Electrical Bad ECU?

stangman16

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Nov 16, 2004
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San Antonio, TX
I have an A9L ECU on my manual transmission 1966 mustang with a Ron Francis EFI harness for Mustangs/Broncos that I cannot get any codes from using my OBD-1 code scanner. When I try the KOEO test I hear a few clicks and get nothing. All zero's on the scanner. I can wait a min or 10 mins and when I push the button to stop the test it will click again and no codes available. Nothing happens with KOER on the code scanner.

The car will start and run poorly sometimes dying twice before I can get it warm and then it will smooth out. I hooked up my quarterhorse and adjusted the HEGO delay from 10 to 20 because of my long tube headers with O2 sensors at the collectors and it seemed to help considerably once it gets warm. I notice from the tunerpro RT datalogging the ACT reads 92° and ECT reads 90° at dead cold and both do not change at all. I still cannot get any codes from this thing with the quarterhorse attached or detached. (in case the QH was bad) I was thinking the computers ground was fried thus causing the poor running and these sensors reading 92° & 90° with the engine cold in the garage and not changing when it got warm. Looks like my TPS volts read 0.00 also according to the datalog.

I have checked this brand new harness:

black/white wire at the self test connector resistance to battery negative = 00.4 ohms (same reading I get when touching both leads together.... not the best mulitmeter btw)
black/white wire and pin 46 on computer wiring connector = 00.4 ohms
computer pin46 to 60 and then pin 46 to 40 both about 00.4 ohms
STI red/white wire and pin 48 on computer connector = 00.4 ohms

Visual inspection inside the ECU and I don't see anyting burned and the capacitors look fine (no bulges or any blown)

Bad ECU or bad code scanner ? I just bought this code scanner brand new...
Attached the Tunerpro RT datalog incase it may reveal something more.
 

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HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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Thief you can't read he said he's done that....I suspect the puter is fine as it will run the engine. Those 2 sensors are critical to it running well, fix those and I bet most other issues will go away.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Thief you can't read he said he's done that....I suspect the puter is fine as it will run the engine. Those 2 sensors are critical to it running well, fix those and I bet most other issues will go away,
I know I miss things sometimes but don't be rude, yeah I went back and read he was in the computer but I still think there is an issue with it or a missing ground.
Does the fuel pump continually run with key on, engine off?
 

stangman16

Active Member
Nov 16, 2004
637
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San Antonio, TX
I know I miss things sometimes but don't be rude, yeah I went back and read he was in the computer but I still think there is an issue with it or a missing ground.
Does the fuel pump continually run with key on, engine off?

No, the FP runs for a few seconds and turns off with KOEO.

Just the fact that my TPS, ACT, ECT are reporting wrong and unable to pull any codes I'm thinking this ECU is toast. I remember changing the TPS and ECT not too long ago. Have another ECT on the way that I can change and check to see if the tunerRT software is showing any change to.
 

HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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Memphis TN
A toasted ECU will not run the engine. The fuel pump running a couple secs at key on is normal, and an indication of a working ECU.
 

Mustang5L5

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Feb 18, 2001
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Just the fact that my TPS, ACT, ECT are reporting wrong and unable to pull any codes I'm thinking this ECU is toast. I remember changing the TPS and ECT not too long ago. Have another ECT on the way that I can change and check to see if the tunerRT software is showing any change to.

Based on what you posted so far, i would not suspect the ECU is bad.

I might suspect wiring though. Those sensors all share a common sensor ground. If there is an issue with it (or pin 46 trace is burned) then will note the sensor you listed are not functioning properly

However if the trace was burned you would lose continuity from pin 46 to both pin 40 and 60.

I assume you visually looked for a burned pin 46 trace as well?

It would look like this on the reverse side of the ecu down by the 60 pin connector.

1628601861443.png


If that is fine, i would test for continuity from pin 46 on the ecu 60-pin (with ecu disconnected) back to the sensors themselves on their sensor ground wire and see what you have there.
 
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stangman16

Active Member
Nov 16, 2004
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Based on what you posted so far, i would not suspect the ECU is bad.

I might suspect wiring though. Those sensors all share a common sensor ground. If there is an issue with it (or pin 46 trace is burned) then will note the sensor you listed are not functioning properly

However if the trace was burned you would lose continuity from pin 46 to both pin 40 and 60.

I assume you visually looked for a burned pin 46 trace as well?

It would look like this on the reverse side of the ecu down by the 60 pin connector.

