Topic-icon ATA 80, Start Valve Indication

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3 years 6 months ago - 3 years 6 months ago #238 by Mark D Francis
I didn't know if this should be put here or possibly starting another Gulfstream topic dealing with just wiring.

Oh well..... this really concerns "old" wiring no matter what aircraft types we're talking about.

This particular GII's problem was not having the "Start Valve" master caution light on when the #1 engine was being spooled up.

Simple-ass circuit, but you sure couldn't see it through all the possible ASC options shown on the wiring diagram.

The switch on the start valve only has two output pins.



Pin "A" is 28VDC coming from (depending on ASC) either the #5 master caution breaker near the floor of the 169 panel (the right engine uses #7) or a separate breaker in the cockpit. Pin "B" is the feed to the master caution, "Left Start Valve" light.

What is the easiest thing to try first???? A jumper on the plug to rule out the start switch itself (even though maintenance had already stated a replacement starter was tried).



No light with the jumper in. Out came the meter. No voltage on pin "A". Shit, now what? Well, the most likely problem would be in the plug...... so let's open it up.



Hey a busted pin!!!
First of all, I haven't seen one of these types of pins in well over 15 years. The last being a 727 firewall plug. The pin actually comes out forward and it is held in by a removable pink plastic ring. Old-old-old technology, but it still needed to be fixed.



After digging through my stash, a replacement pin was installed. Did the light come on???? No.

And here is where the point of this whole post is. "I" broke the wire off the pin when I opened the plug up. Yes, it probably was on the fine line but, it wasn't the problem. The actual problem was an open section of wiring on the other pin about 4" up from the pin itself. Knowing that this was quickly turning into a cluster, we found a replacement plug and fixed everything right (no splices or service loops).

My pucker factor always kicks into gear when I'm dealing with old wiring. It's almost a given that you'll break some when opening up bundles or plugs that haven't been touched in years. I had a nightmare on a DC-8 RMI plug a few years back. Just be aware that bad shit can happen when dealing with wiring that started out in the 60's and 70's (uh.... maybe even the 80's now).
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Last edit: 3 years 6 months ago by Mark D Francis.

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3 years 6 months ago #239 by Mark D Francis
And now a little honesty..... Just how did I find that open section of wiring???

I grabbed a stick pin and started poking my why up the insulation until I found power. Putting holes in a wire is a BAD idea. In this case that whole section was replaced during the repair.

A wire that has been stuck, especially one exposed to the elements, will break down and fail "very" quickly. Even with the knowledge that this aircraft is heading to the bone yard within two years, I would not of left that wire in that condition.

I should of isolated the problem "to" the engine by disconnecting the firewall plug first. I got lazy and lucky with this problem.

Using stick pins is not a recommended troubleshooting practice.
The following user(s) said Thank You: Mike Forti

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