Why would I only be getting 11 volts to the positive side of ballast when the key is on instead of 12? Ballast is bypassed to run MSD. Won't start or fire even when cranking. Started up and ran fine last month.

Bulk head connection? Ign switch?

Thanks

What you should be seeing is the same voltage as at the battery positive terminal, with no more than a .1-.2v loss.

Seeing 11v, without knowing what the battery is at tells us very little.

If you have a drop greater than recommended then odds are you have dirty connections (bulkhead, ignition switch connector, ballast connector, etc) or the ignition switch has dirty internal contacts. Don't forget the negative connections as well.

If your battery is low, charge it up.

Sniper is correct..
Voltage from the ignition switch(start) to the input side of the ballast resistor should be the same as the battery voltage..

Just my $0.02...

I'd check the bulkhead connector and clean that up, and also the ignition switch wiring harness connector to make sure that's tight and clean as well.

Verify the voltage at the battery and go from there. Let us know what you find

I might say .5V worst case due to all the flipping connections, age of the harness etc. Also be aware that the same V is what the regulator uses to determine the charging rate. An excessive V drop will likely lead to overcharging as well . Do not overlook your battery connections and grounds
keep us posted.

Sorry I had 12.5 at the battery so I thought it was odd. I threw a battery charger on it just in case and see what happens in a day or so.

A volt and a half is a lot of drop. I'd be cleaning up the connections and retesting.

battery voltage chart i use.

One other item. Battery temoerature. It makes a difference.
Doug

Does it have an amp gauge? I had an issue with that a couple of years ago. My 77 D100 became very hard to start. I bypassed the amp gauge and it fired right up. Just a thought.

the Amp guauge connections are notorious for coming loose

I had a few minutes yesterday so I thought I'd check the battery terminal on the fuse box first to maybe rule out the ignition switch and it too only had 11 volts. After pushing on the bulkhead connection it got battery voltage but the ignition was still 11.

But still the car started right up and charging and voltage readings were all good. But when I turned off the car I still only had 11 to ballast. It has full voltage when it's running and starts back up fine?

So it looks most likely to be the bulkhead connection. I'll have to work on it later when I get some time. No rush even thou it was 80 yesterday it's still winter and left over salt on all the roads. The wind chill now is -3. Lol

Also does the fuse box itself just clip on the bulkhead block?

I believe it has an amp gauge. It's an ebody rallye dash?

Thanks again

I am not overly familiar with the 70? harnesses but I believe the fuse box and dash harness are one unit that is also the female side of the bulkhead connector. The engine side (ignition and charging) male connectors plug directly into the fuse box I believe.
The clue is in the bolded part of your statement. You have to find that V drop before you wind up with an electrical fire.

I would encourage checking the Alternator battery stud V and comparing it to the battery V with the engine at ~ 2000- 2500 RPM. I suspect you're going to find the alternator output is much higher than the battery V. The ignition side of the ballast is normally used as the "V sense" source for the V regulator.
if it is only seeing the same 11V it will crank up the alternator output to try and compensate. This usually also shows a constant charge condition on the Ammeter.
I am not 100% sure what year make model you're dealing with which hampers being 100% on this.
That 1.5V drop needs to be found and corrected. keep us posted

Tjp 70 Cuda here's a couple of pictures I've found online. The fuse box and bulkhead appear not to connect.

The fuse box looks to be feed from 2 wires coming from separate harness that plug in to lower terminals.


The for sale harness lets you hover over it to see it more clearly.

https://www.partshound.net/product/...y-oe-dash-harness-wcorrect-20-tach-wire/


Back of fuse box

https://timschallenger.weebly.com/blog/back-side-of-fuse-block-after-re-work

As mentioned, not overly familiar with them ( Been a few years )
I would check the alternator output VS. Battery as mentioned and start working your way inside until you find the culprit(s). might start with the Ammeter terminals as they are known to loosen over time. If no luck there, then start at tthe battery and check each connection along the circuit. keep us posted

one other item i find as problematic, are those "bolt on" replacement battery terminals.
if you have those, PLEASE do yourself a HUGE favor and get replacement cables, or at the very least, a pair of quality crimp/solder terminal ends !
this type of battery cable ends are a giant peeve of mine..........
they are also a problem waiting to surface.
nothing makes me question a vehicle's worthiness more than THOSE things !

Always RED FLAG #1

I'm good on #1 I have crimped cables.