TR6 '74 brake failure / oil pressure / ARV wiring question

[TomChar]

I'm trying to figure out the wiring for the brake failure switch. Looking at the diagrams from Advance Auto-Wire, I am really confused as to how this system is supposed to work. From the diagram, it appears that when running, with good oil pressure, the ARV is powered through that switch. If oil pressure drops, does the switch then light both the low pressure and the brake failure light?

Is the ARV related to the gas tank venting at all? My car has dual SCOE Weber's installed, so the valve cover is vented to a breather on the air filters, with no plumbing from the engine to the carbon canister or the ARV.

Thanks, Tom

 

[TR3driver]

I'm trying to figure out the wiring for the brake failure switch. Looking at the diagrams from Advance Auto-Wire, I am really confused as to how this system is supposed to work. From the diagram, it appears that when running, with good oil pressure, the ARV is powered through that switch.

More accurately, the anti-runon solenoid is grounded through the oil pressure switch, only when the oil pressure is above the set point. However, the solenoid is only powered through the ignition switch when the ignition is off. So it only activates when the ignition is off but the engine has oil pressure.

If oil pressure drops, does the switch then light both the low pressure and the brake failure light?

Yes, provided the brake failure switch (on the PDWA) is not closed). If it closes, then only the brake warning light works and the oil pressure light is disabled, regardless of the oil pressure switch.

Is the ARV related to the gas tank venting at all? My car has dual SCOE Weber's installed, so the valve cover is vented to a breather on the air filters, with no plumbing from the engine to the carbon canister or the ARV.

Well, it's marginally associated with gas tank venting, since the tank should be vented to the canister. But with all that plumbing undone, the anti-runon solenoid can't do anything anyway. Might as well disconnect it too.

Ditto for the carbon canister; without a purge line it will simply become saturated with fuel vapor and let it pass through to the atmosphere.

 

[poolboy]

If you have other than the post 72 ZS carbs, you might as well forget about the ARV. It's not capable of doing anything otherwise.
Just get a single spade oil pressure switch such as Auto Zone's PS 123 or NAPA's Echlin OP6612 or which ever switch Moss or the others sell for the pre 73 engines.. Then just connect the White/brown wire to that single spade just like the pre 73 wiring schematic indicates.
The PDWA will still function as it's supposed to.
If you haven't already you might as well just go ahead and remove the ARV, the hose connecting it to the CC, the hose from the intake manifold to the CC and cap the hose nipple on the intake manifold's banjo fitting.
Be aware of what Randall said about the Brown/ red wire on the ARV being hot when the ignition key is "OFF".
You'll want to take precautions that it doesn't ground once disconnected from the ARV...Tape, cap, or heat shrink the exposed end.

 

[TomChar]

Now the conversation becomes educational, as I have no intention to spend the money to restore these functions.

So it only activates when the ignition is off but the engine has oil pressure.

Ah, the definition of run-on. But how does it stall the engine? Does it vent the canister to a port below the throttle plates on the original carbs, preventing additional fuel draw through the idle circuit? I'm guessing the center nipple on the canister is the outlet to the intake manifold side of the carbs? Since this hose is missing (as are the carbs, actually), I actually have no idea to where it originally went.

Yes, provided the brake failure switch (on the PDWA) is not closed). If it closes, then only the brake warning light works and the oil pressure light is disabled, regardless of the oil pressure switch.

On my car, those two lights do work, but the oil light is so dim compared to the BRAKE light, I'd never notice it. Was this system really useful when new? If anything, the BRAKE light might draw my attention to the oil pressure gauge right above it. That and I don't have a low oil pressure sender installed either.

Well, it's marginally associated with gas tank venting, since the tank should be vented to the canister. But with all that plumbing undone, the anti-runon solenoid can't do anything anyway. Might as well disconnect it too. Ditto for the carbon canister; without a purge line it will simply become saturated with fuel vapor and let it pass through to the atmosphere.

Since this solenoid is only activated with the ignition off and the engine spinning - for as long as that would last - when is the CC vented to the engine, to purge the fuel vapors? I noticed a small vacuum port on the valve itself, but it doesn't move with vacuum applied. The solenoid and valve function on my car when I short the wire for the pressure switch to ground.

I would like the CC and the fuel tank venting to be functional, as I despise that fuel smell. Unfortunately the Weber MCHH carbs equipped now do not have a vent port for the fuel bowls.


