Using the brakes kills the engine! Any ideas why?

[bthompson]

Well, this is a new one for me. After spending a little time freshening up Nigel for the fall, I go on the inaugural trip. Every time I touch the brakes, the engine dies. So much for that idea; back home I go. It's puzzling. Starts right up again every time, but every time the brake lights come on -- thunk. Dead.

Unhooked the brake lights: still dies. Unhooked the switch at the pedal: doesn't die.

The natural thing to do is backtrack and see what I had changed -- but I didn't do anything with the electrical system at all, and didn't touch the brake switch.

Here's what I did do: I stripped all the trim and the lights, removed the front fenders and fixed the wheel arches, disassembled, cleaned, and adjusted the rear brakes, rotated the tires, lubed the suspension, put the fenders back on, painted it with a couple coats of Rustoleum, (looks good!) cleaned the reflectors in all the lights, re-assembled everything, and tested the lights.

Everything else works, and I can't figure out what might have happened to cause this problem. Any one else have experience with this? Why would the brake switch kill the spark?

 

[JPSmit]

I suspect it is a ground issue. Not likely spark but ignition circuit. A ground isn't doing its job so the circuit is somhow being completed when you hit the brakes

 

[bthompson]

That's what I figured; most likely something is shorting to ground that shouldn't be? As near as I can tell from the wiring diagram, the ignition and brake wiring aren't interrelated. I unplugged the leads from the brake switch and crossed them by hand with the engine running, and the engine died. So it's not the switch or the lights themselves. No other electrical drain (lights, blinkers, wipers, heater) causes the dead stall. Guess it's time to check the continuity through the brake circuit, wire by wire?

I wish it were anything but electrical. I can figure out mechanical stuff. I'll keep everyone posted.

 

[Guest]

No, check the grounds for the lights especially where you painted. A poor ground there may cause it to "borrow" ground or rob juice from somewhere else like maybe the ignition system. While I understand this makes no sense whatsoever, we are talking about Lucas here...

That and I'd check for pinched wires somewhere.

 

[dklawson]

I agree that you should check all the lamp grounds and make sure they are good.

Repeat your test where you press the brake pedal and let us know what happens to the following things.
Ignition on, car in reverse, hazard switch on, heater blower fan on.
Are the backup lights on, hazards flashing, oil pressure warning light on, and heater fan blowing?
Press and hold the brake pedal.
Are the backup lights, hazards, oil pressure warning light, and heater blower all still working/on?

All those things are sort of connected to your brake light circuit. A white wire from the ignition switch should be powering one side of the fuse that supplies all the circuits above (apart from the oil pressure light which shares the white wire connection at the ignition switch). If all those thing "go out" or dim when you push the brake pedal, something more than a parasitic ground is at fault here. What doesn't make sense though is that IF you have a dead short, one of your fuses should have blown.

All that said, also check each of the fuses carefully. As Kelleysguy said, sometimes electrics don't make sense. I had a car once where when I put the turn signals on the horn made a sickly noise in time with the flashing lights. It turned out to be a "cracked" but not fully blown fuse that somehow was letting current flow through the horn circuit when I used the turn signals. It made no sense at all but a new fuse fixed the problem. Not saying that's happening here... only that automotive electrics sometimes aren't intuitive.

 

[Guest]

I wonder if somehow the breaks got plugged into the ignition circuit on the fuse box but you don't remember doing it.



Like Doug stated, sometimes things don't make sense. Former neighbor claimed to have once owned a early 90's S10 that wouldn't run if the dome light burnt out.

 

[bthompson]

Okay, here's the results of Doug's test. Surprising results!

Blower on, hazards on, in reverse, ignition on. (1976, so no oil pressure light.) So far, so good.

Now, with the brakes on, I lose the blower and reverse lights. Hazards still work. Brake lights work.

