DUI - voltage drop normal

[jahearne]

With the key on, car not running, I get twelve and a half volts to the dizzy. But once the car's running, I get about 7 volts. Is that normal?


Well, I thought I had a problem... a '67 Ranchero that I recently installed my DUI into the stock motor. At first i cheated and powered it directly from the original coil wire and it ran great - no problems.

Later, I add a relay switch, 30 amp from Summit, and I couldn't get to idle or barely start. It would run awesome once I got it started and kept the throttle at least a 1/4 of the way open, but it would barely idle and eventually spit through the carb and die no matter how I adjusted the timing. I messed with it for hours. No luck.

Upgraded to adjustable rockers. Got great static compression numbers. Same problem.

What's going on? Couldn't figure it out. So, back to the original coil wire. I always assumed the original wire was less than 12 volts until I put a volt meter on it. What do you know! 12 1/2 volts. So the heck with it, I'm leaving the original coil wire hooked up to the DUI. Opened the plug gaps to .055 and it runs great. But I was curious if a voltage drop was normal?

 

[fordconvert]

If there is any resistance in the circuit (bad connection, too small of wire, resistor wire?) the voltage drop will change depending on the load.

 

[AzCoupe]

Sounds like you had it wired directly to the alt, since voltage was dependant on engine rpm?

 

[jahearne]

Okay, it's worth investigating further.

I'll run some more tests:

connect directly to the battery and see if there's a voltage drop

check original coil wire for resistance with an ohm meter

look to see where the original coil wire is getting its power from. unless it's connect to an alternator feed under the dash, it's factory wiring. I have wiring manual for '66 Comet\Falcon\Fairlane should be close enough.

I'll probably end up bypassing the original coil wire with the recommended 12 gage no resistors like I did in the last car. I'm just suprised that the relay switch didn't work out.

 

[grocery getter]

did you use the original wire to activate the relay? it likely needs 12 volts. your original wire is likely a resistor wire. which explains the 7 volts under load.

 

[jahearne]

Ya, I did. But you don't need 12 volts to activate a relay, right? and it should make a diffence to the relay if the voltage drops from what I understand...

So that explains the drop in voltage:
resistor with no load = 12 volts
add a load = seven volts

 

[grocery getter]

Ya, I did. But you don't need 12 volts to activate a relay, right? and it should make a diffence to the relay if the voltage drops from what I understand...

So that explains the drop in voltage:
resistor with no load = 12 volts
add a load = seven volts

the relay has a minimum pull in voltage. usually over 8 volts.

 

[jahearne]

I hooked it up directly to the battery and the same deal as when hooked up to the relay switch: it would start/run if I kept at quarter throttle or more, but as soon as I let it idle it would die and/or cough through the carb. Hooked DUI back up to the original coil wire coming from out of firewall/under dash.

I'll test the electonics: http://www.performancedistributors.com/technical.htm
But it fires and runs good with no excessive wear besides I got other issues like leaky accelerator pump and melted vacuum tee.

BTW, look out for bad gas... at the pumps not beer tavern. I filled up with budget Olympian gas and could barely make it up the Waldo Grade it was pinging so bad. Poured half a can 104+ and runs good agian.