Are there wires underhood that lose power with brakes on?

[urbex]

Still tracking down issues with the '86..as per the other thread where I was trying to track down the alternator brown wire, I found that I was losing massive voltage any time the brakes were on. Basically going down to around 11 volts.

I added a junction block and a relay for the fuel pump, and to feed the L terminal on the relay. I grabbed a random unused connector for an ignition power source for the relay trigger. I found today that particular wire lost all voltage any time the brakes were on, which meant that the alternator was also shutting down any time the brakes were on, hence So I probed for another wire for ignition source, and found that it too lost all voltage with the brakes on.

Eventually I said F it, and tapped the positive wire going to the coil. Still just using it to trigger a relay, so the additional load on the coil circuit is minimal. Still, it got me wondering...were there solenoids or something with the factory carb or emissions equipment that tied into brake operation that would explain those wires losing power with brakes? I'm running an Edelbrock carb with all emissions equipment stripped, so I have several connections not used any more.

 

[6872xtc]

The only thing that I can think of would be the cruise control.
Do the brake lights work? I am was thinking that maybe there is a short killing voltage, but I would also think that it would pop the fuse.

 

[diesel4me]

I agree the cruise control would be the only device I can think of offhand that loses power when the brakes are applied..
Even a truck that didn't have cruise,might still have the un-used wire for it in the harness..

I'd avoid using the ignition coil wire to power anything BUT the coil,especially if it has an HEI ignition,it can cause the module to fail if it gets low voltage or spikes in that circuit..

One wire you can use if you need a power source that is only hot with the key in the "on" position and no other,is a green wire that most GM vehicles that had a carburetor used to operate the idle stop solenoid..
If it has an electric choke,leave that alone too,otherwise adding any more load to that circuit may delay the choke opening and cause flooding or over rich conditions..

 

[urbex]

As I said, it's only triggering a relay, so the additional load is minimal. Last time I bench checked one, it took about 0.15 amps to close the coil on the typical 30 amp relay used in cars. I'm not in any way concerned about the effect on the HEI at that point.

Not to mention, the coil circuit isn't regulated outside of the rest of the harness, so anything else in the truck on a different circuit can cause voltage spikes and dips that would also effect that circuit. Heck, all I have to do is fire up one of the big 2 way radios on high power, and bring up a voltage sag on the entire truck that way, lol.

 

[ktmoutfront]

On an 86, isnti the cruise shut off controlled with a vacuum switch at the pedal? No electrical?

I think you have a short in the brake light system that is causing this.

 

[urbex]

Brake lights work fine with truck off, or on but not running, and cause a minimal voltage drop at that time too...no more so than would be expected with running a couple incandescent bulbs. They're also working fine now that I've rewired the relay trigger. The wires I were using underhood lose all voltage with brakes on. I'm sure there's more issues in the harness somewhere, I just wanted to make sure I wasn't chasing my tail before I dive into it deeper. I never played much with the 80's style emissions feedback carbs, and wasn't sure if there wasn't some kind of solenoid that kicked out with brake application to lean the truck out, raise RPMs, etc.

 

[dyeager535]

Brake lights work fine with truck off, or on but not running, and cause a minimal voltage drop at that time too...no more so than would be expected with running a couple incandescent bulbs. They're also working fine now that I've rewired the relay trigger. The wires I were using underhood lose all voltage with brakes on. I'm sure there's more issues in the harness somewhere, I just wanted to make sure I wasn't chasing my tail before I dive into it deeper. I never played much with the 80's style emissions feedback carbs, and wasn't sure if there wasn't some kind of solenoid that kicked out with brake application to lean the truck out, raise RPMs, etc.

There were things used such as a decel solenoid and some kind of kicker tied into AC or something, but that damn CCC setup was so convoluted I never had the time to really dig into it. But even some of the non-CCC vehicles got decel solenoids as I recall. I don't *believe* they were variable voltage.

I cannot think of one device on the truck that uses "variable" voltage, certainly not in conjunction with the brake. Obviously voltage changes slightly as the alternator output changes, but outside of that, system voltage should be the same. Brakes, cruise, neutral start, all are on or off.

On second thought, if CCC, there would be a barometric (MAP) sensor, and that would vary, but I believe that is 0-5V.