Checked the fuse block and the block connections. Re-seated the fuses, which all seem to be intact, and none of the connections were loose. The obvious grounds for the lights seem to be solid. Checked the wiring looms to see if something was pinched or was laying differently than before. All seems to be good there.

Any ideas for the next diagnostic steps?

 

[Guest]

Solid how? Is it clean to fresh bare metal to ground to or you just talking about the wires?

Here's the next thing I'd do. Pull each wire off the fuse box (on both sides), reinstall a few times and wiggle ONE WIRE AT A TIME. I've had gremlins how up that a wire reseat and wiggle cured. How is you ground at the firewall?

 

[bthompson]

Just the wires; they're tight to the ground bolts, but I haven't taken them off and re-sanded them. Over the weekend, I'll run through the fusebox like you suggested, clean all the ground connections I can find, and see if that changes anything.

 

[dklawson]

Since you will be working on/with the fuse box anyway, temporarily disconnect the battery and un-mount the fuse box. Lift it up/off and clean the terminals on the back side of the box and the wire ends. On the top side of the fuse box, you can use a gun cleaning wire brush to clean the fuse holding terminals. Though the fuses all checked OK... maybe try moving them around position to position, or try new ones just to be doubly sure that they are not part of the problem.

I'll look at the wiring diagram again in light of your test results to see if anything makes sense. However, I was expecting you to lose everything when you pushed the brake pedal... not half of the circuits.

 

[Guest]

Just the wires; they're tight to the ground bolts, but I haven't taken them off and re-sanded them. .

Got to otherwise you're wasting time and can't eliminate them as a possible source of trouble until you do.

 

[dklawson]

Sorry... lengthy post to follow.

From the previous posts in this thread we have:
Post 1
Unhooked the brake lights: still dies.
Unhooked green/purple wire at the lights or at the brake light switch?

Here's what I did do: I stripped all the trim and the lights
Dash lights, running lights, brake lights? What part of the lights/wiring did you work with?

Post 2
I unplugged the leads from the brake switch and crossed them by hand with the engine running, and the engine died.

Post 3
1976, so no oil pressure light.
(Really? Should be one. Yes the switch will be different but there should be an oil warning light).
Now, with the brakes on, I lose the blower and reverse lights. Hazards still work. Brake lights work.

********************

I am working from the wiring diagram for a later '78 model so some things may be different but in general....
These electrical circuits all share a more or less common feed from the ignition switch.
A white wire leaves the ignition switch and feeds the supply side of the fuse that these circuits are connected to.
A second white wire connected either to the same terminal on the ignition switch or indirectly connected to the first white wire feeds the ignition circuit, the oil warning light, and the ignition warning light.
(Did the ignition warning light go off when you tried my earlier test? I forgot to list it).

You know that touching the brake pedal causes the engine to die and that it is not the switch itself. The fact that you also lose the blower and reverse lights says there is a big drain on the system. However, the fact that the brake lights themselves still work and that the fuse is not blowing suggests that this is not a dead short of the green/purple wire from the brake switch to the brake lights.

The following items are ALL supplied via green wires either directly or indirectly (via 4-way couplers) to the same fuse supplied by the ignition white wire.
Brake Switch
Reverse Switch
Wiper Washer Motor
Wiper Motor Switch
Wiper Motor
The PDWA/Brake Warning Lamp
The Voltage Stabilizer for the Gauges
The Hazard Switch

And on the white wire itself, the following components are powered (before the fuse).
Electronic Distributor
Distributor Resistor (Drive? not ballast)
Oil Pressure Warning Light
Ignition Warning Light.

Since you are not blowing the fuse, this suggests that a component on the white wire circuit is somehow drawing current and lowering voltage to other items on that circuit. As slow and painful as this may be, you probably need to locate the 4-way couplers and various joints where the circuits above join together and disconnect them one at a time until you determine which circuit/item is pulling the system voltage down.

Are you still running the original Lucas electronic distributor or has this been changed? It could just be a red herring but....

