Okay so this one has got me working, 1990 Mercury, 60,000 miles in well services and almost new condition. The car just quit on the driveway a couple of days ago, though it was very hard to start a couple of days earlier.
The story so far, new fuel filter and pump delivers 39 psi at the Schrader check point, pressure holds good ? 1psi drop in 1 hour. Pump runs on key on, when engine cranking and when grounded through OBD test point.
Good ignition spark at plugs, plugs are dry !!!
Constant 12v at the injector. The heavens opened up with the remnants of hurricane Bill before I could check the injector ground trigger signal. I suspect there will be no signal when I do get to test it.
I have the factory service manual which is as much use as a $9 dollar note for this problem due to lack of circuit schematics, I believe it was written by Ford?s chief editor of obfustication.
Before I ?discover? a zero injector trigger signal tomorrow, does anyone know any good reasons why this signal would be absent ?
My strategy at the moment is to unplug the EEC and having checked the connections replug it up, retry start. Failing that I?ll just disconnect sensors until I get a start attempt (at the moment it?s not even trying).
The injectors are a total bear to get at without removing the plenum, I?m aware that 1 bad injector can shut down the driver and therefore I may need to disconnect all of them to return the trigger signal ? A total PIA.
All suggestions beyond ?Cliff or burn it? gratefully accepted. Any Ford EFI specialists out there ?
Cam shaft position sensor?
Would be favorite as would a crank sensor but the factory manual makes no mention of either. If they exist, do you have any idea where they are located ?
At present I’m thinking the EEC gets a pulse from the coil tach output connector, couldn’t test that either due to the weather, I’m in MA in and poured down this afternoon.
I’m pretty sure there was one on my 91, but I don’t have it or the manual any more. I would think it would be on the front of the engine, if you have one.
Does the engine computer respond to the self-test?
I’ll dig around and see if I can find one but the manual makes no mention of one but as a factory manual it is best described as derisory.
OBD KOEO test returns no codes, but doesn’t give a success code either 
I’ll continue probing tomorrow.
Your car is one of those TFI-IV models that were prone to ignition module failures so maybe that is the cause.
The symptoms can vary wildly and depending on the fault, a test may show nothing as some faults are heat related. In some cases heat may cause them to quit after running a while and in other cases the heat may damage part of the internal circuitry and cause a problem not related to heat.
Another potential cause is the ignition switch. Make sure battery voltage is present at the ignition coil when the key is in both the RUN and the START positions. That should be a red wire with a green tracer.
Have you pulled the codes yet? If the ECM is not receiving a PIP signal from the module a code should be present for that.
Hope some of that helps.
Thanks OK, I did pick up the TFI problem yesterday during some extensive Googling and though I do have ignition spark at the plugs it’s possible the TFI isn’t squawking a PIP signal to the ECM.
I did find a very useful write-up and test procedure here if anyone else runs into this problem : http://sbftech.com/index.php/topic,588.0.html.
I’ll write back when I’ve nailed the problem.
When cranking the engine over does it start and die when the key is released, cough and sputter at all, etc.?
What I’m getting at is that the make and break grounds for the fuel injectors are provided through the ECM and the PIP signal is what is used to determine this.
You might try using a stethoscope, long handled screwdriver, or a NOID light to determine if the injectors are clicking as the engine is cranked over.
If the problem is a start or cough and die immediately when the key is released then this could be related to the ign. switch.
Many Fords have current routed directly through the switch to power the blower motor. This is why that over time an aged/dragging blower motor (more current means more heat) can cause the switch to overheat and fail.
The same thing applies to the Honda ignition switch/main relay problems although in their case it can be caused by a dragging fuel pump or even a partially clogged fuel filter.
Thanks for the input, this is driving me nuts.
The TFI resistances were out of tolerance and it was pushing only 2.5VDC, since it’s cheap I replaced it anyway and the PIP signal is now 4 volts as specified.
BUT - still no start, I have 12VDC at the injectors but I’m only getting 2.5 VDC across the injector plug during cranking - this looks to be either a bad EEC or a bad ground and is probably not enough to trip the injectors. I’m just going to recheck EEC ground resistance, if all is well I think a new EEC will be needed.
And as for the TFI, I thought only Land Rover were stupid enough to park such a heat sensitive component in the hottest part of the engine, I want to meet these people one night, after a really hard day, preferably I’ll be drunk and beligerant…words will be exchanged and that may not be all.