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Running a couple Mopar 440s in my boat and gots a prob when throttling up one of the twins flattens out. Got back to the barn and started checking voltages and found that the bad twin only had 5v on the coil (+) and the good twin had 7.5v on its coil (+). Running 1977 400s steelies with the OEM electronic ignition. I'd like to rip it all out and run either a good point system or a better electronic ignition setup. Any suggestions? Any ideas why the voltage diff on the two motors? What is the (+) coil voltage supposed to be anyway?
Tanks
 

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Doc here, :pimp:

EXACTLY what ignition system do you have?

For instance a ballast resistor on each coil will drop the voltage to 6 to 9 volts on the + side of the coil when the points are closed, and rise to 12 volts when open.

A GM type HEI< needs a solid 12 Volts DC to run, or it will miss or dropout.

Aftermarkets differ as per their spec sheets.

My first line guess, if your running two coils off the same coil + wire in parallel your probably getting major losses in the wire as heat..(two performance coils can draw as much as 80 amps..) They can also act as a ballast drain to the other coil...If this is the case, separate the power wires run a relay from the battery with the coil control energized by the switch, and run 8 to 10 gauge to each coil.

Doc :pimp:
 

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Thanks for responding. Some clarifications... stock ignition Chrysler 1977 marine. Each engine is independent (one coil per motor). Here are some additional measurements that indicate the motor (ENG#1) that drops out may be seeing too little voltage on the (+) of the coil:
ENG#1 ENG#2
(-) .3v .8v
(+) 5.4v 7.5v

ENG#1 seems to be low. Still can't figure out what the source of the problem is.
 

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Doc here, :pimp:

Disconnect coil 2 and measure coil 1...If it comes way up you are current starving both..Or one is bad..Then do the same on 2..

Doc :pimp:
 

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440x2 said:
Thanks for responding. Some clarifications... stock ignition Chrysler 1977 marine. Each engine is independent (one coil per motor). Here are some additional measurements that indicate the motor (ENG#1) that drops out may be seeing too little voltage on the (+) of the coil:
ENG#1 ENG#2
(-) .3v .8v
(+) 5.4v 7.5v

ENG#1 seems to be low. Still can't figure out what the source of the problem is.
How is your positive run to the coils? What gauge wire?

As Doc elluded to, if you have too small gauge of wire it can act as a resistor and drop your voltage.

Run independent positives of 12 gauge or better and see what happens.
 

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Lonestar said:
How is your positive run to the coils? What gauge wire?

As Doc elluded to, if you have too small gauge of wire it can act as a resistor and drop your voltage.

Run independent positives of 12 gauge or better and see what happens.
Doc here, :pimp:

And...In addition to undersized wire..

IF you run both coils from a "Y" connection from the ignition..When COIL #1 is active (Points closed, firing) It will read a low voltage..about 6 volts Because the points are a route to ground..When open , read about 12 volts...

IN addition to that, COIL 2 will be trying to fire when the points go closed..swamping the circuit..Or back-feeding through it's windings..(in other-words, each coil is acting like a second ballast to the other..)

To run both coils , Which can draw up to 40 amps each, or an additive 80 amps..for performance types, from a single feed point (The IGN wire) is not going to support both coils..you need source current (The Battery) and Isolation ( a Relay for each or a STDP)..

To do that, you need to install your Ignition wire to the coil of the relay, #85, Then The ground to #86 ...so the relay "Clicks each time you turn the key on and off...

Next, run 10 or better 8 gauge wire direct to the battery fuse link or disconnect, for each coil..(or set of contacts depending on what type of relay you use) # 87 on each relay..

Next run a 10 gauge wire to the coil from terminal #30 ...and connect it to the + terminal..



This will deliver max current from the source (Battery) to the device (Coil)...

An alternate method would be upgrade the wire, then install two diodes , one on each coil..BUT..they would have to be BIG (40 amp apiece, with about a 500 PIV) and probably expensive..

If you still have problems, check the coils for servicability..to do that, get out your DVOM, and Measure the primary, Set your scale for OHMS, Rx1 , Calibrated 000, and read the meter..It should be more than 000, but less than an ohm..If out of that..It's bad or going..

Next measure the secondary, Set your scale for R X 10 K or higher..It should read between 10,000 and 11,000 K ohms..more of less toss it..it's bad or going..



Grounding on a Boat is hypercritical too..you may have a bad ground system on one engine , robbing ground from the second engine through common devices (coils being one of them..) Check all your grounds..


Doc :pimp:
 
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