circuit that I don't see in the manual. :0)
(Admittedly a long shot, but just for completeness.)
****
FWIW since you can stimulate the fault with just the blower motor I
didn't root around for the turn signal circuit, especially since I don't
have the correct '90 books. I do find it interesting that you can
stimulate the fault with the fan blower OR the turn signals? According
to my book the exterior turn signal bulbs use a different ground,
so instead of it being 50/50 (underhood vs. underdash) it's now a 49/51.
(just a little more under dash because of the flashing indicators
in the instrument cluster and/or associated flasher relay?)
I'm still kinda new here, but I'm pretty sure that somewhere in
this forum there is an online repository for the service manuals?
I'm sure that one of the long time gurus can point you to them if
you don't find them on your own.
Again, I'm just trying to give you a semi-specific troubleshooting
approach, but the correct '90 docs would certainly accuratize
the search.
As long as you can break this at will, you will also be able to track
it down & fix it.
Best of luck --
