computer dead?
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Senior User
Joined: Jan 2003
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From: Marshalltown, IA
computer dead?
If the computer died on an 85 f150 4.9 (feedback carb and eec-iv), would it still run? My timing doesn't advance like it should, and I can't seem to get anything when pulling the codes. I'm pretty sure I had everything hooked up right to pull them with a test light, but the light stays on and doesn't flash at all. It runs ok, just weak from the timing not advancing as rpms increase, it stays around 10 degrees. Anyone have any idea what would cause it to not advance (spout connector is connected)? It has new plugs, wires, rotor, coil, cap, and module. Is there a way to test the computer? Thanks
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Joined: Jul 2003
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From: Chicago, IL
computer dead?
if you cant pull codes and get NO light from the ECM, then theres no power to the comp.. if the light stays ON and does not flash 10, 11 or 12, then its safe to say the comp is fried...
yes you can run without a comp.. just no timing advance. however you can always swap in a vac advance distributor
yes you can run without a comp.. just no timing advance. however you can always swap in a vac advance distributor
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From: Southern California
computer dead?
The manual I use says the computer is protected by a relay. So maybe your relay is bad. Sorry, the book doesn't say exactly where the relay is, but it's probably near the computer. Some computer's are under the seat and other's are located under the dash.
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Joined: Jul 2003
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From: Vancouver, WA
computer dead?
When I was trying to troubleshoot my computer, I found the power relay right next to the computer, but didn't know it because I didn't know what it looked like to recognize it. If this is you, too, go to the parts store and ask your counterman to pull an EEC power relay off the shelf for you to see. Then go home and find and test it. Oh, the frustration we would have saved ourselves if we had known before what we knew after!
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From: Vancouver, WA
computer dead?
Oh, and no disrespect intended 'masta, but I disagree about running without the computer. It'll limp without the computer, but that's about it. The computer controls the air-fuel mixture as well as the timing advance. To ditch the computer effectively you need a carburetor swap, a distributor swap, and a noncomputer ignition system (duraspark II). Replacing the power relay (if that turns out to be the cause) is just a whole lot less time & $.
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From: Chicago, IL
computer dead?
Originally posted by castman
Oh, and no disrespect intended 'masta, but I disagree about running without the computer. It'll limp without the computer, but that's about it. The computer controls the air-fuel mixture as well as the timing advance. To ditch the computer effectively you need a carburetor swap, a distributor swap, and a noncomputer ignition system (duraspark II). Replacing the power relay (if that turns out to be the cause) is just a whole lot less time & $.
Oh, and no disrespect intended 'masta, but I disagree about running without the computer. It'll limp without the computer, but that's about it. The computer controls the air-fuel mixture as well as the timing advance. To ditch the computer effectively you need a carburetor swap, a distributor swap, and a noncomputer ignition system (duraspark II). Replacing the power relay (if that turns out to be the cause) is just a whole lot less time & $.
how exactly does it control the air/fuel mix? through what solenoid?
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Joined: Jul 2003
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From: Vancouver, WA
computer dead?
1987LTDmasta, you've got me on that. I don't know through exactly what solenoid the computer controls the air/fuel mix. And the more I think about it, the more I realize you've caught me making assumptions. 
I just assumed it must be the case, or else what is the point of all the computer sensors and stuff connected to the carburetor? There's a throttle postion sensor, a MAP sensor, EGR on/off solenoid, feedback control solenoid, ... I still feel like I'm missing one but I can't remember what. If all this stuff doesn't give the computer the means to optimize the combustion mixture to near stoichiometric ratio to lower emissions, what does it do?
I've never tried to run my truck without the computer working either, but I heard or read (who knows where) that it'll leave the vehicle in a "limp home" mode. It'll run, but you won't enjoy driving it. Hmmm, maybe that was misinformation, too!
Thanks for shining light on my dim and murky thought process.
I just assumed it must be the case, or else what is the point of all the computer sensors and stuff connected to the carburetor? There's a throttle postion sensor, a MAP sensor, EGR on/off solenoid, feedback control solenoid, ... I still feel like I'm missing one but I can't remember what. If all this stuff doesn't give the computer the means to optimize the combustion mixture to near stoichiometric ratio to lower emissions, what does it do?
I've never tried to run my truck without the computer working either, but I heard or read (who knows where) that it'll leave the vehicle in a "limp home" mode. It'll run, but you won't enjoy driving it. Hmmm, maybe that was misinformation, too!
Thanks for shining light on my dim and murky thought process.
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Joined: Jan 2001
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From: Nebraska
computer dead?
What about the ignition side of things though? Since it's computer controlled, I would think you'd lose everything instead of just timing advance, similar to when a TFI module craps out. That's from my understanding of the system anyway.
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From: Southern California
computer dead?
Hey guys, I reffered back to the manual.
First of all it does say where the power relay to the ECA computer is located. "On the lower right side of the ECA mounting bracket." "It protects the ECA from possible damage due to reversed voltage polarity."
The Feedback Control Actuator controlls the mixture on carbureted EEC engines. It's actually more like a needle valve that controls the vacuum in the float chamber. More vacuum = a leaner mixture, less vacuum = a richer mixture.
