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Thread: OBD EEC-IV test not working or reading codes

  1. #1
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    OBD EEC-IV test not working or reading codes

    Good afternoon, first post here! Grandfather passed away long time ago and left a 1993 Tbird SC it doesn't have more than 30K miles. It was used for a couple of years and fuel pump gave out and was left standing on the driveway for about 3 years. Last week I started to work on the car to revive it, changed the fuel pump, new fuel filter, pumped out old gas and added fresh gas. Now the problem (a couple) is the car only starts with starting fluid, once it starts it keeps running (although not very good and with lots of black smoke coming from the exhaust), going to go get the injectors cleaned next week to see if that solves the issue also waiting on a fuel pressure tester to arrive to see how is the pressure. Also when key is put on ignition the radiator fan turns on immediately and never shuts off, I tried getting codes using the OBD scanner but neither the scanner plugged to the EEC-IV harness works or using the paperclip method produces codes (it doesn't even go to test mode I also tried connecting tester ground directly to battery and nothing). I'm really lost on what else I can try as I cant get any error codes to diagnose the problems, would a bad relay or bad ECM cause this issue? Any help would be appreciated! - Thank you!

  2. #2
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    The most common issue in that example, where it didn't have the additional issues when parked and you can't talk to the EEC now might be power and grounds. Confirm the battery is putting out at least 11.7 volts key off and the alternator is charging just above/at least 14v when running.

    There are two fuses related to the EEC. A 15 amp #6 under the dash, and a fusible link 30 amp G inside the power control box next to the battery...the fuse/breaker map is inside each lid.

    Go over the grounds you can reach, starting at the large black wire that bolts to the header right in front of the battery (I think this is another EEC ground). Then check/replace the ground that runs from the pass. side motor mount over to the body, being sure to remove any green corrosion at the mount points.

    The ground at the EEC is the center bolt, and if corrosion found it's way into that part of the harness, there may be issues on the computer itself.

    Fan run with key on...engine running or just key on? Might be just a symptom and fixed just by getting the EEC to talk to you. Might be a misbehaving IRCM.

    As for the injectors, I'd expect them to have plugged up sitting for that amount of time, with output being compromised to one degree or another. You might need to pull them and have them checked/serviced/replaced. Again, tho, might be fixed just by getting the EEC to talk to you.

  3. #3
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    Battery and fuses are good, ill go over the grounds and see if I see anything, don't think it has corrosion most of the car is rust free. Fan runs when key is on and when running.

  4. #4
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    Key on fan running in my experience means the EEC is bad or not sending a signal. Same thing happens when an aftermarket performance chip gives out.

    EEC is located behind the foot kick panel on the passenger side. It's kind of tucked up into the body and roughly the same height as the middle of the dash.

    -Tim
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  5. #5
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    Quote Originally Posted by marcosrodoca View Post
    ...don't think it has corrosion most of the car is rust free.
    These cars live/die on their electrics and suffer when not driven. Circuits can be compromised without the car rusting Corrosion can hide not just inside a connector, but also inside wire/cable sheathes, so just keep your eyes open.

    If you feel like digging, free up the power control box next to the battery and look at the wires underneath to see if any there are corroded or show signs of overheating, etc.

    One other thing...the multi-wire ignition switch down on the side of the column can cause a multitude of issues, so put it on your list to inspect. Work the key and see if it shows signs of coming open/apart.

    When inspecting harness connectors, it might be helpful to use some electrical contact spray on the metal bits and a bit of connector grease on their gaskets before reassembling, since you're already in there. You can apply dielectric grease to any large bolted/clamped connections but it's not recommended on smaller light clamping force push on connectors. You may have already found out, but the connectors once aged are brittle and the retainers/keepers can break easily, so be patient when pulling them apart.

    So far it sounds like you'll be getting up close/personal w/the ECU, search here for tips on getting to it/getting it out, or yell if you need details. Good news is you can still find a replacement, if it comes to that.

  6. #6
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    Quote Originally Posted by Tim Groth View Post
    Key on fan running in my experience means the EEC is bad or not sending a signal.

    -Tim
    Would the A/C switch being left ON also cause the fan to stay ON?
    Check out my 1993 White Auto SC page for video on how to ..
    1990 35th anniversary SC restoration project
    1990 Blue Auto SC

  7. #7
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    Thanks for all the tips guys look forward to start troubleshooting, I gave a 5 min glance when I came back from work and saw a cable that seems to have burned the cable jacket and was corroded didn't really have much daylight to keep inspecting, will post back with pics when i get the chance. Any way to troubleshoot the ECU (other than buying a new one and swapping) to see if its bad? Anyway thanks again for the help guys, will keep you posted!

  8. #8
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    Quote Originally Posted by marcosrodoca View Post
    Any way to troubleshoot the ECU (other than buying a new one and swapping) to see if its bad?
    Try this: http://mnharms.com/tutorials/disnostart/ - you'll need an LED test light (computer safe). Harbor Freight sells one for $10 I like, but I had to replace it's clips with something more reliable.

    There are methods in the factory manuals, but they usually require a breakout box to follow their pinpoint tests, etc.

    I usually just do a swap with a known good spare, after making a good faith effort to confirm that there aren't any conditions that might risk frying the replacement itself. Keep in mind that you might be able to source a used ECU here, or, refurbs can be had for not too much $$. Hopefully you just need to rehab a harness or two.

