Honda won't start on hot day (how to diagnose a bad Main Relay)

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Honda won’t start after sitting in sun on a hot day :ohcrap:

I just fixed this problem on my daughter's 90 Accord, after many misses (chasing ignition). To test if problem is MAIN RELAY: While the car is refusing to start, roll back carpet in trunk to reveal top of fuel pump/sender unit. Connect volt meter to YELLOW wire at connector. Turn ignition to start. Voltage should be present for about three seconds. If no voltage, confirm the diagnosis by applying 12V to yellow wire for a few seconds. Engine should now start and run. Failure seems to be a solder connection on the relay circuit board that fractures, resulting in increased resistance, especially when relay is hot from car sitting in sun. ECU then fails to sense voltage when ignition is turned to START. The shop manual does not anticipate that this part might operate intermittently and thus is of little help in diagnosing problem.

There are pages on the Internet that describe re-soldering these connections, if you want to try repairing your own part. One such is
See also my own web page on making this repair:

Main relay is located under the cowl, attached to underside of dash near the fuse block. Replacement part costs 45 - 80 bucks. The connector is a seven-pin block (row of four and row of three) with doubled-up wires on some pins. Look for a yellow wire at one end and black wires at the other end. I had to remove the tin box for the cruise control computer to get access. Remove 10 mm nut holding bracket and muscle relay out with bracket attached. If you are doing this work during the heat of the day (when the problem presents), you may want to wait until evening or next morning before you go under the dash. See some helpful photos at:

Earlier, after $200 in ignition parts had not fixed anything, my daughter suggested I call Click and Clack about the problem. I laughed. Believe it or not, I was listening to their radio program while I was finishing this repair and a listener called about her car not starting on hot days. They asked if it was a Honda. No, she said, it was a '90 Acura. Without hesitation, they said replace the main relay; it was a well-known problem on Hondas (and Acuras) of that vintage. My daughter might have also suggested searching the Internet, which I did not think to do until after I heard the call from the Car Show listener.

posted by  gravistar

That's the only major rampant problem i've ever found with 4th gen Accords...I've had two and on this one the only thing I've ever had wrong with it was the hot-day hard start problem. It will start given a few tries but it's annoying...It only happens in 90+ degree weather. I got the relay replaced and installed for $140 if i remember correctly, Problem solved.

posted by  thunderbird1100

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