Then it says this about the 'limp home mode':
LIMITED OPERATION STRATEGY (LOS) MODE
The LOS mode munctions during engine start, or upon failure of the ECA detected by a "safeguard" circuit in the ECA. This mode allows continued vehicle operation (with reduced performance) until repairs can be made. In this mode the actuator functions are set as follows:
(Well now, it doesn't say if the "safeguard circuit" is operational if the protective relay is tripped).
Here is the list of sensors on EEC-III engines:
- Engine Coolant Temperature (ECT) Sensor
- Throttle Position (TP) Sensor
- Crankshaft Postiion (CP) Sensor
- Exhaust Gas Oxygen (EGO) Sensor
- Barometric and Manifold Absolute Pressure (BMAP) Sensor
- EGR Valve Postion (EVP) Sensor
- Air Charge Temperature (ACT) Sensor: EFI engines only.
Then there are the actual controllers:
Feedback Control
Throttle Kicker
Dual EGR Control Solenoids
Cannister Purge
Dual Thermactor Bypass and Diverter Solenoids
I get all this information from obsolete copies of the Mitchell Manual, which I found at the public library. I photocopied the stuff specific to EEC-III. It's the only manual that fully addresses all aspects of electronic engine controls. It seems most manuals give detailed information about Dura Spark II and then jump to EEC-IV. EEC-III is the middle child.
First of all it does say where the power relay to the ECA computer is located. "On the lower right side of the ECA mounting bracket." "It protects the ECA from possible damage due to reversed voltage polarity."
The Feedback Control Actuator controlls the mixture on carbureted EEC engines. It's actually more like a needle valve that controls the vacuum in the float chamber. More vacuum = a leaner mixture, less vacuum = a richer mixture.
Then it says this about the 'limp home mode':
LIMITED OPERATION STRATEGY (LOS) MODE
The LOS mode munctions during engine start, or upon failure of the ECA detected by a "safeguard" circuit in the ECA. This mode allows continued vehicle operation (with reduced performance) until repairs can be made. In this mode the actuator functions are set as follows:
- Ignition Module Timing: Minimum spark advance (10 deg. BTC).
- Feedback Carburetor Actuator (FBCA): Locked at last controlled postition. On startup, the FBCA is driven full rich and then slightly lean.
- Exhaust Gas Recirculation (EGR): No EGR.
- Thermactor Air (TAB): Bypass (dump) position.
- Canister Purge (CANP): Canister sealed, no purge.
- Throttle Kicker (TK): Low RPM idle.
(Well now, it doesn't say if the "safeguard circuit" is operational if the protective relay is tripped).
Here is the list of sensors on EEC-III engines:
- Engine Coolant Temperature (ECT) Sensor
- Throttle Position (TP) Sensor
- Crankshaft Postiion (CP) Sensor
- Exhaust Gas Oxygen (EGO) Sensor
- Barometric and Manifold Absolute Pressure (BMAP) Sensor
- EGR Valve Postion (EVP) Sensor
- Air Charge Temperature (ACT) Sensor: EFI engines only.
Then there are the actual controllers:
Feedback Control
Throttle Kicker
Dual EGR Control Solenoids
Cannister Purge
Dual Thermactor Bypass and Diverter Solenoids
I get all this information from obsolete copies of the Mitchell Manual, which I found at the public library. I photocopied the stuff specific to EEC-III. It's the only manual that fully addresses all aspects of electronic engine controls. It seems most manuals give detailed information about Dura Spark II and then jump to EEC-IV. EEC-III is the middle child.
Thread Starter
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Senior User
Joined: Jan 2003
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From: Marshalltown, IA
computer dead?
I'm considering going back to the duraspark II ignition, just wish I would have a long time ago. I've already put in a reman carb and distributor, and am still kicking myself for not doing it then. Any tips on testing the power relay? Thanks again guys.
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Senior User
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From: Marshalltown, IA
computer dead?
Can the power relay be tested or is it just something I should replace? Also, anything I need to check for that may have caused it to go bad? I must be stuck in limp home mode, my timing doesn't advance at all. Not sure about how the carb's reacting, it seems to run ok aside from the timing not advancing.
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From: Southern California
computer dead?
Well I tried to get a look at my ECA. It's wedged under the seat and I couldn't get a very clear view of it. I didn't see any little square **** kind'a thing I would expect a relay to look like. So either it's somehow integrated into the ECA and I can't see it or it's under the dash or on the firewall. Once you find it I'm sure it can be tested or replaced. I guess now it's up to you to find it and tell us where it is and what it looks like.
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Senior User
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From: Marshalltown, IA
computer dead?
I found the relay, it was right next to the computer, under the dash on the drivers side. I'm not good with electronics, is there a way to test it or should I just go get a new one? My local parts store has one in stock, so if it's too hard to test mine I'll just buy a new one, only like $18.00. Any advice will help, thanks.
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 7,298
Likes: 45
From: Southern California
computer dead?
You could test it's resistance with an ohm meter. If it shows 'open circuit' then you'll know for sure it's bad. If it shows resistance, you won't know for sure if it's good or bad unless you know what the spec is for the relay.