87 Mustang w/ 89 underdash harness: No spark! Help? - Ford Mustang Forum
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post #1 of 3 (permalink) Old 08-19-2012 Thread Starter
Buckallred's Avatar
Joined: Jan 2012
Location: Sacramento
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87 Mustang w/ 89 underdash harness: No spark! Help?

Problem: I bought an 87 Mustang that some [dipshit tweaker] tried to convert from stock speed density to a MAF system. They did it wrong, way wrong, it ran like ****. After ******* around with it for way too long, I want to convert the car back to its factory (non-MAF setting) so I can actually drive the ******* thing.

The tweaker installed a new ECU (A9L) and hacked into the wiring harness in several very ****** ways, thus rendering the car almost undriveable.

I obtained new wiring harnesses: engine harness and underdash harness. I BELIEVE the new harnesses I got were from an 89. I installed them both.

The engine harness appears to be same as the stock (1987) one. The underdash harness has several additional plugs, at least some of which I believe are for an airbag (that my 87 does not have). There's obviously nothing to plug them in to.

After installing both harnesses, I have power to the fuel pump (and there is good fuel pump pressure), and I have power to the distributor and power to the coil. BUT, NO SPARK!

Question: why would the 89 harness allow power to the distributor and coil, but not to the actual sparkplug? Is there some aspect of the 89 harness (or the A9L ECU computer) that would not allow the spark to get all the way to the plug? Since the car ran before with the A9L computer, I don't see why it would not allow a spark now. It must be something with the 89 harness, right?

My only idea at this point is to get a correct 87 harness and install it. Then (assuming the car will run), I will also need to get a non-MAF ECU computer, in other words a stock 87 ECU computer.

As always, all input and advice is greatly appreciated.
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post #2 of 3 (permalink) Old 08-19-2012
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Location: Sudbury
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I'm not sure about the harness aspect, but if all the wiring was changed for the MAF sensor, then you'll need to change it all back for the SD ECU to work, at which point it would be better off to just correct the wiring and use the current MAF system.

If your TFI is getting powered up though, you the problem will be from the coil on out. To my knowledge the harnesses should be the same for engine wiring. It could be something as simple as forgetting a ground wire, but I would check for TFI function at the coil.

1986 Mustang GT T-Top, SCAT 331 stroker w/Probe SRS pistons, TFS 170 heads, TFS track heat intake, 30 lbs. injectors and matching Pro-Flow MAF, BBK 75mm TB, T-5 to AOD conversion, MAF conversion, full length subframes, Bassani ceramic coated 1 5/8 shorty headers, BBK high flow cat x-pipe, Bassani quiet thunder 2 1/2" catback, 3.73 gearing.
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post #3 of 3 (permalink) Old 08-19-2012
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If the PIP sensor inside the distributor is bad, you will not get spark. Spark starts at the distributor via the PIP sensor. Goes through the TFI to the ECM via the PIP terminal, back to the TFI via the SPOUT wire, then out of the TFI to the coil via the IDM wire. Then from coil the distributor cap.

That being said, make sure you have 12v while cranking on the 3 and 4 wire from the top on the TFI, is so. Check for a PIP signal on the top wire on the TFI with a volt meter, while cranking the engine over. if you dont have a pulse, chances are the PIP sensor is bad.

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