For the oil pressure switch location, I have an idea of where it would be based on the location of the wires. Here's a pic of the block below the distributor, there are two plugs, and then a large plug like thing just above the oil pan. Was the oil pressure switch located in one of these openings?


Thanks all,
Tom

 

[poolboy]

Randall is more than capable of answering those questions.
For my part, I will provide a picture where you might be able to see the OP sensor switch in my 73. The wiring was 'modified ARV delete' as I described.

 

[TR3driver]

Ah, the definition of run-on. But how does it stall the engine? Does it vent the canister to a port below the throttle plates on the original carbs, preventing additional fuel draw through the idle circuit?

Yes, exactly.

I'm guessing the center nipple on the canister is the outlet to the intake manifold side of the carbs? Since this hose is missing (as are the carbs, actually), I actually have no idea to where it originally went.

The original carbs had a special port used for the purge line, which was above the throttle plate but below the venturi (so exposed to a constant, low level vacuum with the engine running). The center port of the CC was plumbed to both of those ports, plus the rocker cover.

On my car, those two lights do work, but the oil light is so dim compared to the BRAKE light, I'd never notice it. Was this system really useful when new? If anything, the BRAKE light might draw my attention to the oil pressure gauge right above it. That and I don't have a low oil pressure sender installed either.

Marginal at best, IMO. As you say, it's mostly the brake light that gets my attention. At least on a TR6 you have an oil pressure gauge to check; Stags did not originally come with oil pressure gauges, leaving the driver to puzzle out whether the brakes were failing, or the engine. (Scared me the first time I saw it.) But it was a US safety requirement (I believe) that there be a bulb test function for the brake warning light and this is how BL chose to implement it for a few years. In 76 they finally went to having a bulb test relay (probably because they also needed a bulb test for the EGR service indicator).

Since this solenoid is only activated with the ignition off and the engine spinning - for as long as that would last - when is the CC vented to the engine, to purge the fuel vapors? I noticed a small vacuum port on the valve itself, but it doesn't move with vacuum applied. The solenoid and valve function on my car when I short the wire for the pressure switch to ground.

The purge line is always open (although IIRC there are supposed to be restrictors on the carb ports that limit the flow slightly). The vacuum port on the valve goes to manifold vacuum and gets used only to kill the engine after the key is turned off.

I would like the CC and the fuel tank venting to be functional, as I despise that fuel smell. Unfortunately the Weber MCHH carbs equipped now do not have a vent port for the fuel bowls.

As long as the bowls are vented into the air filters, you probably won't get much smell from them (unless hot soak is boiling the fuel, a common problem these days). Without support from the carbs, I think you're stuck on that point. (Although a really clever and determined person might figure out how to add support to the carbs, perhaps by blocking off the existing vent and drilling a new one to be fitted with a solenoid.)

But for the tank vent, all you really need is a source of vacuum for a purge line (like that banjo fitting on the intake manifold), with a suitable restriction so it doesn't mess up the mixture too much. Lots of cars from the 70s and 80s used little restriction modules for this and that, or I'm sure you could fab one on your own. It doesn't need to be any more than a bit of brass that fits snugly inside a hose, with a hole drilled through it.
https://www.oreillyauto.com/site/c/...+restrictor_-1_3062&keyword=vacuum+restrictor
Or you could even use a needle valve if you wanted real-time adjustability
https://www.mcmaster.com/?m=true#angle-needle-valves/=wjg2ja

That hex head "Whats this" in your photo is the oil pressure relief valve. It keeps the oil pressure from going too high.

Some years ago I got to watch the initial startup of a rebuilt Toyota motor where the relief valve had gotten blocked off by an unwise substitution of parts from different years. It would literally blow the oil filter can off the fitting just a few moments after starting! I don't think the professional mechanic who did the rebuild was very happy with me for pointing out the problem (even though I also showed him how to fix it). He quit the job soon afterwards Although that might have had something to do with the shop owner docking him for the 15 or so quarts of oil that got dumped on the floor, plus 3 ruined filters.

 

[TomChar]

I ordered a single terminal pressure switch ($7), and I'll keep the CC and the VC plumbed to the carb air filters. Playing with the OP & PDWA wires, both lights work, but if I ground the PDWA wire, the BRAKE light doesn't light up; and there isn't power at the wire with the ignition on. I haven't a chance to look around, but I imagine those lights are connected under the dash right there by the lights. Any thoughts on where my break might be?

