I have a 1994 F150 4.9L straight 6 automatic transmission. The check engine light comes on a few seconds after I start the engine. My mechanic says the computer needs to be replaced. Is the computer the same for an automatic trans and a standard? Will other year models interchange?

Any advice is greatly appreciated.

 

Welcome to the site..

Manual and Automatic will not interchange. Was he able to pull any trouble codes out of the PCM? What was the reason he gave for a replacement other than the CEL?

 

He said there were no codes to report so he took the computer out and found where it had scorched in places. Before I took it to him I read where I could run a wire from the diagnostics cable to the negative battery terminal and count the blinks of the engine light. From that I came up with 117 which was the ETC sensor. I replaced it and cleaned the connector, neither of which made the light go off.

 

'Took it to him', like towed? Doesn't run, correct?

A PCM after 94 won't work their OBDII systems 95 up.. You should be able to be backward compatible to 93 ect. as long as the truck PCM is optioned/calibrated the same.. If the PCM smells or shows sighns of burning/heat, chances are your shopping..

 

It runs, I drive it everyday. When he called me with the diagnosis he said he could put it back together and it would still drive. I was there in 15 to 20 minutes and it was ready in the parking lot. I thought that was fishy. Any other suggestions?

 

Well, if you got a DTC or or LFC for 117 for the ECT Sensor Circuit Below Minimum Voltage, then you may want to have one of the aftermarket parts houses do a free scan to be sure on the DTC, also make sure there aren't any others present since your mechanic worked on it. Then, if only 117 had returned after the PCM was reinstalled, you would need to do some pinpoint test at the ECT connector to see if the circuit is active.. Make sure you check the harness near the ECT, and the connector for corroded pins or any pushed out as a start point..

 

My local after market parts houses had no OBD I equipment to check my truck so I bought an OBD I code scanner.
I still have code 117 (ECT sensor - singnal too low) showing as well as 118 (ECT sensor - signal voltage too high) and 556 (fuel pump relay -primary circuit failure). I used a voltage meter to check at the ECT sensor and there is continuity but no reading on the meter. I used an electronic meter and a meter with a needle set on DC 15V. I'm not quite sure I did it correctly.

Any suggestions are greatly appreciated.

 

Generally when you get both codes for 117/118 the ECT sensor would be suspect, but since you said the PCM had some 'heat' issues it could be bad.

You should at least see a 5v ref voltage on the Gray/Red wire at the ECT connector with the testers black probe grounded and the red lead testing/probing on the GY/R of the connector. The ECT is a variable resistance switch, and as the coolant heats up the resistance increases, voltage drops on the other leg (Light Green/Red) and the PCM uses the return value to calculate for running changes. You can prove out the ECT sensor by monitoring the voltage through the ECT sensor as long as there is an input voltage. DTC 118 sets if the self test voltage goes above the max. 4.6,, and 117 sets if voltage drops below the min. of 0.2 volts. If you have a ref voltage (you should) you could use a test lead and attach it to the GY/R terminal of the connector,, and attach the other end of the test lead to the corresponding terminal end on the ECT sensor. Then, ground your testers black probe, and using the red probe, check the voltage on the 'ECT sensor' where the LG/R would normally have connect to.. Monitor the base voltage cold, then completely warm the engine to see what the voltage value ends with..

 

I checked the voltage at the ECT connector with the testers black probe grounded and the red lead testing/probing on the GY/R of the connector and there is no signal. Is there a way to check the signal at it's source? If so, how can I do that and are there diagram's available?