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Can someone tell me how my MSD 6a was ever working?

D

dodgestang

Active Member
So I drove my car to work....and got stuck there with a car that would crank with no fire (no spark).

I've done all the diagnostics and could reach a conclusion so I decided I would do the normal shotgun.....completely remove the MSD setup and then reinstall it after checking every wire for continuity and hoping that maybe I would find an inline fuse somewhere.

So here is how it was wired:

Primary Red: Positive post of solenoid
Primary black (ground): ground to block where alt is grounded. Confirmed good contact

Switched Red: This is where it gets puzzling....it comes out of the box and gets split into 2 reds....one goes to the electric choke and the other goes to a temp sender? No where in this 'circuit' does this red line every go to a key on source? Was this whole thing working by some lucky back feed or reverse ground issue?

White: taped up tight and not grounded.

Magnetic pickup lines to dizzy: plug in and confirm intact with ohm meter.
 
As answered on that other site.... :roll

Possiable back feed from ist cluster.

Red switch wire from box should have been connected to the old + side coil wire, white wire is for tach lead.

Wire your choke + lead to Sta lug on alt, and make sure ground lead on choke is connected.
 
"Mach1Rider" said:
As answered on that other site.... :roll

Possiable back feed from ist cluster.

Red switch wire from box should have been connected to the old + side coil wire, white wire is for tach lead.

Wire your choke + lead to Sta lug on alt, and make sure ground lead on choke is connected.
Click to expand...

Maybe my old eyes don't work like they used to, but testing the wires with a meter somehow they show a little over 10v with the key on.

I think it is a moot point anyway, I have the box out and attempted to bench test it.

I hooked up the positive and the 12v switch to positive on the battery
I hooked up the negative to the negative on the battery
I hooked up the coil to the wires from the box
I plugged in the magnetic pick up wire
I short the pickup wire....and then unshorted...and then shorted....and I can't get a spark to pop from the coil wire shorted to ground.

It actually does look like the box went bad.....and its never the box. I suppose I will go buy a new one tomorrow, wire it in, and then scratch my head some more when it doesn't start still ;)
 
"Fast68back" said:
Why not pull the box from the 65 and test the 68?
Click to expand...

I learned a long time ago not to scavenge parts from a together, working, running, car to trouble shoot another car....because then you end up with 2 cars that don't run ;)

I ordered a box and it will be here Tuesday.
 
"Mach1Rider" said:
Wire your choke + lead to Sta lug on alt, and make sure ground lead on choke is connected.
Click to expand...
This does not work. The STA lug doesn't get the full 12V that the choke needs to function correctly.
 
I have a 3G alt and my choke (Summit carb) wired to the stator on the alt. It pulls off in less than 2-3 minutes (depends on temp). Works fine - and it's worth noting that Ford wired some models that way from the factory using a 2G alt. So it does work with those - I have no experience with the 1G.
 
"Starfury" said:
This does not work. The STA lug doesn't get the full 12V that the choke needs to function correctly.
Click to expand...


I will have to differ with you on this.
There is 2 of my cars wired this way and they both work perfectly.
While I have tried and used a switched 12v source to the elec choke, it caused the choke coil to open up prematurely creating
a cold stumble when driving till warm up.
Ford wired it this way for a reason.
 
Thats a horse that gets beat every time. Some claim it needs 12V and the other will claim that Ford did it from the factory for years. I tend to wonder if the choke on the OEM Ford cars were made to take less than 12v? Another kicker is the output of the Sta post is in A/C and not D/C.

How was your choke hooked up dodgestang? If hooked to the stator then most likely that is how your MSD box was working and confirms why you were getting 10V.
 
"buening" said:
Thats a horse that gets beat every time. Some claim it needs 12V and the other will claim that Ford did it from the factory for years. I tend to wonder if the choke on the OEM Ford cars were made to take less than 12v? Another kicker is the output of the Sta post is in A/C and not D/C.

How was your choke hooked up dodgestang? If hooked to the stator then most likely that is how your MSD box was working and confirms why you were getting 10V.
Click to expand...

Not hooked to the alt (at least not directly). After more digging poking and proding the power is coming from a red wire with a white stripe that connects into the stock wiring harness with a stock looking plug. I know the old coil wire everyone says is a pink resitor wire but if I was a betting man I would say the power for the MSD box, the choke, and this temp sender were all being piggy backed off the old coil wire. The new MSD box is here and I hope to find sometime to wire it up tomorrow...and then hopefully report back good news.
 
Red/white stripe is the temperature sending unit line. That line would have about 5-6V on it if it came from a stock CVR. If you have aftermarket dash gauges, it would probably have 12V. If it was hooked up as well to the TSU, the colder your engine, the better the MSD would run. With the engine hot, the voltage would drop due to the drop in voltage through the sending unit. This all assumes a gauge system. If it was an idiot light affair, the chances would be good that the MSD would run until you had a big overheating event that would trip the idiot light.
 
Yup, Red/White is temp sending and coil wire from that same harness is red/green. Oil pressure is White/Red
 
"Midlife" said:
Red/white stripe is the temperature sending unit line. That line would have about 5-6V on it if it came from a stock CVR. If you have aftermarket dash gauges, it would probably have 12V. If it was hooked up as well to the TSU, the colder your engine, the better the MSD would run. With the engine hot, the voltage would drop due to the drop in voltage through the sending unit. This all assumes a gauge system. If it was an idiot light affair, the chances would be good that the MSD would run until you had a big overheating event that would trip the idiot light.
Click to expand...

There has been some conjecture that the relay in the MSD may have gone bad and that it might have always been on.
Literally it stopped working after the longest trip the car was ever one and it sat in heavy traffic just moments before I parked it and peaked up to around 210-215 (it had cooled down again 180 prior to parking). It has both the stock temp gauge and an aftermarket triple pack...and both of them 'work'.
 
Now the 50ct question..... Are you going to correct the wiring?

Or wait till the new box goes out . :roul

Its nice to have a working choke and running engine. :wor

I'll just sit by :part and watching while you remove the inst cluster.
 
"buening" said:
Thats a horse that gets beat every time. Some claim it needs 12V and the other will claim that Ford did it from the factory for years. I tend to wonder if the choke on the OEM Ford cars were made to take less than 12v? Another kicker is the output of the Sta post is in A/C and not D/C.
Click to expand...

Electric chokes are operated by a coiled bi-metalic strip... Current passes through the strip and it heats up due to the resistance of the metal. But the two metals used in the strip heat at different rates, which causes the metal to bend and pull off the choke. When the metal cools off, it goes back to its original shape and activates the choke again.

This device is not sensitive to AC vs DC, nor is it particularly sensitive to voltage fluctuations... but given the basic V=IR formula a lower voltage will take longer to heat the strip and pull off the choke. This would be an advantage for using the STA terminal in cold climates, but even in San Diego the STA terminal pulls off the choke in a reasonable amount of time.

This is also why you want the electric choke energized only when the engine is running; otherwise the choke can deactivate before you even start the car...

-Rory
 
I just managed to find 30 minutes in my schedule and ran out to the garage. I hard wired the new MSD box to the positive/negative terminals, shoved the 12v switch wire under the battery cable....and the car roared to life.

So....for once.....it actually was the box that was the problem.
 
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