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73-87 Chevy _ GMC Trucks => Electrical => Topic started by: brlarl on July 22, 2012, 06:09:16 PM

Title: A/C
Post by: brlarl on July 22, 2012, 06:09:16 PM
I swapped the complete engine, tranny, and  wiring harness out of a 1990 Chevy Suburban into a 1985 c-10.  I drove the truck around for about 2.5yrs with no problems at all but I parked the truck in a garage for about 2 yrs.. Now that I'm wanting to sell the truck the A/C system will not power up. I have no power at all to the blower. I found a few burnt wires at the relay and replaced the harness and the relay. I also had a few burnt wires at the dash control so I changed both harnesses. (soldered all the harnesses)I changed the resistor as well as the firewall relay (I think it controls the compressor). All the fuses are good under the dash but I do not have 12V at the dash control at either harness. I do have 12V at the relay mounted near the blower but nothing happens from there. Can anyone share any ideas on my problem? Are there any fusible links for the a/c system on a 1990 Suburban? Any help would be great I need to sell the truck soon and want to fix this before I do to make sure I get top dollar for the truck. Thanks Guys
Title: Re: A/C
Post by: 78 Chevyrado on July 23, 2012, 01:08:30 AM
Open up the new relay you installed and check for a burnt up resistor inside.  if it IS burnt up, return it, buy another one, and BEFORE installing it snip out the resistor and use it like that. (the same one that burnt up in the other one.)

I had this on my truck, took 3 relays before i figured it out.  turned out the replacement relay had an extra resistor the stock relay did not and every time I turned it on it burnt up the extra resistor.  I snipped it out of the last replacement and it works perfectly.  if you do snip the resistor, make sure whats left doesnt contact anything it shouldn't like the case, etc.
Title: Re: A/C
Post by: brlarl on July 23, 2012, 07:33:58 AM
I did open the relay but did not notice if there was a resistor in there but I will check again today.  Thanks.  Any other input out there?  Thanks Guys
Title: Re: A/C
Post by: brlarl on July 23, 2012, 08:22:41 PM
78Chevyrado this is what I have in the relay, is this what you speak of or is this a diode? Thanks
Title: Re: A/C
Post by: bd on July 23, 2012, 08:52:58 PM
That's a clamping diode.  The end of the diode with the little silver band should be connected to the positive (+) terminal of the relay control circuit.  If the silver band is connected to negative (-), the diode will pop like a fuse when the relay is energized. 

The purpose of a clamping diode is to prevent the relay coil from 'spiking' the control circuit with high voltage when the relay is switched OFF.  It protects switches and electronics.
Title: Re: A/C
Post by: brlarl on July 23, 2012, 09:06:16 PM
bd do you have any ideas about what my issues could be caused by if this relay does not have any resistors 78 Chevyrado speaks of? I have 12Vat the relay going in but nothing coming out or at the dash switch. All fuses are good. Thanks
Title: Re: A/C
Post by: bd on July 24, 2012, 01:09:20 AM
Allow 78 Chevyrado some time to respond to your post of the relay pic.  Not everyone is on the forum everyday.  In the meantime...

...I have no power at all to the blower.  I found a few burnt wires at the relay and replaced the harness and the relay.  I also had a few burnt wires at the dash control so I changed both harnesses (soldered all the harnesses).  I changed the resistor as well as the firewall relay (I think it controls the compressor).  All the fuses are good under the dash but I do not have 12V at the dash control at either harness.  I do have 12V at the relay mounted near the blower but nothing happens from there....

If the wire connections to the selector and blower switches in the control panel were burned, chances are the switches are damaged as well.

