Electrical 89 fox 5.0 crank no start when cold

First thing to verify is that you are using the right small wire next to the " dog house " connector. The proper fuel test connector and the hood light connector look almost exactly the same. The fuel pump is a ground and the hood light is constant 12v. If you ground the fuel pump test connector, with the key on, the fuel pump should come on.

Edit; and what the Thief said. ECU Exchange is a good idea. There's a jumper that could be wrong depending on what harness you got off ebay. It also could hurt the computer. I'd pull the computer out and open it up to check it for burnt traces or leaking caps 9/10 the caps are bad.

If it sat for 10 years do your self a favor and order a new tank, fuel pump, sending unit, filler neck gasket and fuel filter, any car that sat that long that I've worked had a badly rusted tank. You can get it all for about $250 with a decent 255 or 320lph pump if you shop around. Won't take much to kill a new pump if you just swap one in..... Blow the lines out from the fuel rail side .

if you can't pull codes its either wiring or the ecu, need to remove the board from the case and check both sides. Best bet is to show pictures of the harnes you installed, there pretty easy to identify by the body plugs connectors at the driver side firewall area. Have you checked the timing with it running? without the spout and with the spout plugged in?

The spout is the little 2 prong plug right by the distributor module just follow the harness you will see it. you remove it to set base timing at 10*, when you plug it back in the computer now controls the timing and should raise it past 10. If you swapped the distributor and just tried to line it up and didn't use a timing light its a good chance the timings not correct. The fuel filter is in front of the tank/ spare tire well sheet metal under the car

That computer needs to be rebuilt. The blue capacitor is leaking all over the board.

There's another single wire little plug around the dog house connector area. Usually there are 2 single wire connectors...one is gray and the other is black ? Or brown ?

The a9p is the correct computer. I didn't see anything else but that capacitor on the computer board.

The traces that most often short are on the other side of the board, need to remove it from the case to inspect them or can test with a dvm.

I'm guessing that's an 87-88 harness converted to mass air based on the test port only time I see the screwed up test port wiring is on pre 90 cars with harness swaps or mass air conversions... Your sti is the tan wire with the red strip, 4th spot on bottom of the connector, It should lead to pin 48, the white wire with the red strip should be sto but is usally black/ white. should lead to pin 46. i would probe those and verify the pin out with a dvm, check both for voltages then try to pull codes with the test lamp method and those jumped.... This why I hate dealing with mixed matched wiring, I've dealt with too many problem cars over the years with mixed and matched stuff lol.

And the two connectors in the last picture are for the wiper motor..

@DruryFoxbody

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Foxbody Diagnostic connector

Foxbody Diagnostic connector close up view



If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10




Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Reader/dp/B000EW0KHW Equus - Digital Ford Code Reader 3145.
Or for a nicer scanner see www.midwayautosupply.com/Equus-Digital-Ford-Code-It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.

Order it at Walmart for a better price and free shipping






Computer will not go into diagnostic mode on 86-90 models 5.0 Mustangs .

Revised 23 May 2020 to include warning about jumpering the dark brown connector with a black/orange wire.(12 volts) to the test connector.

Disconnect the battery positive terminal before making any resistance checks.
The voltage drop in the ground cable will cause incorrect resistance readings.

How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. Since it is a dedicated ground, it passes through the computer on its way to the computer main power ground that terminates at the battery pigtail ground. It should read less than 1.5 ohms when measured from any signal ground on the engine mounted fuel injector harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

Engine mounted fuel injector wiring harness sensors for a 5.0 mustang


What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

OR

If an O2 sensor harness from an automatic transmission Mustang is used with an A9L manual shift transmission computer. The 12 volts from the automatic transmission starter circuit will damage the A9L computer.

The STI (Self Test Input) is jumpered to ground to put the computer into test mode. Jumpering it to power can produce unknown results, including damage to the computer. The ohm test simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring:
With the power off, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms.



If that check fails, remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46 on the computer wiring connector: it should be less than 1.5 ohms. More than 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

While you have the computer connector disconnected from the computer, turn the ignition switch to the Start position and look for 12 volts on pin 46 of the computer wiring harness. If you see 12 volts then you have an automatic transmission O2 sensor harness. That will damage the A9L manual shift transmission computer.

See the pictures below for help finding and fixing the burnt computer trace.




The fix is some careful soldering of a small jumper wire across the burnt section of copper trace.



If the first ground check was good, there are other wires to check. Measure the resistance between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms. More than 1.5 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.




Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every sensor on the engine except the MAF is connected to it.



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine

See the graphic for the 10 pin connector circuit layout.