1628601861443.png


If that is fine, i would test for continuity from pin 46 on the ecu 60-pin (with ecu disconnected) back to the sensors themselves on their sensor ground wire and see what you have there.


I checked the ecu inside again. No blown capacitors, the backside like you show looks perfect.

Tested the ecu again between pin 46 and 60 is 00.2 ohms and 46 and 40 is 00.2 ohms.

Tested pin 46 to the ECT sensor with the sensor removed and Im getting 00.2 ohms and when i switch the DVMM to continuity it buzzes.

New ECT sensor. Logged into quarterhorse again still showing 90 deg temp at dead cold engine 92 for ACT and 0 on the TPS.

Brand new harness from Ron Francis. I even added 2 ground straps from his connection point on the lower intake bolt to the firewall and to the upper intake.
 

Mustang5L5

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New ECT sensor. Logged into quarterhorse again still showing 90 deg temp at dead cold engine 92 for ACT and 0 on the TPS.

If you set your meter to resistance and check the two pins on the ECT sensor at cold, what reading do you get?

Values should be this

50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
 

stangman16

Active Member
Nov 16, 2004
637
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San Antonio, TX
I get 26.6 on the one in the car and 24.3 on the other i took out. Guess that means it really is 90deg at dead cold? Thanks for clearing that one up. When I get back from work I will warm the engine up and see what readings I get and compare to what the quarterhorse is telling me. I do not believe the temp changes even when the engine is warm as from what I see in the data log.

I checked the TPS voltage at dead cold KOEO and it is reading 0.83v where the quarterhorse is reporting it is at 0... Let me see if TunerproRT has some setting to show decimal places. Maybe it's just not showing the whole value. I'll feel like an idiot if this is the case but I sure hope it is.

Still doesn't explain why I cannot get any codes to dump with and without the Quarterhorse attached. I hear the FP run for a second or two and couple of relays click. Wait for a long time and still shows 000 on the code scanner. The code scanner is brand new... Don't have a CEL in this old car, perhaps I can snag a test light to try the old-school way to get codes.
 

Mustang5L5

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Can you prob the TPS wires and confirm you have your 5v reference voltage at the sensor? You’ll want to test to the sensor ground.

orange to black would be your 5v reference.

green to black would be your output voltage
 
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Frank cortez

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A toasted ECU will not run the engine. The fuel pump running a couple secs at key on is normal, and an indication of a working ECU.
Hi i have a 91 gt with a A9P and currently i dont have spark and fuel pump keeps priming i changed the fuel filter replaced relays and idk if i fried mu computer since this problem started when i changed my ign switch with battery connected and key turned took a while and some fuse links burnt and fixed those but im sure the fuel pump used to prime well and now it just keeps priming
 

stangman16

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Just to update on this situation. I bought another ECU off ebay. It ended up being a dud so I paid ECUexchange to fix the ECU for about $100ish not including shipping to them, and when I got it back I was able to pull codes.

Plan on spending another $100ish to send this ECU to them to fix, just to have a backup on hand, and I will reply back here with what they said was wrong with it in case it helps anyone else out..

I do appreciate all the help with this - thank you!
 
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Diddimus

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A toasted ECU will not run the engine. The fuel pump running a couple secs at key on is normal, and an indication of a working ECU.
Surprisingly I watched a Utube video done by a highly acclaimed service technician start and run a car with the computer disconnected and sitting in the passenger seat! I don't remember the make of the car but it wasn't an old one. He did that specifically to make a point.
 

General karthief

wonder how much it would cost to ship you a pair
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Surprisingly I watched a Utube video done by a highly acclaimed service technician start and run a car with the computer disconnected and sitting in the passenger seat! I don't remember the make of the car but it wasn't an old one. He did that specifically to make a point.
I call BS on this without specific info.
I watch a fair amount of utube.
 

7991LXnSHO

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A toasted ECU will not run the engine. The fuel pump running a couple secs at key on is normal, and an indication of a working ECU.
That’s Not an absolute truth. When I had a bad computer, it would start and run fine at WOT, but was stinky rich at idle and spit out different codes most every time it was scanned. The ECU exchange on EBay fixed it right up and tested it with quick turn around time.
 
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91GTstroked

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A toasted ECU will not run the engine. The fuel pump running a couple secs at key on is normal, and an indication of a working ECU.
Interesting you say that. I had a couple bad capacitors and main power supply according to ECU Exchange. My car still ran with these problems.
 
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