QuestionQUESTION: 2001 Sonata, 135K miles.
I disconnected the positive battery cable over night (about 10 hours). When I re-connected this morning, the starter engages, rapidly turns the engine but no signs engine starting.
3/4 tank of fuel. Good battery voltage.
Did my actions affect the ECU? If so, how do I recover?
ANSWER: Disconnecting and reconnecting the cable should not affect the ECUs ability to operate. If the engine is turning faster than normal, you should suspect a broken or stripped timing belt or a crank sensor. Otherwise, you'll want to perform normal diagnostics to determine why the engine doesn't start even though it cranks.
-- Does the check engine lamp illuminate when the ignition is turned on?
-- Check diagnostic trouble codes.
-- Do you have spark at the spark plugs?
-- Do you have fuel pressure?
-- Is the timing belt intact?
Also, I'm curious why you left the battery cable disconnected overnight. This seems an abnormal thing to do unless there was already some sort of problem.
---------- FOLLOW-UP ----------
QUESTION: Thanks so much for the response.
The reason I disconnected the batter cable was to attempt to reset the ECU. The car would drive for 15 miles or so and then drop to third gear and stay there, it would also give a check engine light. This occured routinely (daily) during warm weather but not during the winter. Thinking it may have been a radiator problem, I flushed the radiator. Problem did not clear.
I took it to a local shop, and found the Crank Sensor was bad. I am now replacing the Crank Sensor, Timing Belt, and Water Pump.
I have learned my lesson. It is better to take it to the shop for diagnostics than to try snd fix it on my own.
AnswerMost likely, there's still some sort of problem lurking out there. The dropping into 3rd gear is indicative of a transmission-related problem. It's not necessarily a large problem, but it's somehow related to the transmission.
When you disconnect the battery, that will clear the diagnostic trouble codes, potentially making it impossible for you or a shop to determine the problem. When the check engine lamp illuminates you should check the codes:
-- with a code reader you've purchased for yourself,
-- at a place such as AutoZone that will check diagnostic trouble codes for free, or
-- at a shop that will diagnose your problem.
Once you've learned the trouble code and what it means, and if possible, retrieved the data from the time the trouble occurred, you can then clear the code to see if/when the check engine lamp will return. This way, you have the information about the nature of the problem.
I'm glad things seem to be better.
