Main Relay Repair How-to
03-07-2012
#1
Registered!!
Thread Starter
Join Date: Jan 2012
Posts: 34
Likes: 0
Received 0 Likes
on
0 Posts
Rep Power: 0 
Main Relay Repair How-to
Just repaired my main relay. Now it starts perfectly every time, even when hot. Not sure if this info is out there already. I didn't see anything more than 'replace,' but if not, here it is:
The main relay is, as they say, directly above the hood release (in a 1990 DX model). It is a PITA to get at. It is hidden very well. Took me many attempts to find it. The plastic panel needs to be loosened and pulled out of the way so that it can just barely squeak through--that is, after the extremely hard to get at 10mm bolt is removed. When I put the relay back, I didn't bother to bolt it in place. I suppose I could later.
The failure on mine was cracked solder connections. I am guessing that this will be the failure for most, if not all. Vibration and heat will fracture the solder connections. Thermal movement of the components will make and break the connection as the car warms up. The coils are robust, as is the resistor, and the coils don't draw much current, and so it is unlikely that the diodes will become stressed. The resistor is wire-wound and should measure 42 ohms. It is unlikely to drift, but could in rare circumstances open up. The diodes, when checked with the diode check function of a DVM, should measure a forward voltage drop of between .6 and .7 volts. In reverse it should be infinite, or OL. Any numbers in reverse indicate reverse leakage.
Without performing all of these tests, the very least you could do and probably be fine is to remove the cover from the relay by bending the two side tabs. Then reheat all of the solder connections and add more solder to them so that they are good and strong. Then reassemble and be on your way.
The relay contacts can pit and burn, so they could fail at some point. I didn't bother to test mine. Still working well at 265k.
...So most likely, considering the typical symptoms and what I found in my relay, the most you will have to do is freshen up all of the solder connections and then continue using the relay.
The main relay is, as they say, directly above the hood release (in a 1990 DX model). It is a PITA to get at. It is hidden very well. Took me many attempts to find it. The plastic panel needs to be loosened and pulled out of the way so that it can just barely squeak through--that is, after the extremely hard to get at 10mm bolt is removed. When I put the relay back, I didn't bother to bolt it in place. I suppose I could later.
The failure on mine was cracked solder connections. I am guessing that this will be the failure for most, if not all. Vibration and heat will fracture the solder connections. Thermal movement of the components will make and break the connection as the car warms up. The coils are robust, as is the resistor, and the coils don't draw much current, and so it is unlikely that the diodes will become stressed. The resistor is wire-wound and should measure 42 ohms. It is unlikely to drift, but could in rare circumstances open up. The diodes, when checked with the diode check function of a DVM, should measure a forward voltage drop of between .6 and .7 volts. In reverse it should be infinite, or OL. Any numbers in reverse indicate reverse leakage.
Without performing all of these tests, the very least you could do and probably be fine is to remove the cover from the relay by bending the two side tabs. Then reheat all of the solder connections and add more solder to them so that they are good and strong. Then reassemble and be on your way.
The relay contacts can pit and burn, so they could fail at some point. I didn't bother to test mine. Still working well at 265k.
...So most likely, considering the typical symptoms and what I found in my relay, the most you will have to do is freshen up all of the solder connections and then continue using the relay.
03-08-2012
#2
Registered!!
Thread Starter
Join Date: Jan 2012
Posts: 34
Likes: 0
Received 0 Likes
on
0 Posts
Rep Power: 0 Re: Main Relay Repair How-to
The fractured solder connections appear as fine annular rings. There will be a very fine jagged circle separating the solder into two parts; one that's attached to the wire or lead that's soldered in place, and the other that's attached to the foil on the circuit board.
I'm not sure which connections were fractured on my main relay because the actual relays are enclosed in plastic, and I didn't take the time to disassemble them. From what I saw at a glance, I believe that it was something to do with the 'start' circuitry that bypasses the resistor fed coil and feeds the coil full current. My guess, as the car would run just fine indefinitely, is that the start circuit opened when hot. The resistor would hold the relay closed, but without the bypass, the current provided by the resistor was not enough to re-close the relay when the car was re-started. Just a guess. I really have no definite idea, because I didn't take the time to look. I built up solder on all of the connections to be safe, and put it back together. Works perfectly now.
So nice, since earlier today I was stuck in the middle of the Scrub-a-Dub parking lot for over 3 minutes. Turned the engine off for the wash to conserve gas. Figured that it would start at the end of the wash, but it wouldn't. Thankfully the wash conveyor gave the car one last good kick out of the garage and I was able to roll into the lot and sit there and wait until my car would start again.
I'm not sure which connections were fractured on my main relay because the actual relays are enclosed in plastic, and I didn't take the time to disassemble them. From what I saw at a glance, I believe that it was something to do with the 'start' circuitry that bypasses the resistor fed coil and feeds the coil full current. My guess, as the car would run just fine indefinitely, is that the start circuit opened when hot. The resistor would hold the relay closed, but without the bypass, the current provided by the resistor was not enough to re-close the relay when the car was re-started. Just a guess. I really have no definite idea, because I didn't take the time to look. I built up solder on all of the connections to be safe, and put it back together. Works perfectly now.
So nice, since earlier today I was stuck in the middle of the Scrub-a-Dub parking lot for over 3 minutes. Turned the engine off for the wash to conserve gas. Figured that it would start at the end of the wash, but it wouldn't. Thankfully the wash conveyor gave the car one last good kick out of the garage and I was able to roll into the lot and sit there and wait until my car would start again.
03-09-2012
#3
Registered!!
Thread Starter
Join Date: Jan 2012
Posts: 34
Likes: 0
Received 0 Likes
on
0 Posts
Rep Power: 0 
Thread
Thread Starter
Honda Civic Forum
Replies
Last Post
