Greetings SpaceCityErlan,
Welcome to the GMT400 forum, where folks from all over the world compare notes on what it
takes to get these hardworking beauties to work like they used to back when they were new.
recently installed a new stereo on my dads 1989 Chevy Silverado. It’s the Jensen cm9mir floating screen stereo. Used the gm02b wire harness. Everything matched. Stereo works fine. The only thing that happens is when you lock the door while the car is on the stereo reboots itself. It is the only time it happens. What could causing this issue? Is this a normal issue on these trucks?
Good question. If your question was restated as, "Are
logically unrelated electrical loads
affecting each other a
normal issue on these GMT400s?", then my answer would be no, when
they were new this was unexpected behavior.
However, 35 years after your Dad's truck rolled off the assembly line, we may have one or more
marginal connection(s) that's causing a 'brown out' condition to the net power to your radio.
The trick with any electrical issue is to figure out where the problem lies in the fewest number of tests.
(Especially when the Factory Service Manual doesn't have a listing that matches your specific symptom.)
So I propose that we define the problem as tightly as possible, and then devise a short series of
test scenarios to prove/disprove our theory. And as we collect more data on this failure, we reserve
the right to restate the problem definition in order to point ourselves in the right direction.
****
Without further ado, here's a series of steps that I would try if this were my truck:
1) The first thing that would be nice to know is how far down does the nominal (12.6-14.5v)
battery voltage have to sag before the internal CPU will reboot on your Jensen CM9MIR?
Is it 9 volts? 8.5 volts? I quickly scanned the internet & came up empty handed. (But if
there's a Jensen Tech Support Line that you can call/text/email then I think that this is
a legit question.) For the sake of argument I'm going to pick 8 volts as the one where
the internal regulator for the CPU power can no longer keep things chugging along.
(Keep this mind in the back of your mind for later.)
2) I'm assuming that the 'gm02b' wiring harness allowed you a 'plug-n-play' installation.
If this is the case, I'm going to make the further assumption that this harness is going
to connect your aftermarket radio to the factory
RADIO 10A fuse. Please correct these
assumptions if you have a different power feed setup.
3) As for the Power Door locks, the factory wiring ties them into the
ACC/BAT 30A fuse.
In other words, with the factory wiring the Power Door locks and radio do not share their
power feeds from the same fuse. (!)
If all is healthy in fuse block land, the large, short duration current draw by the door lock motors
should NOT cause a large enough voltage drop on the radio power circuit (down to 8 volts)
that would cause the Jensen to reboot. (!)
4) When you read the following, remember that your radio works on the net difference
voltage between the positive 12v connection (YEL, circuit #43) and the ground. (BLK, circuit #150)
When a voltage drop (brown out) is experienced by an electrical device, most folks assume
that the positive voltage sags, and chase that.
But there are situations where the positive voltage is strong & steady, but because of a shared
marginal ground, when one logically unrelated device tries to sink a large amount of current
through a ground connection that's not clean & tight, this causes a voltage to develop across
this unwanted resistance. So the other electrical loads also trying to use this same ground at
the same time are adversely affected.
In English, IF the door locks AND the radio are sharing the same ground, then when the
door locks are operated, it's possible for the radio to go from 12 volts across it to <8 volts
as a transient voltage sag, creating the reboot condition.
So let's see which ground the factory wiring has the Radio connected to:
You must be registered for see images attach
Looks like G202 is the ground for the radio. Now let's check the Power Door wiring diagram:
You must be registered for see images attach
Aha! The Power Door Locks
share the exact same physical ground as the radio. So the
logically unrelated electrical devices affecting each other could be the root cause of what you
are experiencing.
Now to figure out where G202 is located. Thankfully the FSM has an illustration showing us
what to look for:
You must be registered for see images attach
****
Now if I could *guarantee* that cleaning/tightening G202 would solve your problem I would have just written that. But I don't know
if you have a shared marginal ground problem, or does the entire fuse block brown out when the door locks are operated because
of a bad connection between it and the battery?
So here's how we are going to troubleshoot your radio in the fewest number of attempts:
1) Locate G202 & check for tightness. Loose or tight, I'd like you to disassemble it, clean up
all mating surfaces back to new condition, put it all back together, and see if the problem
no longer occurs. If fixed, great!
2) If the problem persists, temporarily disconnect the yellow power wire to the RADIO fuse
and temporarily run a fused jumper wire from the radio straight to the battery. If now the
door locks can now be operated without causing the radio to reboot then we need to go
completely through the power path and figure out where the unwanted voltage drop is
occurring.
**
3) If jumpering the radio +12v power feed directly to the battery doesn't solve the reboot issue,
then reconnect the positive to the yellow wire and connect the radio ground directly to the
negative terminal on the battery. If the problem is now solved, now we know that we need
to further investigate the quality of the grounds in this truck.
Of course since this is an aftermarket radio, and we don't really know if the Jensen radio
is compatible with a slightly glitchy 35 year old vehicle, it's perfectly OK to connect both
positive & negative radio leads straight to the battery (with fused jumpers) and verify that
it will stop the rebooting nonsense when you operate the door locks, and then removing
one jumper at a time so that we can figure out if the power side or the shared ground is the
cause of the malfunction.
Hope this gives you a few ideas on how to test your way out of this weird interaction.
The good news is that you typically can't fix weirdness until you can first break it at will.
So given your problem statement you actually have the hardest part of all this behind you.
Best of luck with the hunt. And please be sure to return and tell us what you discovered.
Cheers --
PS:
**The full power path for the Radio power is: Battery > Junction Block > Fusible Link >
circuit #2 > C100 > S202 > C203 > Ignition Switch > C203 > circuit #4 (BRN) > RADIO fuse >
circuit #43 (YEL) > Radio. So I'm really hoping that performing step #1 (cleaning G202) is
the fix, and we don't have to go through step #2. It's definitely doable, but you will spend
a lot of time in the attached power distribution and Radio prints. :0)
EDIT: Observing the radio performance while working the Power Door locks will do the job.
But if you have access to a analog voltmeter it would be easy to monitor how much of a needle
swing operating the power door locks will create. If you could find out how far the power to
the Jensen has to fall in order to cause a reboot, and then capture that voltage drop with
a voltmeter would be very cool. Nothing quite as nice as a premeditated fix on an
vehicular electrical gremlin like this. :0)