Not sure, but I'm going to assume that your A/C wiring is similar to an '85-'87 square. 
You state you have no power anywhere except at the blower relay (I presume on the 10-ga red wire).  Look behind the instrument panel in the area to the right of the steering column... there should be a ~16-14-gauge solitary brown wire feeding through a single-wire connector.  This should be the power lead that feeds your A/C selector and blower switches.  This brown wire comes directly off the 25-amp Heater-A/C fuse in the fuse block.  Use a test light and probe the brown wire for 12-volt ignition power.  Check the in-line connector & terminals for heat damage.  If the circuit is dead, probe both sides of the Heater-A/C fuse.  Then, give us an update.
Title: Re: A/C
Post by: brlarl on July 24, 2012, 07:45:10 AM
I forgot to mention that I did change out the switches.  I noticed that brown wire you mentioned last night when I was sitting in the truck starting at the dash.  I did check it for power but I'm so caught up in fixing it that I do not remember if I had the key turned on.  I do remeber that there was no power so either the circuit is burned from the fuse box up or I did not have the key turned on!  Either way I will check it again tonight with the key ON and report back.  LOL  :o  Thanks BD for your input it is much appreciated!
Title: Re: A/C
Post by: brlarl on July 24, 2012, 06:02:01 PM
Ok I tested the wire to the right of the cluster.........no power detected.  I traced the wire to the fuse block and no power on either side of the fuse. I traced the wire that tied to the hot side of the fuse and followed it to the ignition starter switch and found the the wire harness and switch had burned. I replaced the switch and harness. Now I have power at the selector switch at the dash but nothing on the speed selector. I still have power on the red 10g wire at the relay mounted on the box near the blower but the blower does not kick one. also the blower does work as I jumped it directly to 12V. Any ideas? Thanks
Title: Re: A/C
Post by: bd on July 25, 2012, 01:15:50 AM
Here's the skinny on how the blower circuit works.  Read through it while you're "looking" at the wiring in your truck, or the details will probably put you to sleep  ??? :

You stated you restored power to the selector switch, via the 16-ga brown wire from the fuse block...

There are two additional 16-ga brown wires connected to the selector switch, each has a single white stripe.  One of the brn/wht wires attaches directly to the blower resistor (bypassing the blower switch) to power the blower motor with constant low speed; hence, low fan speed is not controlled by the blower switch.  The other brn/wht wire feeds the blower switch with 12 volts.  As long as the selector switch is not set to OFF, and the ignition is switched ON, you should have 12 volts on the brn/wht wires at the selector switch and blower switch!

Besides the brn/wht feed wire, the blower switch has a 16-ga tan wire (medium-low speed that runs to the blower resistor), a 16-gauge light-blue wire (medium-high speed that runs to the resistor), and an 18-ga orange wire (high speed that runs to the blower relay and directly energizes the relay, bypassing the blower resistor). 

All fan speeds, except high, are routed through the blower resistor and then on to the blower motor, via a 16-ga dark-blue jumper wire that connects the blower resistor to the blower relay - and powers the blower motor through the blower relay, as long as the relay is not energized.  In other words, by default (on any fan speed except high), the blower relay connects the dark-blue jumper wire and resistor to the 10-ga purple wire running to the blower motor.  Life is good. 

But, because of the nature of the resistor wiring, don't try to puzzle through the 'logic' of the various blower speeds or you'll end up institutionalized.  All of the wires off the blower switch (except orange) will be 'hot' all the time, because of back-feeding through the resistor.   :o   And, life will suddenly seem very bad!   :(

Finally, when the blower switch is set to high speed, the 18-ga orange wire energizes the relay, and the 10-ga purple wire is disconnected from the dark-blue jumper wire and blower resistor, and connected to the 10-ga red wire powered directly off of battery through a fusible link.  The 18-ga black wire connected to the relay is the relay coil ground.

Whew!   :P

Hopefully, this explanation will help you sort out the remainder of your problem.

Yet, I'm surprised at the number of burned connections and components you encountered.  Once you restore proper blower function, you should measure blower motor and compressor clutch current draw - shouldn't be over ~20 amps for the blower and under 3 amps for the compressor.   8)
Title: Re: A/C
Post by: brlarl on July 25, 2012, 09:07:30 AM
I will research based on your info. bd do you by chance have a schematic of the a/c wiring on the 90 Suburban? I believe it's the same as the trucks, everything has been right on where you said it was. Thanks
Title: Re: A/C
Post by: bd on July 25, 2012, 11:51:56 PM
I will try to dig one up for you, but it will be in pdf format - tomorrow at the earliest.  Good luck with your project.
Title: Re: A/C
Post by: brlarl on July 26, 2012, 07:09:37 AM
Thanks bd!
Title: Re: A/C
Post by: bd on July 26, 2012, 09:13:38 PM
Here's the diagram in pdf format.  Any luck since your last post?
Title: Re: A/C
Post by: brlarl on July 26, 2012, 09:27:02 PM
Thanks BD, no I have not had time to look at it but will do so the weekend.  I'll keep you posted. Thanks Again!!!!
Title: Re: A/C
Post by: brlarl on July 28, 2012, 12:07:09 PM
BD can you tell me how the two dark wires receive power? They are the ones that go to the compressor and the pressure switch.  Also the brn/wht wires get their power form the selector switch once the switch is turned on, correct? Is the brown wire the power source for the selector switch? Sorry for all the questions, I guess I'm not too good at electrical like I am mechanical.  Thanks Again.
Title: Re: A/C
Post by: bd on July 28, 2012, 01:54:45 PM
Ignore the hand drawn notes on the supplied diagram; they are non-factory circuit modifications that do not apply to your vehicle.

