67 base, voltage loss when cranking

mckiecougar said:

Hey there,

I have a 67 base that is fairly new to me, i've just completed an entire engine rebuild and along the way added electric fans and a new DUI - hei distributor.

Finally got everything ready to go for cam break in and I haven't been able to get a start as wired... My fans and HEI are run off of separate relays from the back of the ignition switch 12v post. Everything seems to work as it should when key is in ON and ACC but when I go to crank the engine voltage drops to 0 at my HEI and my gauges / lights all die. The starter however does crank over...

I managed to complete cam break in by just adding a jumper wire from battery terminal to the distributor BATT directly, like this it all runs perfectly.

I'm at a bit of a loss as to what might be causing my voltage drop when cranking? Any thoughts?

Bad ignition switch? Bad ground somewhere?

My starter solenoid does not have wire 32 (Blue / Red) connected to the S terminal, PO had cut it somewhere along the harness. The car did run however prior to me pulling the engine for rebuild...

Thanks for any help!! Will try to fetch some pictures when i'm back at my garage Monday.

Click to expand...

For a stock 67 cougar, Circuit #262 brown wire needs to be connected to the I post on the starter solenoid (assuming of course the PO didn't hack the entire ignition circuit which we know the PO has done in the past since you say there is no circuit #32). Circuit #262 will be powered from the Starter solenoid directly when cranking the engine.

Circuit 262 then goes back to the firewall where it connects to the ignition circuit's #16 (red-green) which goes to the coil, providing power when starting. (Note: the power draw by the starter motor consumes 100+ amps and thus power provided to the I circuit will measure roughly 9 volts, not 12 volts. This is why the resistance wire is bypassed during starting and not to give a brief 12 volt jolt to the ignition systems when starting!!!!)

When the car starts and you return the ignition switch to the "Run" position, power to the coil is now provided via the ignition switch C or I post on circuit #16A at a reduced voltage of 8 to 9 volts. If you have an XR7, the power from the resistance wire goes to the Tachometer then to the firewall. If you do not have power at the coil in run position with an XR7, place a bypass wire across the tachometer and retest for power at the coil. Note if the car is running, and you put a Voltmeter on the Starter solenoids I post, you will also read 8 to 9 volts because the wires are all connected at the firewall thus backfeeding power from the resistance wire.

Now after re-reading your original post, you should have wired your HEI relay to the C (or I post) because the switch connects the battery power to both the Start and Coil when starting. The Accessory post is only connected to the battery when in Run or Accessory mode. You may want to try this first especially if the circuit #262 is missing at the starter terminal as described above.

Once you figure out what works for your situation, do yourself a favor, document how you wired the relays to the ignition and which wires were removed from the starter solenoid from a stock system AND put it in your glovebox. This way should you need to work on your ignition system in the distant future (hey I'm certain you will get your Cougar running) or the next caretaker of your Cougar will know exactly how your car is wired and why!!!

Coach Jack

mckiecougar said:

Coach Jack!

Thank you so much for the detailed and thorough response. I had suspected that my next step might be re-routing the relays to the I post on the starter solenoid - great to hear that reconfirmed! That will be my first next step once I get back to it (hopefully tomorrow).

I imagine so but will that 8-9 volts at the I terminal on the starter solenoid be steady / constant whilst running? I don't want it dropping again and having my fan relays not be powered for some reason once coolant temp is hit.

This all makes sense but I had been confused, for some reason all of my digging so far had told me that I would have 12v constant at the ignition switch post in ACC, RUN and START... If that isn't the case then clearly this is my problem!

Good advice on making a fresh diagram to keep with the car, I certainly will do once I sift through some of the PO's uhh... Handiwork...

Will report back once I reposition the relays. Cheers!

Click to expand...

Remember, your relay trigger wires will switch on the battery power to your ignition system or cooling fans, but the load will be passed thru the relay via the battery (always use a circuit breaker or fuse to protect between your battery and load in your case ignition and cooling fans).

You can certainly use the I post on the starter solenoid as the positive trigger wire to your relay (even though it will only have 8 to 9 volts when running) or you can wire the positive trigger wire to the I post on the ignition switch ( this will have 12 volts when the ignition switch is in the Start and Run positions). Either way you choose is up to you. I power the relay for my pertronix ignitor via the starter solenoid's I post.

I would leave your trigger wire for the fan relay attached to the ignition switches Acc post. The Acc post will be powered when the ignition switch is in the Run and Acc positions. Why do I recommend this? Because you may want the fans to run when the engine is off especially if the engine has heat soaked, then all you have to do is turn the ignition switch to the Acc position. You could always get fancier placing a timing circuit into a trigger wire so that your fan relay trigger stays powered for X minutes after the engine is turned off, that way the engine fan would stay running until the temperature cutout switch or the timer expires whichever comes first. Or you can simplify this and install a toggle switch to turn on the power to your trigger wire ( just dont forget to turn it on after starting your car)!!!

Just remember to document it!!!

Coach Jack

I thought about your cooling fan and put together this pdf file showing how to wire the fan so that power to the fan turns on when you turn the ignition switch to the Run or Acc position which then triggers a second relay that powers the fan when the sending unit exceeds its set value. In addition, the second relay will power the fan when the sending unit exceeds its trigger value even when you turn the Ignition switch Off until the temperature sending unit senses the water temperature drops below its trigger value. With the switch Off and the sending unit drops below its set value, all power to the fan is OFF.

The one drawback to this design is that your fan will be running with the engine off and will draw down the battery until the temp cools down. Some people kill the fan once the Ignition switch is turned Off, but I personally don't like that design. All modern cars keep running the fans while the ignition is Off and the engine is hot. You could get real fancy and turn on a timer as another trigger to the second relay to let the fan run for x minutes after the Ignition switch is turned off so you get the best of both worlds.

And to make this complete, if your car has air conditioning, you need to add a separate ground to bypass the Sending unit so that the fan ALWAYS turns on when the A/C is on.

Coach Jack


https://www.mercurycougar.net/attac...118712/?hash=b460b9024d507149f7f5c05728fd1f9e

Good to hear that you got your car running!

Funny thing is I was working on my 67 recently replacing the timing cover, water pump which entails removing the smog pump, alternator and brackets. When I reassembled the front of the engine and went to fire the engine, it would crank, fire off then immediately die. Hmmm, turns out I accidently disconnected my I wire at the starter post, pushing the fitting back on the I post, it immediately ran. Sounds familiar.

As far as the taillights, they are very sensitive to the power being generated by the charging system. I was driving my Cougar at night (which is a rarity cuz I'm blind in one eye and I can't see out of the other), and stopped in the left turn lane at a stop light. Car idling, Lou Rawls crooning on my stereo, headlights on, Brake light on and left turn signal on. Then the headlight dim a little, and the left turn signal stops blinking. WTF!! I personally rebuilt my alternator just a year ago so I knew it was fine. One thing I hadn't changed on my Cougar was the voltage regulator. Next day bought the motorcraft solid state voltage regulator, retest under the same condition and life was GOOD!!

Which leaves me to one of my latest Coach Jacks rules of auto wrenching:

When replacing an alternator with an external voltage regulator, always ALWAYS ALWAYS replace the voltage regulator!

Now go out and enjoy your ride!!!

Coach Jack