No Start Weak Spark - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

Aussiblue · 08-01-2004, 08:22

The NB (3.9 MPEFI) wouldn't start Thursday morning and my wife called the RAC who said their was no spark and it was the TFI module.

Well an inline spark tester (globe thingo that fits between plug and HT lead) shows that that there is a spark albeit a very weak one that can't jump even a small plug gap. I have tested both the TFI module and coil and found:

a) TFI module is OK although just in case I replaced it with a spare I had (from last time the RAC said I had a dud one) with no change. The resistances at each point on the old TFI module once removed were also to specs.
b) The coil itself test OK at the coil and the HT lead from the coil is OK but if I try and test the primary resistance at the TFI connector I get 1.7 K ohms instead of the .4 to .5 ohms the Ford Factory Manual says says. The wire between the coil and pin 4 of the TFI connector is also 1.7 K ohms so it seems there is a high resistance in this circuit which leaves me to suspect a fault in the harness, ECU or ignition switch.
b) The KOEO self diagnostics yields no fault codes.
c) The battery is fully charged.
d) A noide light placed in the low tension coil plug glows brightly when I try and start so it seem to be getting power OK.
e) The fuel pump and relay is working. The ECU relay also clicks so I assume it's working.
f) The injectors are getting power pulses (noide lights flash).
g) The small pink wire from the TFI with the white connector that sits on the ISC solenoid is not broken.

I have done all the electronic tests at the TFI and coil per both Gregory's and Ford factory manual and other than the high resistance in the primary circuit to the TFI connector and very weak spark everything checks out OK. I have't been able to the checks at the ECU and ignition key yet because it's raining and while my car is in the shed I can't open both doors to get at either. So while I am waiting for some fine weather I thought I would ask if anyone else had any ideas of the liklely cause.

Aussiblue · 08-04-2004, 06:55

I must stop working on my cars when I have had little sleep. It has dawned on me that despite the fact that the coil resistances check out OK it must be a dud if a noide light connected to the primary plug flashes brightly but there is no spark from the HT coil lead. I will buy a coil tomorrow and take the opportunity to reposition it somewhere where I don't need a trained octopus to reach it.

AusXH · 08-04-2004, 07:35

Its almost a two man job to make it easy. One person holding the coil from underneith whilst the other using 3/8" sockets and extensions (they fit between the runners) simply undoes/does up the bolts. Its certainly not the best job to have to do.

Good luck and I hope it solves your problem.