Problems with codes and possibly the computer

[RYC CUKR]

So I am just a little frustrated today.

The 88, which is a MAF converted car, has started to misfire a little when you are just cruising, its fine if you are on the throttle but under light throttle it starts to miss. So I figure that I will dump the codes and see what it tells me. Well it doesn't tell me crap cause it won't dump codes. Plug the tester in and it doesn't do anything. Just sits there with the light on. No blinking no nothin.

I did a search and followed jrichkers advice in another thread. Pulled the battery and tested out the black/white wire on the test connector to ground. Got around .3 ohms. So that looks good. So I pull the computer out and and test the single wire, I believe it is white or tan w/red. and it ohms out fine from the computer plug to the test plug so its good. So I decide to test out the pins on the computer. I get either OL or roughly 3.3Mohms when I read between pin 46 and either pin 40 or 60. I used a nice Fluke 77 digital multimeter to do the testing. So it looks like the computer might be bad. Kinda bites but I have a 93 GT that is just sitting right now so I will just steal the computer out of it. Pull the battery and pull the computer and I go to check it just for grins and guess what. No continuity between pin 46 and pins 40 or 60 either. So now it looks like I might have TWO bad computers.

Just wondering if anyone can confirm if I am right or wrong before I go out and get a replacement. And what could cause them to go bad and what do I need to check before I put another in to keep it from crapping out.

I am just about ready to get the can of gas out and light this sucker on fire.

 

[RYC CUKR]

Alright,

It looks like the A9P out of my 93 is good. It was late last night and I was testing between the wrong pins. So the A9P reads good between 46 and 40 or 60. About .3 ohms or resistance. The dedicated ground wire reads about .2 ohms of resistance when tested from the EEC test connector to the computer connector. I also tested all the other wires, except the fuel pump relay, back to the computer connector and they all test good.

I installed the A9P and it runs better, still has a little bit of a miss in lower RPM but not nearly as bad. It also idles much better. It doesn't sit there and surge like it used to. Whoever did the conversion never bothered to hook up to the VSS or the fuel pump so it has the tendency to want to stall when you so I will have to fix that but that is no big deal.


The only problem that I still have is it won't dump the codes. I tried with my tester and with paperclip and test light. When you jump the single wire to the ground in the test connector you can hear relay click and all but thats it. The test light goes out and thats it. It doesn't blink or anything.

Anyone have any thoughts on this.

 

[jrichker]

Use the test light/check engine light method of dumping the codes.

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link 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.

See http://www.troublecodes.net/Ford/

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.

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

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.

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

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see http://www.midwayautosupply.com/pc-7208-90-equus-digital-ford-code-reader-3145.aspx – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

 

[jrichker]

You may have already seen this, but just in case you haven't, here it is. Note that the 91-93 cars have different wire colors, so that may have some effect on your troubleshooting process.

Computer will not go into diagnostic mode on 91-93 model 5.0 Mustangs

How it is supposed to work:
The grey/red 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 anyplace on the engine harness with the battery
pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector grey/red 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. Only an experienced electronics technician can open
the computer up & repair the trace if it burns up and creates an open circuit.

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
(grey/red 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 grey/red wire and pin 46 on the computer wiring connector : it should be less than
1.5 ohms. More that 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 that that and the computer’s internal
ground has failed, and the computer needs to be replaced.

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 that 1.5 ohms is a
wiring problem

The following is a view from the computer side of the computer connector.

Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the grey/red 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

 

[RYC CUKR]

I tested out all the wiring. Its all good. Everything reads around .2 or .3 ohms or resistance. I tested the original computer in the car and it tested out bad from pin 46 to pin 60 and pin 40 and it tested out with OL or open line so that one is bad. The trace must have gotten burned off the board. So I swapped the ECM from my 93, which I tested before I put it in and it read about .2 ohms between 46 and 40 or 60. So that computer tested out good. But it still won't dump codes.

So the wiring is good, the computer or at least what I have checked is good but it still won't dump codes. I am thinking about trying to get an A9L, since I have to get a new ECM anyway and swapping it in and seeing if that works. Other than that I am completely stumped.

 

[RYC CUKR]

ttt