|The MGA With An Attitude
IGNITION TROUBLE SHOOTING, Diagnostic Tree - IG-106A
On 3/20/2010, W. David Houser wrote:
Car ran fine coming into garage and then it didn't. Replaced coil, no change. .... switch on, fuel pump works, lights, getting gas...no spark out of coil".
This is a more direct version of the prior page, just follow the directions.
Always do the diagnostic work before throwing money at it.
Basic stuff first, start here:
Spark and fuel circuits: ET-101D (click for diagram)
Grab test light. Switch on.
. . A3 = light --- If not, fix the feed wire.
. . SW = light --- If not, fix the feed wire.
. . CB = no light when points closed --- If light, no ground through distributor.
. . . . Broken flex wire (2 of them), or loose base clamp.
. . CB = light when points are open --- If not, shorting across points in distributor.
. . . . Common wiring error, points wire shorted to ground on screw post.
. . Points of course must open and close.
Use ohm meter.
Check resistance in coil primary winding (SW to CB, or + to -).
. . 2.7 to 3.3 ohms = good
. . 1.6 ohms = wrong coil
. . 50 mega ohms (open circuit) = bad coil.
Check resistance in coil secondary winding (HT terminal to one side terminal).
. . 7,500 to 10,000 ohms = good
. . 50 mega ohms (open circuit) = bad coil (but it might still run).
Check resistance, distributor side terminal to engine block with points closed.
. . 50 mega ohms = points open or wire disconnected.
. . Near 0 ohms is good.
. . Greater than 1/4 ohm (250 milli-ohm or more) = burned points.
If that checks out, then check for spark at coil HT wire (not plug wires).
Very weak spark from coil = bad condenser
No spark out of coil = bad coil
Spark from coil but no spark at any plugs = bad rotor or broken center contact in cap.
Spark on some plugs, not others = fault of cap or HT wires or plugs.
. . Check for spark on each HT wire to engine bolt (not through plugs).
. . Some wires no spark, check resistance of wires end to end.
. . . . 0 ohms = solid wire, good
. . . . ~5000 ohms per foot of length = resistor wire, good
. . . . ~50 mega ohms (open circuit) = bad wire or bad end connectors.
. . Check resistance of plugs from top connector to center electrode.
. . . . 0 ohms = solid plug, good
. . . . ~3000 ohms, resistor plug, good
. . . . 6,000 ohms or more = resistor plug, faulty
If that's okay, then spark on some plugs but not others = bad cap.
. . Check resistance of each post inside to outside.
. . . . 0 ohms = good post.
. . . . 50 mega ohms = corroded aluminum post
. . . . . . Replace cap. Try to buy one with copper posts.
. . If posts are good, visual check inside of cap
. . . . Crack or tracking inside of cap = bad cap.
If you haven't got it yet, read these pages:
Ignition Coil Testing: IG-108
Ignition Trouble Shooting: IG-106