...Is the brown wire the power source for the selector switch?

Yes.  All power is controlled directly by (routed through) the selector switch.  Ignition power enters the selector switch on the 16-ga brn wire from the fuse block. 

...can you tell me how the two dark [grn] wires receive power?  They are the ones that go to the compressor and the pressure switch.

The selector switch routes power:  (1) to the blower switch and low speed blower resistor terminal via the two 16-ga brn/wht wires, and (2) to the A/C low-pressure (cycling) switch via the 16-ga lgt-grn wire.  From the 'cycling' switch, power continues on to the A/C compressor clutch, and the ECM via two 16-ga drk-grn wires.  A/C circuit feed to the ECM allows for improved idle speed control and slight adjustments within the fuel/timing matrix for improved economy and drivability.

...the brn/wht wires get their power form the selector switch once the switch is turned on, correct?

Correct.
Title: Re: A/C
Post by: brlarl on July 28, 2012, 02:18:21 PM
Ok BD, I might have found my issue but still need your help. I was sitting in the truck looking at the power selector switch and noticed that the brn/wht wires at the power selector were in the far right slot of the plastic harness. But, when I look at the power selector switch itself there is no copper prong extending out of the switch to make contact with the wires in that position (so no power going to the speed switch via the brn/wht wire). I removed the lt green and brn/wht wire from the harness and moved them over one spot to the left to make contact with the copper points in the switch and when I flipped the switch the 25A fuse blew, so that wasn't right. Looking at the switch and harness from the drivers seat can you tell me which order from left to right should the wires be? I tried using the schematic you sent be but unfortunately I could not increase the size of it without distorting the picture so I could not read it. Thanks Again!!
Title: Re: A/C
Post by: brlarl on July 28, 2012, 02:30:57 PM
BD, I do not think that the schematic aplies to the 1990 Suburban wire harness I have in my truck. Looking at the schematic and comparing it to the hi-lo selector harness the colors do not match up. Any ideas? Thanks
Title: Re: A/C
Post by: bd on July 28, 2012, 03:13:18 PM
...the brn/wht wires at the power selector were in the far right slot of the plastic harness.  But, when I look at the power selector switch itself there is no copper prong extending out of the switch to make contact with the wires in that position (so no power going to the speed switch via the brn/wht wire).  I removed the lt green and brn/wht wire from the harness and moved them over one spot to the left to make contact with the copper points in the switch and when I flipped the switch the 25A fuse blew....

Is it possible they gave you the wrong switch?  Compare it to the old one.

...Looking at the schematic and comparing it to the hi-lo selector harness the colors do not match up.  Any ideas?

Post some pics of the control panel face and the switches, resistor, relay, connectors, wiring, etc, so I can see what you're seeing.  In a new unit of time, tell me what does and what doesn't work.  Maybe we can still sort it out....
Title: Re: A/C
Post by: brlarl on July 28, 2012, 03:32:22 PM
Will do shortly. Thanks
Title: Re: A/C
Post by: brlarl on August 01, 2012, 12:12:51 PM
BD, sorry it took so long to re post, trying to get this done along with buying a new house.  But here are the pictures that I have.  For some reason I do remember seeing a power switch that has a copper posistion all the way to the right with a blank side just to the left of that one and then three other spots to the left of the blank spot that have the copper extensions.  If that make any sense.  Can you confirm whether Chevy has more than one Power selector switch for the A/C.  Do you know if they are diffrent from the Trucks to the Suburbans?  Thank Again
Title: Re: A/C
Post by: brlarl on August 01, 2012, 12:15:48 PM
BD, here are two more pics.  Let me know if you need pics of anything else.  Thanks
Title: Re: A/C
Post by: bd on August 01, 2012, 01:14:56 PM
Your wiring is identical to the supplied diagram and posted descriptions.  Wire colors are the same.  However, the diagram has errors in the circuit numbering of the blower switch terminals.

Post pix of the selector switch and connector (unplugged) to illustrate the details of the physical position of wires in the connector and the terminals on the switch.

