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Category: Classic Car
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Experience:  ASE Certified Master Tech with Advanced Level (L1) - 20 Years Exp. with multiple makes of vehicle
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1978 ford f150: spark plugs..voltage on coil negitive side..volts

Customer Question

My truck 1978 ford f150 351w .I'm not getting fire to spark plugs. I have a breakless ignition modulator. I tested voltage on coil negitive side read 1.98 volts and on positive side 7.98 volts. change modulator same reading. I removed the grounded side of wiring to modulator read 12v . is this normal. It still don,t start.

thanks for time Jesse
Submitted: 7 years ago.
Category: Classic Car
Expert:  AutoDoc replied 7 years ago.


Check to make sure you have Ignition Voltage at the RED/BLUE wire of the ignition module - if OK, then hook a test lamp with Clamp end to Battery POWER and put the shaft tip on the GROUND side of the coil - while cranking, see if the test lamp 'pulse flashes'. Let me know. Thanks, Dave.

Customer: replied 7 years ago.
does is have voltage on white and red/blue same time
Customer: replied 7 years ago.
I have voltage on red blue when I'm starting. I checked for flashing light goes steady no flash. I hope placed tester right. I put on postive side and shaft on neg side of coil.
Expert:  AutoDoc replied 7 years ago.


Don't put the clamp of the test lamp to the coil positive, put it to the BATTERY positive. Yes, RED/BLUE is CRANKING voltage and the constant voltage comes from the the BLUE wire. Having voltage on both sides of the coil at rest is normal (it is a continuous winding) - when the module picks up a trigger from the pick up magnet inside the distributor, it sends a ground trigger to the coil negative side to fire it - this is why your test lamp should flash when the ground pulse occurs. Make sure you have the voltage at the BLUE wire of the ignition module. I am not showing a WHITE wire at the MODULE - I show the colors as ORANGE/YELLOW, BLACK/GREEN, PURPLE/BLUE, GREEN/YELLOW, RED/BLUE and BLUE to RED/YELLOW. Thanks, Dave.

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Customer: replied 7 years ago.
I did test and test lamp didn,t flash . What I do next
Expert:  AutoDoc replied 7 years ago.
Do you have ignition power on the ignition module BLUE wire?
Customer: replied 7 years ago.
Yes I do,but it is white to ignition switch. The book I have is haynes Ford 1973-1979 automotive repair manual also shows white lead.Could this be the module. I do have 12v on 3 leads to distributor, if that helps.I want know if there is 12v on ignition and cranking lead at the same time.
Expert:  AutoDoc replied 7 years ago.
I just checked my Mitchell - for some reason my 1978 shows BLUE and my 1979 shows WHITE, but we are on the same page - As long as you have continuity on the GREEN/YELLOW wire from the module to the NEGATIVE side of the coil, then you have a trigger problem inside the distributor - either the gear broke and the rotor is not spinning, or the shaft bushing is bad and the armature is making contact with the pickup, or the magnetic pickup itself is faulty. Thanks, Dave.
Customer: replied 7 years ago.
should I read voltage on green/yellow wire when removed off coil and switch on.
Thanks for Patience. Jesse
Expert:  AutoDoc replied 7 years ago.


No, just disconnect it from the ground side of the coil and disconnect the plug at the modulator - then connect an OHM meter to the GREEN/YELLOW at both ends and make sure the circuit has some resistance - it is just to check that the circuit is not OPEN (broken wire). Thanks, Dave.

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