Strange electrical issue (missing voltage)

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87e30
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Strange electrical issue (missing voltage)

Postby 87e30 » Thu Apr 24, 2014 7:05 pm

First off this is not e30 or bmw related in any way, so feel free to move to off-topic or whatever. This is a problem we're having with one of our flatbeds that is confusing the shit out of me.

Ok, I'm sure many of us here know what a relay is, and how it works. If you don't know, a "standard" 4 pin relay allows a low current source to control a high current device. The 4 pins are numbered 30, 85, 86, and 87. When you put power and ground to 30 and 85 it connects 86 and 87 together. So we have a relay on our truck with ground connected correctly, and it gets power just fine from a switch in the dash. Power is fed straight from the battery (through a 40amp fuse) to pin 86. Pin 87 goes to the trucks PTO solenoid. If the solenoid is disconnected pin 87 gets a full 14 volts with the truck running when the switch is turned on. If I plug the solenoid back in there is 0 volts on pin 87 with the switch on, and the solenoid does not activate. Now if I run another wire straight from the battery to pin 87 the solenoid activates, and the PTO functions as normal until it is turned off. How is this possible? If you are feeding something 14 volts there's no way it could just go missing.

This is actually the second time something like this has happened on the same truck. The first time I couldn't figure out why it was doing it, but I ripped out about 100 yards of extra ghetto rigged wire that did not need to be in the truck, and everything started working as it should. Whoever wired up the truck before we bought it had some weird power loop with multiple fuses and tapped into one of the stock relays in the truck. I'm not sure what that relay is for, but it should not have been connected to the PTO wiring, so I removed it. There was a fuse wired up coming from that relay that would have 14 volts on it if you removed the fuse. As soon as you put the fuse back in there was 0 volts.

I have to be missing something here. Why on earth would this happen? It just makes no sense at all.
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban
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87e30
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Re: Strange electrical issue (missing voltage)

Postby 87e30 » Thu Apr 24, 2014 8:46 pm

I may have mixed up the pin numbers, but I know it's wired correctly, and I have tried 2 relays. It worked most of yesterday. The driver dropped off a vehicle, shut off his PTO, drove to the next disabled vehicle, and PTO didn't work when he tried to turn it back on. I've been trying to figure this damn thing out most of the day.
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban
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87e30
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Re: Strange electrical issue (missing voltage)

Postby 87e30 » Thu Apr 24, 2014 10:25 pm

Even stranger than the behavior, now it's working again. I found there was voltage at the fuse by the battery, but not at the end going in to the relay when things were not working, so I started pulling out that wire to see if it was shorting somewhere or something when things just started working again. I removed the 40amp fuse and instead wired it to an existing 50amp waterproof breaker. There's 2 50amp breakers on that truck, one is for all of the work/aux/strobe lights, the other was only running to a brake controller that we don't even use. If I remember I'll go back and edit my original post and fix the pin numbers.
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban
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Re: Strange electrical issue (missing voltage)

Postby Yoshi » Thu Apr 24, 2014 10:52 pm

Maybe the wire has been pinched or internally corroded?
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Re: Strange electrical issue (missing voltage)

Postby M20_fever » Fri Apr 25, 2014 3:15 am

Sounds like a bad connection on the trigger wire. Why not just pull the garbage wiring out and run new stuff you know if good?
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Re: Strange electrical issue (missing voltage)

Postby 87e30 » Fri Apr 25, 2014 12:27 pm

I tested the wire with an ohm meter and went inch by inch flexing the cable and saw no changes in resistance, so I'm pretty sure the wire is good. Replacing the whole harness would be several hundred dollars and about a whole day of work. I rerouted the wires so there is now less twists and turns. I'm thinking the fuse holder hay have had something to do with it. It was partially melted due to a previous electrical fire when the driver put in a jumper wire because he didn't have any spare 40amp fuses on him. The fire didn't happen at the fuse, the old solenoid burnt up because of an internal short, which is what was popping his 40amp fuses, but the fuse holder did melt a little. That night sucked. I was laying under his truck in the middle of Newark, NJ with a Jeep half loaded on the bed for 2 hours before I could get his PTO activated so we could unhook the Jeep and get it loaded on my truck. At least we were in a parking lot so I didn't have to worry too much about someone driving over my legs.
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban

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