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Another "Radio is Shorted" thread

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

MrPc
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Another "Radio is Shorted" thread

Post by MrPc » Sun Dec 11, 2016 10:25 pm

A while back my radio suddenly stopped working. It's an SC815 Premium Sound edition with the separate amp.

A quick check found that fuse #7 had blown. The next fuse blew as soon as it made contact.

I got my voltmeter out, and measured the resistance from the fuse block terminal to ground--0 ohms. I pulled the radio out and disconnected it; still 0 ohms. Then I disconnected the amp; still 0 ohms.

I searched the database at this point. There are a couple of reports of the radio fuse blowing with the radio & amp unplugged, but no solutions posted.

So I went and dug up a schematic. It's a 1996 service manual that I downloaded some time ago. The relevant pages are attached below.

Next I disconnected the C/CR connector behind the radio; still 0 ohms. Then unplugged connector 24/13A from under the dashboard. Now I'm getting somewhere--the short from the fuse box terminal to ground disappears and becomes an open circuit. Measuring from the unplugged connector pin #1 to ground is 0 ohms.

So the short circuit is in the cable harness, on one of the two black and red wires connecting 23/13A pin #1 and C/CR pins #4 and #12. The schematic indicates that there's nothing else connected to these wires between the two connectors.

Today I took the dashboard cover off hoping to take a look at the cable routing and maybe find some damaged or melted insulation, but couldn't see anything wrong. I wiggled the cable harness in various places to see if I could make the come and go but nothing happened.

I need to drive the car this week so I put everything back together with the radio disconnected.

So, my questions to the group:
  • Have anybody else seen this? It's been reported a few times on this board. Is there a common cause/solution?
  • Does anybody have any tips or suggestions on how to find a short in a wiring harness? Does anybody know exactly where these wires run?
  • Does this schematic look correct? Could there be another device attached to the black and red wires on that harness that's shorting them out?
  • Anybody see any mistakes or flaws in my reasoning?
Of course I can snip those wires and run a new power wire to the radio, but I would rather not do that until I know what's going on. If there is a sharp metal edge rubbing on my cable harness I want to fix it before it cuts more wires.

I'd love to hear peoples thoughts---what do I do next?
radio_schematic.jpg
Radio schematic. The short circuit appears to be on one of the R-SB wires between 24/13A and C/CR
harness routing.jpg
24/13A is underneath the dash, and runs up behind the instrument panel and down to the radio.
24_13A.jpg
Location of 23/13A underneath the dash. When I unplug this connector, the fuse stops blowing.
connectors.jpg
Other connectors in the radio circuit.
Thanks!
Paul
=====================
Red Red '96 855R, 169k

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sleddriver
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Re: Another "Radio is Shorted" thread

Post by sleddriver » Mon Dec 12, 2016 12:08 pm

You didn't mention independently powering up the radio, so you're assuming the problem lies in the wiring harness instead?

I went through an extensive wiring harness/ECC controller check back in Jan of this year. Loads of time to do it. Divide and conquer. Then divide again. Rule out as much as possible.

You're on the right track. I print out those wiring diagrams and blow them up to about 18 x 24 so I can make notes and meas.
1998 V70 T5 226,808 miles. Original Owner.
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Re: Another "Radio is Shorted" thread

Post by BEJinFbk » Mon Dec 12, 2016 2:09 pm

Something else to isolate things - Did you entirely disconnect the amp or just the 6 pin DIN?
I'd confirm that your amp isn't blowing fuses by bypassing it entirely. Unplug the amp bypass
harness and connect your SC directly to the car harness. If everything works, you blew the amp.
'98 V70 R - Well Equipped for Life Up North... ;)

MrPc
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Re: Another "Radio is Shorted" thread

Post by MrPc » Mon Dec 12, 2016 8:38 pm

Thanks for the replies.

The radio and amp are completely disconnected. Connectors 24/13A and C/CR are unplugged--see the first diagram. The short to ground is in one of the two wires that runs between them. These are the ones marked R-SB in the schematic.