 

[poolboy]

Do you have a copy of the real wiring schematic for a 74 ?
If not get it here:
https://www.advanceautowire.com/tr2506.pdf
You'll see how it's wired.

 

[TomChar]

I have that schematic. I'm asking if anyone knew the physical location of the wiring connections. I won't be able to crawl under the dash until later tonight (maybe), but I was just wondering if anyone knew of the probable location where the PDWA circuit lost power (if this is a common failure). The schematic indicates the brown /purple wire connecting the two lamps, and then leading to the PDWA switch. The lamp connection is good, as indicated by grounding the OP switch wire, but then somewhere between there and the end of the brown/purple wire by the PDWA it's broken. I was asking where that connection was made; I'm assuming it's at one of the two lamps behind the dash.

 

[poolboy]

"B" is not brown.."N" is brown. Don't be looking for a brown wire.

 

[TomChar]

Yup, it's black, that's what I meant.

Actually, under the hood they're all black from filth. I can barely make out the stripe, and whether it was red, pink, purple or any other color I wouldn't know unless you told me. I had to clean the wire with a rag dampened with brake cleaner, and I'm sure that didn't help color retention any either.

 

[poolboy]

The schematic also indicates the B/P wires from the lamps and PDWA switch are joined in a splice, possibly wrapped with harness tape.
You might want to wait until you get the single terminal switch and connect the W/N wire to it, before proceeding.

 

[TomChar]

After a little more cleaning, I realize I'm not looking at the correct wire by the PDWA. I can't find that black / purple wire anywhere under the hood. At the lamps under the dash it's clean, easily identified and does have 12V on it with the ignition on.

Back under the hood, there are three wires that exit from the main loom in a small tube near the PDWA and the oil pressure switch; white w/ purple (hot with key off, insulated terminal, from ARV); white w/ brown (hot with ignition on); and a black wire. The black wire is grounded all the time, which makes me believe that it went to the P terminal of the three blade oil pressure switch. This is the wire I thought was for the PDWA, and was originally plugged into the PDWA switch when I pulled it off for the brake job awhile ago.

So I can't find the black w/ purple wire anywhere. Poolboy, you mentioned the splice buried in a harness somewhere. Any idea where that splice might be? Or does anyone know where under the hood that wire should be located?

Thanks again.

 

[poolboy]

There's a bundle of spliced wires, against the firewall, inside the cockpit. but rather than unwrapping the bundle, since you can identify the wires at the lamps why not just splice into the BRAKE lamp B/P wire and connect it to the PDWA switch ?

 

[TomChar]

There's a bundle of spliced wires, against the firewall, inside the cockpit. but rather than unwrapping the bundle, since you can identify the wires at the lamps why not just splice into the BRAKE lamp B/P wire and connect it to the PDWA switch ?

Well, that's true. But working under that dash in that tiny little space leaves a lot to be desired. And that's with the seats out. Getting at it right at the lamp is a good idea, but as I look at the lamp, how does one get the lamp fixture, or even the lamp itself, out? I can't figure how it's attached!

 

[poolboy]

The lens that says BRAKE just slides into the dash from the front.
The lamp bulb slides into the BRAKE lens from the rear, behind the dash.'
If by doing that splice, you eliminate the OP warning part of the circuit, you won't get the BRAKE light illuminated unless there is a brake fluid pressure imbalance.
You may have to include the OP warning lamp's B/P wire into the new splice to get both the OP and the BRAKE to light up when there is no or less that 5psi OP.

 

[TomChar]

I guess I just gotta push it, push it real good.. Dang, I hate that song!! Stupid commercials. Thanks, I'll probably do that. Still wondering where that wire went tho...

 

[poolboy]

The B/P wire that goes onto the PDWA's switch, had a bullet type female socket, not a spade like most of the other wires, if that helps

 

[TomChar]

My PDWA switch has two pins in it, and a female spade connector fits perfectly over both pins. Again, no idea whether the switch is original either.

 

[poolboy]

I had to rewire mine when I got my 73 TR6 and that's what I used, a spade slipped on the 2 pins
My first TR6 was a 74 and was original as far as the wiring, or so I believe, and it had the bullet type socket stuck on one of the pins, the other pin had a short piece of cut off wire on the other pin.
From what I've read on other Forums that cut off piece of wire on the other pin was common, leading to a lot of questions as to what it was originally connected to...I don't think anyone knew...the PDWA warning switch and BRAKE lamp worked regardless of that other wire being cut.