    This is a good time to mention the value of factory manuals, which are still available, often for fair prices. Most electrical work requires at least an EVTM for your year SC. A matching Engine/Emissions Diagnostics Shop Manual, and the 4" thick factory Shop Manual for all the other systems, etc. eBay is a good source for them.

  9. #9
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    Ill try to troubleshoot using those steps thanks! I ordered the service manual, EVTM and Powertrain/ Emmisions diagnosis manual from eBay, found them pretty cheap so decided to buy them all. Here is a picture of the burned cable i found the other day, dont really know what it does but ill try to follow it to replace it. I did verify some grounds and sure enough they do have a bit of corrosion so ill be cleaning those before I start throwing parts at the car.Name:  20190225_154743.jpg
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  10. #10
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    That is a specially shielded ignition ground back to the computer from the crank sensor. The shielding helps reduce EMI and aggravated signals, etc.

    See the image w/numbered tags here:
    http://www.sccoa.com/forums/showthre...05#post1059505

    The munged section you show that is part of the main harness between the firewall and the top of the engine, separate from but connecting to the upper/front harness referred to in that linked post.

    Once those manuals are in hand you'll be able to learn more, of course.

    Wow...the green on that ganged ground is impressive Note it has a twin on the other side of the radiator. That pair of ganged grounds BTW, includes those used for the IRCM, fan, ABS pump motor, headlights and wipers.

    Take that as a hint of what exists on several other locales, sadly. Did you eyeball the ground where it's bolted to the body from the pass. side motor mount yet?
    Last edited by KMT; 02-25-2019 at 06:23 PM.

  11. #11
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    Was able to verify the grounds, both of the grounds by the radiator are corroded, still haven't checked the grounds by the motor mount, the grounds inside the car by the PCM look good as new, ill start by cleaning the grounds by the radiator today and see if that fixes anything. If I can at least get the scanner to communicate and get some codes ill be happy.

  12. #12
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    Quick troubleshooting tip - it can sometimes help to install temporary grounds to help isolate various issues.

  13. #13
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    Went over a couple of grounds, they arent really as bad as they look, once i took them apart they looked pretty good, gave them a quick scrub and put them back. Took the ECU harness out, looks brand new no signs of corrosion on the pins or harness also went and verified the grounds on the harness with a continuity tester (PINS 20 40 60, got the EVTM so i was able to verify that) they are good, so atleast no grounding issues on the ECU, going to be checking the IRCM harness next.

    Searching on the forums i found that if you have the fan and fuel pump always on when key is on its probably a bad ECU, so went and checked to see if fuel pump was on and sure enough the fuel pump is always on when key is on ignition. I was also seeing that it could be a bad IRCM as this contains the EEC relay inside. So im leaning towards replacing the ECU, will probably order one online, cant find many junked SC Tbirds here in Puerto Rico, know of any good online store that has SC tbird ECUs? What do you guys think replace ECU or IRCM first? By the way thanks for all the help!

  14. #14
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    Quote Originally Posted by marcosrodoca View Post
    What do you guys think replace ECU or IRCM first?
    Practically speaking, I think EECs are generally available refur'b, while IRCMs not so much. You might have luck finding either used, just be sure to match numbers.

    Never hurts to have spares of either, tho. Might want to start with whichever you can locate first. I'm still trying to decide which I'd expect to more likely fail when sitting...the IRCM is technically more exposed to the elements than the EEC, so... Might be one of them was marginal when parked.

    I've seen a lot of ICRMs get stink-eye'd, but only one actually fail, and I have 5 spares. Sometimes just manipulating the harness fixes the problem, making it appear that the swapped unit was bad when it wasn't. Keep in mind too that there can be multiple issues, where you're actually making progress as you go along, but won't see success until the last fix, so don't assume the others weren't effective individually.

    Did you check the ignition switch down on the side of the column yet? Did you open and clean those ganged grounds on the header panel where the leads connect, in addition to the bolts that clamp them onto the header?

    Can you check for EEC power out from the IRCM pin 24? Did you try a luv tap on the IRCM to see if that wakes up the relays? Did you pop the lid on the EEC to do a visual? Has the car been welded on or hit by lightening?
    Last edited by KMT; 02-27-2019 at 12:00 AM.

  15. #15
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    Well was able to work like 40 minutes on the car until it started to rain... tapped the IRCM to see if it had any stuck relays, nothing, wiggled a couple of harnesses nothing either, kept checking for bad grounds, burned wires and besides the first wires I showed the car is in pretty good shape. Did check power out from IRCM pin 24 and is giving out the 12V so I can assume the IRCM is atleast powering up the ECU. Took out the ECU to get the model number to order a new one, also opened it to inspect for any damages, cant really see anything extremely burned except one microproccesor and a capacitor (which seems that it heat up so badly that it melted the tin below, or maybe whoever assembled it did a ~~~~~~ job) . I think with that damage it may be reason enough to order a new ECU. Im going to order from Advance Auto https://shop.advanceautoparts.com/p/...369/20700776-P , is this pretty much plug and play or do I have to take it to someone to get it programmed? Cant believe that after 25 years that computer looks so clean inside I attached a couple of pics of the ECU inside. Havent been able to verify the ignition switch yet, and no the car has never been welded on or ever been hit by lighting that i know of.

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    Last edited by marcosrodoca; 02-27-2019 at 09:44 PM.

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