EDIT: To elaborate a bit on one connection mentioned above, the hazard switch connection of the green wire should just be a pass-through for the turn signal circuit. It is not a connection relative to the hazard lights themselves.

 

[bthompson]

Thanks for all the work, Doug! No reason to apologize; it's all good info.

Some unexpected family stuff came up today, and I couldn't get to the Midget, but I expect to be able to get my hands dirty tomorrow.
__________________

Here are some clarifications for your last post:

- For the initial tests, I unhooked the GP wire at the brake lights (engine dies,) then unhooked the GP and G wires at the brake switch (engine runs,) then crossed the brake switch wires (engine dies.)
- Before painting, I removed all the exterior lights; head, running, tail, markers, license, and reverse. The wires were unhooked at the fixtures; the looms, routing, etc. were not (consciously) messed with.
- No oil light, honestly! It's an early '76, with dual pressure/temp gauge. (Early style bourdon tube/hard plumbed.) Two idiot lights on the dash: ignition and high beams. No amber light in sight. :chuncky:
- The ignition light did NOT go off during the test. Neither did the wipers or hazards. (The tach seems to go dead, but that's hard to tell 'cos the engine is failing at the same time.)
__________________

Partly from the DPO, partly from me, some of the wiring is "nonstandard." I don't see how this could directly affect the problem, since it's been running fine 'til now, but I notice that some of these are 'green wire' circuits. So in the interest of full disclosure:

- The OPUS ignition has been replaced with a 25D distributor with Pertronix.
- Coil resistor disconnected.
- Brake warning light disconnected.
- Door jamb switches disconnected.
- Windshield washer disconnected.
- Seatbelt buzzer and light disconnected.
- Catalyst light disconnected.
- Cat counter disconnected. (Single counter model - no EGR counter)
- Running-on valve disconnected.
- I think that's all of them!
____________________

So it looks like the next stage of the game for me is to re-sand and get good grounds, take out the fuse box and sand down all the connections, make sure the 'box connections are all solid, and give the wires a good wigglin'.

Then go through the bullet connectors with a bore brush.

Then start disconnecting one subsystem at a time.

I'll keep you posted on my progress. Thanks for contributing your brains to this...it's encouraging to not be 'alone' in this. Hooray for the internet and the BCF!

 

[bthompson]

Success! And wouldn't ya know it, it turns out to be my own stupid fault after all.

I cleaned out the fuse box, giving a good polish to all the connectors. (And in the process, found out how the terminals disassemble from the box, which made cleaning much easier!)

I cleaned the grounds under the hood, and those !@#$% 4-way bullets under the dash. No luck so far.

So I started checking continuity, and traced the green-purple wire all the way back to the brake lights. The loom was black with grunge, so I got some thinner and cleaned the wires off before cleaning the bullets.

Oops. I had a wire mis-labeled from when I took the tail lights off! One of those black "ground" wires turned out to be the green-purple lead...so I had one brake light grounded, and the side marker grounded to the tail light lead. The freaky thing is, the lights all still worked normally. I suppose the brakes caused an electrical "backwash" that cancelled juice flowing the other way? I dunno...electrons are still a mystery.
Anyway,Doug and Billy for the input and help! It's running stronger with the clean connections, so that's a bonus too!

 

[dklawson]

Great! I am glad that you found it and that the problem was easily fixed.

However, I don't understand why this did not cause the fuse to blow.

 

[bobhustead]

Probably the ignition system and something else (brake lites ) were effectively connected to the same power source and the non ignition circuit had less resistance than ignition did. On closing the brake lite circuit, the juice all took the path of least resistance. Weird problem though; I hadn't a clue. Bob

 

[1974MGMidget]

Glad everything worked out. And, most of us have made those maddening errors. But, to follow up on dklawson's comment, it wouldn't hurt to check to see that the proper fuses are there, and not one with a too high rating.

 

[Guest]

Told ya it was your fault...layful: