Honda won't start in hot weather (how to diagnose bad Main Relay)

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

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 between Solid Yellow wire (be careful; yellow wire w/white stripe is fuel guage sender) at pump connector and a known good ground. Turn ignition on. If voltage is present for about three seconds, relay is OK. If no voltage, confirm the diagnosis by applying 12V to yellow wire for a few seconds. Engine should start.

Failure seems to be that solder connections on the relay circuit board gradually fracture and develop resistance. This resistance goes even higher when relay is hot from car sitting in sun. Eventually, resistance is high enough that, when ignition is switched on, insufficient voltage is available to operate relay, thus not sending essential pulse of power to fuel pump.

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

The main relay is located under the cowl, attached to underside of dash near the fuse block. Replacement part quotes ranged from 45 to 80 bucks, so shop around. 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 gain access. Remove the 10 mm nut holding bracket and take relay out with bracket attached. See some helpful photos at:

:doh: I was feeling frustrated from my earlier attempts to find and fix this problem. The fact that I prefer to work on cars in the cool of the day (when this problem never presents) might have been a factor. 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 replacing this relay on her car (having diagnosed it with the test described above). Someone called in 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 claims this proves women are better than men at solving car problems because they know to call the Tappett Brothers rather than waste time and money trying to fix it themselves. She might have also suggested that I search the Internet, which I did not think to do until after I heard the call from the Car Show listener.

posted by  gravistar

gravistar, you are my hero. Really.

We have been having this exact problem with my parents' 1993 Accord DX. I'll have to check that relay out. Great writeup!

posted by  ZanCRX

Ok, I am also having problems with my honda not starting in warm weather and in the winter when the heat is run. A new rebuilt distributor was put on, new fuel filter, rebuilt alternator.
The mechanic also said he put a new relay switch in the car, however the problem still exists. Any suggestions?
I don't think he put the swtich in. Is there a second switch that might have gone bad?
The car acts like it wants to crank. The fuel pump can be heard etc...the spark plugs are good also.
I'm really fustrated.
Like I said it does this normally during hot weather. If I air out the car, the car eventually starts. Anyone?:ohcrap:

posted by  soapmom1

My Honda feels way smoother when the weather is cold.

posted by  MetalManiac

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