Engine won't fire when tach wire hooked to coil

[usraptor]

I asked this question on C.C.com and the only answer they came up with was a grounded wire.  We've checked that and it does  not seem to be the problem so I thought I'd try here to see if anybody has any other ideas/suggestions.  1970 Cuda with 440, 4-speed and rallye gauges.  When we installed the rebuilt engine I converted to an electronic ignition.  Summit racing electronic distributor, coil and ECU.  This is the second dist. setup as I returned the first one thinking it was defective.  Had the same issue with this set up. When the gray tach wire is connected to the negative side of the coil, the engine will not fire. Doesn't cough or anything, just turns over.  When you remove the gray tach wire from the coil the engine fires immediately.  My son and I ran an OHM test ( I think that's what you call it-he's the mechanic) from the bulkhead connector to the connection that connects to the coil and it was okay.  We then connected the engine side bulkhead connector and then tested the tach wire from the coil connector to the rubber 90 degree connector that goes to the back of the Rallye tach itself and again it tested good. When the engine is running the temp, oil pressure, gas gauge and clock work correctly.  When I bought the car the engine was out of the car so I don't know if the tach worked before.  However, a few years ago my son and I ran a bench test on the gauges with a 6 volt battery (similar to Cody's video) and all the gauges, including the tach,  started to sweep/move when hooked to the battery.  I've read that the original rallye tach will not read correctly with an electronic ignition and either it needs to be transistorized ($300) or I need to buy some kind of in line adapter (approx $100) from either Mallory or Mopar Performance for the tach to read correctly.  However, I haven't read or found anything about the engine not firing with the tach wire hooked up, only that the tach won't be accurate.  Unless somebody has another idea, my next step to make a temporary wire (bypassing the original tach wiring) and run it from the neg side of the coil directly to the back of the tach and see what happens.  Any other suggestions/ideas would be appreciated.

[Cuda Cody]

I could get up and go out in the garage to double check to be sure but I think there is a cardboard washer on the back of some of the connections on the dash to stop them from grounding out.  Not sure if the gray wire you speak of needs a cardboard / paper washer, but I can check it you want?  Might be the bad ground you're looking for?   

[Cuda Cody]

@usraptor  is the gray w/ red stripe wire the one you talking about?

[anlauto]

I had a case where the grey tach wire got pinched between the dash and steering column mount. grounded out and caused problems

[usraptor]

@usraptor  is the gray w/ red stripe wire the one you talking about?

No, the one in your picture that has the rubber sleeve and is bent at 90 degrees.  If I remember correctly it pushes onto a post at the back of the tach.
On the engine side there is a gray wire that comes from the bulkhead to the coil.  I don't remember a red stripe on that wire but it may have faded.  I'm not sure if it's the original wire, it may have been changed, but I can't really tell as it is weathered.  But I do know that it has a newer, non-factory,  connector on it where it connects to the neg side of the coil.  Don't really remember what color the wire is inside the car that runs from the bulkhead to the tach. 

Cody, I don't remember seeing any cardboard washers.   Where would they be?  If I had a bad ground wouldn't that effect the rest of the gauges?  Also, don't know if it makes a difference but this is a rallye dash, not a standard dash.

Alan, thanks, I'll check for that.

[Cuda Cody]

It sounds like something is grounded that should be.  Double check to make sure you have the little washers on the back of the gauge cluster so they don't make a ground.  You can also check and see if that the blue wire is grounded by doing an ohm test.

Your coil wires look like this right?  The blue to the POS and the black & gray to the NEG?

[screamindriver]

 The tach itself could be a problem if you're sure the coil trigger wire isn't grounded...What were the ohm readings when you were testing ??? And no, if the tach itself is sending the coil trigger wire directly to ground it won't affect the other gauges...

[usraptor]

Yes Cody, that is what my coil looks like.  I'll check the back of my gauges for those washers.  It's been a couple months since we did the test so I honestly don't remember if they were there.

Scremindriver, at my age you expect me to remember what the ohm readings were?  I can't even remember what I had for breakfast.  No, really we did the test a couple of months ago so I don't remember but will recheck. 

Just had a thought.  We didn't do an ohm test on the tach itself.  Would that tell me if the tach is bad?  As you not doubt figured out by now, the electrical end of this is not my strong suit.

Thanks for the suggestions.  Soon as I get the engine re-installed (it's out for a header swap- hopefully next week if my transmission stamps ever get here) I'll start checking all this stuff out again and report back. 

[Cuda Cody]

Lets us know what you find on the back of the gauges. 

A thought for a test.  Take the blue wire of the back of the gauges (the one with an 90 degree angle at the end) and do an ohm test to the dash frame or the back of the cluster.  If you get an reading at all that would tell you that the wire is grounded.  And I don't think it should be grounded.   

We'll help you get this solved!!!   

[usraptor]

Thanks Cody.   

[70chall440]

Lets us know what you find on the back of the gauges. 

A thought for a test.  Take the blue wire of the back of the gauges (the one with an 90 degree angle at the end) and do an ohm test to the dash frame or the back of the cluster.  If you get an reading at all that would tell you that the wire is grounded.  And I don't think it should be grounded.   

We'll help you get this solved!!!   

should not be grounded.

[usraptor]

Okay finally got the car down from the lift and checked the wiring. First the ohm report.  From the coil connection to the firewall bulkhead connection plug I have .5 ohms. Inside the car, from the bulkhead connection to the other end of the red and white wire I have .7 to .8 ohms. 

Also looked at the back of the wire connections on the tach and the blue wire connection has a cardboard type gasket but the red and white wire does not.  Does anybody know where I can buy one of these or have suggestions on how to fab. one?

I disconnected the blue wire and tested it against the dash frame and back of the tach gauge housing and got a reading of 4.25 ohms.  So I guess that means I have a short in the blue wire?  I'll look in the service manual but can anybody tell me where that blue wire originates from so I can trace it?

[Cuda Cody]

I just tested my blue wire on the back of a rallye dash (tach) and with it set to 200 ohms I get 25.9 when I ground to the dash frame.  So maybe yours is fine?   

Can you post a photo of the back of you cluster?  And when we say the red wire, I thought the one next to the blue was grey?  I'll get you a wiring diagram.

[usraptor]

Dash is in the car now so I can't get a picture.  I have some on my old computer, I'll see if I can find them. 

Re the wire color. the wire from the coil connection to the engine compartment bulkhead connector is gray.  Inside the car the wire from the bulkhead connector to the tach (bayonet connection) is white with a red tracer. 

[usraptor]

Okay, here's some pics of the back of my dash wiring at the tach before it was installed in the car.  The pictures make it look like there is a cardboard washer on the white/red tracer wire but it's not there now??  Do you think that is my problem?