 

[diesel4me]

Have you peeked under the dash to ensure no wires are getting pinched by the brake pedal ?..
I had a stereo that would cut out sometimes when I stepped on the brakes--found the speaker wires were hanging down low enough to get caught in the brake pedal cluster and getting pinched!..

 

[Blue85]

There is also a circuit for transmission lockup that runs through a switch on the brake pedal. There should be 3 things on the pedal - the cruise control shut-off, the TCC lockup switch and the brake light switch. The two switches have opposite logic. When you press the pedal the brake light switch closes while the TCC switch opens. Pretty easy to unplug them one at a time and see how the symptoms change.

Anything else we should know about this truck, like a transmission swap, a diesel or a California model?

 

[urbex]

Started life as a 305, PO claims it now has a 350 from an 80-something Camaro. Auto trans, should be a 700R4 based on the square 4 pin plug on the driver's side. I haven' pulled the casting number yet to verify motor...sure feels like an 80-something V8, lol. RPO sheet doesn't say anything about California spec.

Forgot about the TCC lockup...been many years since I last ran an auto trans, and even then it was a full manual non lockup 400.

The two plugs I used was one brown 2 pin Weatherpak style near the heater box on the firewall, with I think was a blue wire and yellow wire. The other was near the brake booster, black two pin connector with female pins similar to that in a fuse panel, with a blue wire and a blue/black stripe wire. I can get pictures when I get home.

Coil power was spliced to a heavy pink wire, which I know is common GM ignition switched power, though I don't know if it was actually supposed to be the coil power originally.

 

[Blue85]

The age-old question is whether it's better to be lucky or good. I believe you're 0 for 2 so far on this one.

One of your attempts must have used the lockup vacuum switch (under the guy's thumb) connector - which of course is powered through the brake switch. That connector must be what you describe as "like a fuse block", as it mates with the two male spades on that switch.

I grabbed a random unused connector for an ignition power source

You can see why this isn't recommended, right?

Near the heater box/ PS valve cover is a purge solenoid that is powered through the transmission and therefore switched by the brake switch. It is a 2-pin weatherpack, but I thought it was red.

If you don't have a light bulb for the field connection, you can use a resistor. I've heard that direct connection of "L" can damage the alternator.

 

[urbex]

You can see why this isn't recommended, right?

Neither is splicing half the crap underhood with 18gauge red wire, and loosely twisted wire ends wrapped in cheap electrical tape...along with many missing components, but seeing as how I don't have any control over what the nimrods before me did....

Wiring diagrams only go so far when you're dealing with crap like this, and I'm not really in the mood to pull an entire 30 year old chassis harness to attempt to rework it in 110 degree Phoenix heat.

 

[rampage]

Neither is splicing half the crap underhood with 18gauge red wire, and loosely twisted wire ends wrapped in cheap electrical tape...along with many missing components, but seeing as how I don't have any control over what the nimrods before me did....

Wiring diagrams only go so far when you're dealing with crap like this, and I'm not really in the mood to pull an entire 30 year old chassis harness to attempt to rework it in 110 degree Phoenix heat.

Where’s your enthusiasm?

 

[Blue85]

If it's all standard equipment underhood, sourcing an original harness might be quicker and cleaner overall. It's hard to know why somebody did so much hackery, but it could be as simple as not bothering to undress the harness from the engine before pulling the engine. If you have regular '86 Q-Jet, HEI, A/C, etc. a stock harness should drop in and just plug to everything.

 

[urbex]

No, it's not. AC has been changed, serpentine set up now, Edelbrock carb (though I'll be doing an EFI conversion later this year, may just do the LS treatment), gauges will be swapped out later this year, and there will be many electrical things added inside. I'm not putting another 30 year old OEM harness in, and new OEM replacements are stupid expensive. I've already done a couple dozen full engine/chassis rewires using aftermarket harnesses, along with 3 EFI motor swaps in different vehicles.

 

[urbex]

Oops...apparently I can't edit posts...anyways, all emissions/smog stuff has been removed, so I'm just going to end up with a bunch of unused wires/connectors underhood again anyways if I go OEM unless I unwrap the whole harness and cut out unneeded circuits. It will end up being far cleaner and more effective to just start with an aftermarket harness that I can modify easier to handle the different needs.

 

[Stomis]

The only other thing I can think of other than Cruise/TCC that would disengage with brake application is the EGR. But that shouldnt be a direct switch but rather logic handled by a computer you said you dont have anymore.