To answer your question sleddriver, I didn't independently power the radio because the short remained after I took the radio out.
Divide and conquer.
I think I've got divide figured out. Conquer could sure use a little help! :lol:

Anybody know where that harness runs, and where any likely abrasion points are? How do you find out where a short is in a harness? I already took the top of the dash off--anybody know how a way to get a better look at the harness? Take out the dash and dismantle the harness? Any chance I've got the wrong schematic (the book said '96, my car is a '96...) and there might be another connection/device between the two connectors causing the short?

I'm all ears. Snipping those wires and running a new one directly is beginning to look pretty good right now...

Paul
=====================
Red Red '96 855R, 169k

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Re: Another "Radio is Shorted" thread

Post by JimBee » Mon Dec 12, 2016 9:56 pm

Are you sure the short on pin #1 isn't in the connector itself?

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Re: Another "Radio is Shorted" thread

Post by BEJinFbk » Mon Dec 12, 2016 11:42 pm

Do you know about the amp bypass/add-on harness and how it works?
There's a base wiring harness in every 850/x70 and another harness
with a second set of C/CR and C/CS connectors, both male and female,
for the amp harness. ( Yup - 4 more, total ). Makes it all Plug N Play.

They're wrapped in foam, buried in the dash and intercept the wiring
between the base harness and the HU to insert the OEM amp.

With the amp harness out of the picture, you reduce even more.
And, if the fuse holds, you can plug the HU straight into the car
harness and confirm those two segments of the system are OK.

There's gotta be a simple fix here - Resist the urge to bastardize! ;)
'98 V70 R - Well Equipped for Life Up North... ;)

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Re: Another "Radio is Shorted" thread

Post by sleddriver » Tue Dec 13, 2016 11:50 am

MrPc wrote:....I didn't independently power the radio because the short remained after I took the radio out............Snipping those wires and running a new one directly is beginning to look pretty good right now...Paul
Thus my suggestion to try it and see if it solves your problem. At this point doesn't need to be "final", just crude & effective to see if the radio powers up.

As to all of your other Q's: There is no magic to figure this out. Unwrapping a factory harness for inspection can be a RPITA. Wire doesn't usually break by itself though, but where connectors join it, bends occur, something rubs over time, insulation creeps exposing copper, connectors corrode, loosen. Too many to list.

How you discover this is just to gut it out, suck it up and go after it. Until enough frustration sets in that you decide running new wire is the solution staring you in the face.

Your time. Your choice.
1998 V70 T5 226,808 miles. Original Owner.
M1 10W-30 HM

MrPc
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Re: Another "Radio is Shorted" thread

Post by MrPc » Wed Dec 14, 2016 9:03 am

I took a close look for shorts inside the connectors on both ends of the harness, but I don't see anything wrong.

As for the amp bypass harness, I think I know what you mean. Since my car has the amp, the harness comes out from behind the instrument panel and at from C/CR connector it goes to another little harness that goes to both the radio and the amp. You can see this in the schematic I posted. And it is/was wrapped in nice sticky self-distruct-on-contact foam. The schematic for the radio without the amp shows the harness coming out from the dashboard and the connector C/CR plugging straight into the radio only, without the little harness. (I haven't posted the non-amp schematic.)

At this point in my debug effort, this connector is unplugged, the amp and radio are out of the circuit, the amp/radio harness is out of the circuit, and connector 23/13A is unplugged.


So the short is somewhere between 24/13A and C/CR, which is the cable run through the instrument panel, going in above the driver's left knee and coming out behind the radio.
gut it out, suck it up and go after it
Yep. The solution is to grab the bull by the horns. I think I may run a wire to get the radio going temporarily as I wait for the weather to warm up, and pull the dash then. Perhaps plan for a preemptive evaporator replacement while I'm there.

Thanks for the suggestions, guys!

Paul
=====================
Red Red '96 855R, 169k

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