Not sure where to post, car won't start with brake pedal pressed

-
Tuckerjr

Tuckerjr

Well-Known Member
Joined
Jan 11, 2011
Messages
95
Reaction score
3
Location
Georgia
Hi guys,

My 74 dart sport has been sitting several months after replacing a head gasket earlier in the year. I was finally able to get back to work on it, but it will not run with the brake pedal pressed. It will turn over but not start until I take my foot off of the brake. It cranks and idles fine with my foot off the pedal, but as soon as I press the pedal it dies as well.

I was thinking maybe I have a vacuum leak in the brake booster. If this is a possibility, how do I verify that the booster is bad? Any other thoughts as to what it might be?
 
Disconnect the vacuum line to the booster and plug the vacuum line or fitting at the engine.

You mentioned Head Gasket, did the engine to chassis ground get installed?
 
Checked the engine grounds, they are both tight. Will take them off and clean them to be sure. I removed the hose and plugged the brake booster and intake connections, still dies when I press the brake.

Since we eliminated the booster does this mean it's electrical? A short of some kind maybe.?
 
I cannot imagine that the brake light circuit is somehow related, unless some wiring has melted IN the harness(es) and cross shorted. Someone mentioned above--might be pedal movement---which is somewhat close to the ignition switch wiring---might be pulling on a wire connection, etc. Get under the dash, do "wiggle" test. Inspect the column connector for the ignition switch, and get a test lamp and meter and some jumper leads and actually do some testing

A couple of things:

This might not be related, but 74 has the obnoxious annoying Federally mandated "seat belt ignition interlock." Look on the left fender apron for a box about the size of a horn relay with a push button. This is the reset for that interlock system. The yellow start wire goes through that. Find the two "yellowish wires" and splice them permanently together

Also, partly because of the interlock, 74 has a unique wiring system. I created a 74-only electrical system from the factory 74 manual. If you don't have one you can download the electrical here:

Go down to post 64 and click the black icon in the box, "save as"

Free service manuals

Also go over to MyMopar and download the two page 3rd party diagrams. These are NOT as complete and detailed, but can be easier to follow

Get your meter/ light and go over both parts of the column ignition connector and see if you have power on the "start" wire. In the electrical section I posted, there is a section after the diagrams that pictures all the various connectors

ignswitch.jpg


The "start" wire is the top terminal yellow. This should be hot anytime you twist the key to "start."

Then go out to the bulkhead connector and the start relay, see if you have power on that yellow at the start relay, and if not, go to the bulkhead connector and see if you have it there

FUNCTIONAL PATH

The functional path which the starter relay takes is...........ignition switch--out on yellow---through the firewall connector---through the seat belt interlock---to one "push on" flag/ spade terminal on the start relay (coil)--through and out the remaining flag terminal--and either down the firewall to the automatic neutral safety switch, which is grounded in park/ neutral, or on a "stick" car, back into the interior through a grommet, and to the clutch pedal safety switch which grounds when pedal is depressed
 
Last edited:
I finally found the issue I believe. It looks like one of the wires to the msd was getting power from a connector that loses voltage when the brake pedal is pressed. I ran it to a different wire that only gets 12v when the ign is switched on, and now it stays running.

Next thing is to see if I can trace that connector and see where it leads, I'm just curious.

This whole wiring harness is a spliced up mess.
 
^^ OK here is the deal. ^^

The original ignition switch wiring is something you MUST understand and it is not complicated.

Look at the diagram of the ignition switch I posted above. You are interested in J2A, Dark BLue, and J3, Brown.

These are the ONLY wires going through the bulkhead connector from the switch, that are to be used for ignition. The DARK BLUE is the "run" circuit and is powered ONLY in the key "run" position. THIS GOES DEAD during cranking

During cranking the BROWN supplies power. This wire originally went to the coil+ side of the ballast resistor

IF YOU HAVE an MSD you must chase both these down at the bulkhead connector, make certain the dark blue remains connected to run such things as the VR and alternator field. Jumper the BROWN into that and run that off to your MSD "small red." If the ballast resistor wiring is intact, and the original coil+ wire is as well, you can just bypass the resistor and use the old coil+ wire to the MSD "small red." This wire is sort of a "trigger" wire to power on the MSD

Then the big black on the MSD must go to a good ground, and the big RED must go straight to a good battery connection, such as the starter relay "big stud."

MAKE CERTAIN that ONLY the wires from the MSD ARE THE ONLY wires hooked to your coil. DO NOT connect a tach, a radio capacitor, or anything else to the coil
 
Last edited:
67Dart273 said:
I cannot imagine that the brake light circuit is somehow related, unless some wiring has melted IN the harness(es) and cross shorted. Someone mentioned above--might be pedal movement---which is somewhat close to the ignition switch wiring---might be pulling on a wire connection, etc. Get under the dash, do "wiggle" test. Inspect the column connector for the ignition switch, and get a test lamp and meter and some jumper leads and actually do some testing

A couple of things:

This might not be related, but 74 has the obnoxious annoying Federally mandated "seat belt ignition interlock." Look on the left fender apron for a box about the size of a horn relay with a push button. This is the reset for that interlock system. The yellow start wire goes through that. Find the two "yellowish wires" and splice them permanently together

Also, partly because of the interlock, 74 has a unique wiring system. I created a 74-only electrical system from the factory 74 manual. If you don't have one you can download the electrical here:

Go down to post 64 and click the black icon in the box, "save as"

Free service manuals

Also go over to MyMopar and download the two page 3rd party diagrams. These are NOT as complete and detailed, but can be easier to follow

Get your meter/ light and go over both parts of the column ignition connector and see if you have power on the "start" wire. In the electrical section I posted, there is a section after the diagrams that pictures all the various connectors

View attachment 1715792404

The "start" wire is the top terminal yellow. This should be hot anytime you twist the key to "start."

Then go out to the bulkhead connector and the start relay, see if you have power on that yellow at the start relay, and if not, go to the bulkhead connector and see if you have it there

FUNCTIONAL PATH

The functional path which the starter relay takes is...........ignition switch--out on yellow---through the firewall connector---through the seat belt interlock---to one "push on" flag/ spade terminal on the start relay (coil)--through and out the remaining flag terminal--and either down the firewall to the automatic neutral safety switch, which is grounded in park/ neutral, or on a "stick" car, back into the interior through a grommet, and to the clutch pedal safety switch which grounds when pedal is depressed
Click to expand...
Hey Thanks! I needed one too,just incase but yes, thanks to Y'all here, that sob is permanently bypassed. I had it bypassed another way, and ya had to hold the key to crank for about 1 .5 -2 seconds before she'd roll over
 
You must log in or register to reply here.
-

Back
Top