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PostPosted: June 11, 2018, 3:38 pm 
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Location: No. Nevada
I cannot overstate how badly I despise EFI and it's myriad sensors and connectors!
Makes Webers very simple by comparison.

So I have a 1990 Chevy truck, 4.3 V6 and TBI.
Less than 60K miles and still a PITA.
Keeps throwing error code 15, coolant temp sensor.
Replaced the sensor and the code went away for while.
Now it's back again.

Seems I need to check continuity of the yellow temp sensor wire to "C10".
Fine, but WHERE is "C10" to be found?


Only two connectors at the ECM and NO yellow wire, no ability to READ the connector numbers there anyway.
A troubleshooting guide that gives no clue WHERE to find the $#@!^&! connection is useless!

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PostPosted: June 11, 2018, 7:09 pm 
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Location: San Tan Valley, Arizona
Looks like "C10" ic the grey or clear connector pin 10 on the left side (looking at the back of the connector) see the picture


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PostPosted: June 11, 2018, 7:22 pm 
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The only computer I can see is mounted on the master cylinder and only has two connectors.
Searching the web is seems there may be another ECM behind the glove-box?
So the connector I need to access requires me to tear apart the dash?
Gaaa, modern cars are made to be scrapped, not repaired!

Either I have a broken wire or a failing ECM.
Suspect the ECM as the truck runs fairly well with the temp sensor unplugged but will not start with it connected, but sometimes will.
Would be easier if the issue was not intermittent. :BH:

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PostPosted: June 11, 2018, 7:25 pm 
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Modern?!? Its 28 years old! :shock:

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PostPosted: June 11, 2018, 7:35 pm 
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a.moore wrote:
Modern?!? Its 28 years old! :shock:


And still too %@&! new!

Happiness is never owning any car made after 1970!
Unless you build it yourself so that you know every intimate detail.
Then if it's still a PITA to work on you have only yourself to blame.
I didn't build this #@$, GM did with Gov. Org. mandates to "Help" them. :ack:

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PostPosted: June 11, 2018, 7:52 pm 
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Location: San Tan Valley, Arizona
ECM should be easy to get to. Just remove the glove box, it will be on the right.


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PostPosted: June 12, 2018, 1:23 am 
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RichardSIA wrote:
The only computer I can see is mounted on the master cylinder and only has two connectors.

That's the anti-lock braking system comp, look behind the glove box for the ECM.

This is a 5.7L but the troubleshooting should be the same.
https://www.youtube.com/watch?v=RnpLfiUUzTU

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PostPosted: June 12, 2018, 8:19 pm 
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If the sensor is out of tolerance, a small resistor in parallel or series could bring it back in. An adjustable pot could also be used to readjust. It could be that the cheapo sensors are never exactly right for tolerance but work well enough not to set a code for a while.

If you really want to go back to basics, you could put a carb on the existing wet flow intake with a simple to make adapter and swap the distributor for an 85 astro hei but the gear would need to be swapped also since you have a roller camshaft. Low pressure pump and a short tube to replace the intake pump, though it will pull through the intank pump. If you have the automatic, TV cable hook up and adjustment along with lock up control with a hobbs vacuum switch (or just a toggle) will add to the list.

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PostPosted: June 12, 2018, 11:00 pm 
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Zero volts at the yellow wire to the temp sensor, supposed to be close to 5V.
So I have to pull the connector and check for a broken wire.
Funny thing is that if I connect to the temp sensor with the engine running idle goes up and the engine runs ragged.
How does that happen when there is NO power at the yellow 5V wire?
If I leave the connector hooked up to the temp sensor the truck will not start. :chev: :BH:
Starts and runs pretty well with the temp sensor disconnected, but may die at any time.

Manual trans and I am tempted to ditch the ECM but then would have to deal with reducing fuel pressure, fooling whatever else is involved with the codes, etc.
Oh, and :chev: cheaped out on the 4.3 in trucks, no roller cam! :(
New ECM is $90 - $1,000 with around $300+ being common.
Yes I really did see some listed at $1,000! :shock:

Not fond of this truck, when I get the El Camino going it will be disposed of.
Much prefer my 1981 one-ton flat-bed (No computers!) except that it's a pain to park in modern lots.

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PostPosted: June 12, 2018, 11:06 pm 
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RichardSIA wrote:

Fine, but WHERE is "C10" to be found?




Dude..... I'm right here.

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PostPosted: June 12, 2018, 11:09 pm 
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Two things:

-you can just run new wires from the ECU rather than try to find the broken part
-Those old ECUs use big enough components that an electrical shop may be able to replace what is popped for far less than a replacement ECU.

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PostPosted: June 12, 2018, 11:14 pm 
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Weather is cooler this evening so I'm about to pull the glove box, find "C10", and check continuity.
$90 at Autozone and I'm under a ridiculous time constraint right now so if I do have continuity the ECM is failing, and I will bite the bullet to save time.

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PostPosted: June 13, 2018, 7:35 pm 
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If it is zero volts plugged into the sensor, unplug it and check again across the two terminals of the plug. The sensor may have shorted to ground internally.
Grounded sensor would provide a low voltage, setting the code, indicating the engine is extremely cold to the ecm, richening the mix, raising the idle.

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PostPosted: June 13, 2018, 7:43 pm 
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Zero volts yellow (C10) to ground.
Will try again this evening to get to this, truck is outside so too %@^^ hot out.

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PostPosted: June 14, 2018, 12:13 am 
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Finally got the glove box out.
Found some sort of $#!*% alarm? type device was spliced in, in the way of course.
Got it out and got the C/D ECM connector off.
So then of course the wires on my continuity tester are too short to reach both ends of the yellow wire.
One in the dash the other at the front of the engine.
Why I hate late model cars, EVERYTHING is made unnecessarily difficult.
Web claims some parts stores can test the ECM, but I bet they cannot as it's "Too Old" now. :BH:
Back at it again in the morning.

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