I have a Model 986374-A Multiplex that I am hooking up to a Delco AM/FM Model 986118 via the 12 pin connector

I sourced these two components from separate sources and wanted to make sure that the two worked properly before removing the perfectly good radio that is presently in my car, as it is fairly awkward install involving the removal of lots of other components in the dash.

For these reasons I bench tested the two units and they appeared to be working perfectly fine, for a brief minute or so, using a 3 amp fuse on the positive lead from the battery.

Then the unit stopped, and blew the 3 amp fuse.

It is possible that I may have crossed two of the alligator clips that I was using to connected the speaker wires to the terminals in the male wiring harness (that contains 6 terminals for the speakers).

Now when I reconnect things as before, the fuse blows.
When I hook up just the radio, without the multiplex, it does not blow.

Do you suspect that I have done terminal damage to the Multiplex, or is 3 amp simply the wrong fuse?

The Multiplex installation instructions do not specify a fuse, instead it just reads "Replace existing fuse [3 Apm] with the fuse provided".

Thank you. Your insights would be most appreciated.

Hi Brent,
Since no one else has taken this up, I will give you as much as I have.
You seem to be unsure if 3 amp is adequate for your multiplexer because you don't have adequate documentation. I am not sure where you would find documentation. A 3 amp seems like a reasonable size.

The best I can do is go from your story. On the first try your multiplexer played OK for a few minutes, then blew the fuse. Now if you try it, it blows the fuse imediately. From my experience this indicates that something has gone bad wrong in your multiplexer. If it was merely a fusing problem you would get the same behaviour as the first time you plugged it in.
You may want to try it one more time, turning it on with the volume turned way down. If it is a fusing problem the current draw will be reduced and this will tell you that the 3 amp is sized too low.
If it blows under that condition, then it is troubleshooting time on the multiplexer, and I do fear that your nose will be your best tool.

Best Regards,

Bill Grimm

When these components were powered up on the bench, was there a ground jumper to tie the 2 chassis to 'common' as they would have been if they were bolted into a car?
I'm wondering if the interconnecting cable wasn't intended to be providing a ground path for the multiplexer and something in the multiplexer has shorted (transistor?). Any burnt odor or burnt components? What car is this in?

: When these components were powered up on the bench, was there a ground jumper to tie the 2 chassis to 'common' as they would have been if they were bolted into a car?
: I'm wondering if the interconnecting cable wasn't intended to be providing a ground path for the multiplexer and something in the multiplexer has shorted (transistor?). Any burnt odor or burnt components? What car is this in?
:

Thank you for your questions and suggestions.

Yes there is a cable that provides a common ground to both the Radio and the Multiplex along with a common +ve connection

For certain, there was no odour or puff of smoke that occurred.

There was however, a small amount of fluid beneath the multiplex unit that I assumed got there when I spilled some rust-away that I used to clean the 12 pin connector earlier in the day. Could this fluid be anything else......such as an electrolyte from one of the components in the Multiplex? I realize that I am demonstrating my non-existent knowledge about the inner workings of electronic devices.

I will try to see what happens if I turn down the volume on the Multiplex, in terms of whether I it blows a fuse or not.

If this has the WonderBar search tuning, is the tuning drive motor off? If it were stuck on and mechanically stalled, it could cause excessive current draw. When you power it up, is there any 'click' heard from the drive motor relay? It should not be on, but I've seen these stick on.
Investigate the fluid source, but I don't think it's conductive.

No, this is not a WonderBar radio, but that was a good suggestion.

In response to an earlier question, this radio is for a 1966 Corvair convertible. Pictures of which are here if you are curious:

http://img5.imageshack.us/gal.php?g=dscn0985n.jpg

I am going to bench test the radio/multiplex this morning, with the volume setting at low, as was suggested, and see if that blows the fuse or not?

:No, this is not a WonderBar radio, but that was a good suggestion.
:
:In response to an earlier question, this radio is for a 1966 Corvair convertible. Pictures of which are here if you are curious:
:
:http://img5.imageshack.us/gal.php?g=dscn0985n.jpg
:
:I am going to bench test the radio/multiplex this morning, with the volume setting at low, as was suggested, and see if that blows the fuse or not?
:
Have you placed an Ammeter in the power lead and measured the current?
Lewis

"Have you placed an Ammeter in the power lead and measured the current?"

No, but I will do that over the weekend.

This morning I hooked the system up again and had the volume on low. The radio went on briefly and the speakers were producing sound. Then the (3 amp) fuse blew after about 10 seconds.

Hi Brent,
I just powered up a similar combination radio and multiplex on the bench and noted that the combination draws just over 3 amps at 12 volts. Since your radio does operate correctly for a brief period, I would suggest using a higher amperage fuse. I note that some Buick and Pontiac radios of the same type and year use 7.5 or 9 amp fuses even without the multiplex hooked up, so those might be a suggestion.

Meade

:I have a Model 986374-A Multiplex that I am hooking up to a Delco AM/FM Model 986118 via the 12 pin connector
:
:I sourced these two components from separate sources and wanted to make sure that the two worked properly before removing the perfectly good radio that is presently in my car, as it is fairly awkward install involving the removal of lots of other components in the dash.
:
:For these reasons I bench tested the two units and they appeared to be working perfectly fine, for a brief minute or so, using a 3 amp fuse on the positive lead from the battery.
:
:Then the unit stopped, and blew the 3 amp fuse.
:
:It is possible that I may have crossed two of the alligator clips that I was using to connected the speaker wires to the terminals in the male wiring harness (that contains 6 terminals for the speakers).
:
:Now when I reconnect things as before, the fuse blows.
:When I hook up just the radio, without the multiplex, it does not blow.
:
:Do you suspect that I have done terminal damage to the Multiplex, or is 3 amp simply the wrong fuse?
:
:The Multiplex installation instructions do not specify a fuse, instead it just reads "Replace existing fuse [3 Apm] with the fuse provided".
:
:Thank you. Your insights would be most appreciated.
:

Meade and others:

Thanks very much for your suggestions and insights>

I am VERY, VERY PLEASED to report that the unit is working just fine now, with a 6 amp fuse.

I received this piece of advice from Tony at another forum dedicated to Corvairs (Virtual Vairs):

The multiplex unit uses pretty much the same current as the radio and the radio will use around 1.5 amps as a rule. The second rule is to use a fuse that's about twice the working current of the device it's supposed to protect. Any fuse between 6 and 10 amps would likely work out ok although a 6 amp fuse might be not so easy to find. 10 amp, not a problem. The usual failure in a Delco car radio of that vintage (or the multiplex) ends up drawing over ten amps in any event (shorted output transistor or filter capacitor) so even the 10 amp fuse would blow if something did go wrong.

tony..