What does/doesn't work at this point?
Title: Re: A/C
Post by: brlarl on August 01, 2012, 01:58:08 PM
Ok. I'll do it after work.  FYI the way the wires that are currently in the power selector I moved them.  The brn/wht wires were all the way to the right then an empty space just the the left of them then the green wire next.  So from right to left its the brown wire then an empty space then the green wire then an empty space with then the brn/wht wires.    I tried to switch the wires according to the schematic and it would blow the 25A fuse.  I'll post the pics later.  Thanks
Title: Re: A/C
Post by: brlarl on August 01, 2012, 02:02:37 PM
Nothing actually works, If I turn the A/C on the engine does idle up but no blower is blowing.  If I jump the pressure switch the compressor will run, but other than that the blower does not turn on.  I think it has to do with the dash wiring.  I may have switched wires when I added the replacement harnesses not realizing it.  Thanks
Title: Re: A/C
Post by: 78 Chevyrado on August 01, 2012, 07:39:23 PM
I'm sorry the reply took so long man.  I haven't had time to do anything lately.

(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=24232.0;attach=20367;image)


That looks like the diode mine had, when i hooked it up, turned on the key that diode fried and i had to replace the relay again, but cut the relay out of the new one before hooking it up to power.  if you get power to the relay and the diode doesn't fry, it's fine. 

it only did that in my 78 though, I never had a problem on my 87.  i just thought of that, maybe being newer it won't be a problem. 
Title: Re: A/C
Post by: bd on August 01, 2012, 10:12:01 PM
BD, here are two more pics....

(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=24232.0;attach=20444)

Look at the connector that attaches to the selector switch.  From right-to-left in your image you'll notice that each wire position has a letter, A-E.  "A" should be empty, "B" should be the two brn/wht wires, "C" should be grn, and "D" or "E" should be brn from the fuse block (D & E, if both are present, are electrically connected inside the switch).  It appears that your wire positions may be incorrect. 
Title: Re: A/C
Post by: brlarl on August 02, 2012, 08:50:08 AM
BD, here is a better picture of the wire harness showing the letters.  Thanks again.
Title: Re: A/C
Post by: brlarl on August 02, 2012, 08:58:14 AM
BD, What about the speed selector switch, do the wires look like they are in the correct positions?  Thanks
Title: Re: A/C
Post by: brlarl on August 02, 2012, 05:55:53 PM
Ok BD, I set the power selector wires the way you described and this is where I'm at...........When I turn the power to "Max" one of the 25A fuses blows BUT if I remove the plug off of the speed selector the fuse does not blow and the blower is actually running on low. I'm guessing it has something to do with the speed selector circuit. The saga continues.........Any ideas? Thanks Bd.
Title: Re: A/C
Post by: bd on August 02, 2012, 10:24:41 PM
What about the speed selector switch, do the wires look like they are in the correct positions?

Wire connections to the blower switch appear correct.

...When I turn the power to "Max" one of the 25A fuses blows, BUT if I remove the plug off of the speed selector the fuse does not blow and the blower is actually running on low.  I'm guessing it has something to do with the speed selector circuit....

* Which 25A fuse blows?
       Tip - during diagnosis, try substituting a 20 to 25A circuit breaker for the fuse.
* Does the fuse blow only on "MAX" setting?
* Will the fuse also blow on "NORM," "BI-LEV," and "DEF?"
* Does the blower switch position (low, med-low, med-hi, or high) have any bearing on whether the fuse blows?
* Plug the blower switch back in, then unplug the resistor - any change?
* Plug the resistor back in and unplug the A/C low-pressure switch - any change?
* Do you have a volt-ohmmeter?  How about an ammeter?

Title: Re: A/C
Post by: brlarl on August 03, 2012, 10:41:57 AM
Which 25A fuse blows?- I'm not sure which one of the two, will check after work today.
       Tip - during diagnosis, try substituting a 20 to 25A circuit breaker for the fuse.
* Does the fuse blow only on "MAX" setting? No, all settings.
* Will the fuse also blow on "NORM," "BI-LEV," and "DEF?" Yes.
* Does the blower switch position (low, med-low, med-hi, or high) have any bearing on whether the fuse blows? No
* Plug the blower switch back in, then unplug the resistor - any change?Will check after work.
* Plug the resistor back in and unplug the A/C low-pressure switch - any change? Will check after work.
* Do you have a volt-ohmmeter?  How about an ammeter?I have only a volt-ohmmeter.

Thanks BD.

Title: Re: A/C
Post by: brlarl on August 05, 2012, 10:24:43 PM
Hey BD.....for some reason miraculously somehow I restored power to my blower and it works in all levels. I'm not too sure would I did but it works. Just hope it stays working! Thanks for all the help BD!
Title: Re: A/C
Post by: bd on August 05, 2012, 10:59:33 PM
One of the blower resistor wires may be intermittently shorting to ground at the firewall pass-through or pinched or (?).  Glad it's working again!