Lude Generation

The UK Honda Prelude Club

Main Relay Repair (3rd Gen, maybe others)

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As much of the Lude knowledge we have in one handy place
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 Postby dwallis92 »Thu Oct 12, 2017 6:52 pm

I've been on this forum all day browsing round and posting a couple things, loving it so far and looking forward to getting involved. So anyway, I had an issue with hot-starting recently and it annoyed me because it really didn't make sense at first. In the end I managed to figure it out and fix it basically for free which is awesome. Apparently it's a common problem with many 80s/90s Hondas so I thought I'd post a little background and show you how to fix it too, it might help someone one day. Heads up, you do have to solder, but don't worry it's an easy one and not as daunting as you might think.

The car would start and run fine, never cut out bogged down or died on me at all, but once the engine got warm if I turned it off and tried to start the car again it just wouldn't fire up. Almost like the fuel pump wasn't working. Sometimes if I removed some fluid from the rad and topped up with cold water it would work, which to this day baffles me and I think it's just a coincidence. I flushed the cooling system, replaced water pump and thermostat, and was thinking it was something to do with my radiator when the car started cutting out on me.

The cutting out turned out to be the battery, which was under warranty so I went and got a brand new one for free :D hehe. But still it didn't want to start when warm. I started checking fuses and relays and noticed that when the problem occurred I wouldn't hear any noises coming from the main relay (next to the interior fuse box). So I ripped the sucker out and tried to get a new one. Only 1 local parts supplier had one, and they wanted about £180 for it! No chance. I found 1 on eBay from the US, shown below:

Image

But even £87 delivered is too much for some piddly relay so I thought "hey!! might as well try fix it". I popped off the casing and let's just say the problem couldn't have been more obvious if it had jumped up and slapped me in the face with a wet fish. I didn't get a picture of it but basically the solder on the 5 points (circled in red below) was clearly cracked. At this point I didn't even need to test the components in the relay, just needed to reflow the solder. Real easy. I started by cleaning the surface with solvent cleaner, then pasted a generous amount of my favourite flux (Kingbo RMA-218) and was staring at this:

Image
EDIT - I reflowed more points than were damaged, this was just personal preference and took me all of about a minute to reflow all points.

Good flux makes soldering really easy. Once my iron had warmed up I touched it to the solder point that I wanted to reflow and applied some solder. Your iron is at a good temperature when it melts the solder in about 2-4 seconds of contact. If it's too hot you risk damaging components, if its not hot enough the solder won't flow correctly and you end up with ugly work that probably won't last. I melted the bad solder, and applied a reasonable amount of solder. Because relays get hot you want a decent quality solder. If you have a little spare silver-based solder lying around use that it will last forever, but if not DO NOT buy any. Rosin Core solder wire with an Sn60/Pb40 composition will work perfectly, and it's actually my "go-to" general purpose solder. You can pick up a 100g reel of it for about £5 and will only probably use a couple of inches on this project.

After you have finished soldering, make sure you clean away all excess flux and residue with a good quality solvent cleaner, I like 99% pure MEK as it evaporates really quickly and doesn't leave any marks or residue. You should be left looking at something like this:

Image

Beautiful! From dull and cracked to shiny and perfect, you couldn't ask for better work and it's so easy thanks to good flux. It's all cheap too, You can buy an iron for £5-10, RMA-218 flux for £5, Sn60/Pb40 Solder for £5, and 1l of MEK for £10 all from Amazon!! I brought all that stuff a few years ago for working on circuit boards, and it's a bit 'overkill' for this application, but it's cheap enough to buy that you would probably spend just as much on low quality products. There's no need for you to use these specific products if you have other stuff you can use, but if you don't have flux, solder and solvent cleaner I recommend spending the the £20 on the stuff I use so you can solder like a pro with little to no effort.

Thanks for giving this a read and I hope it can be of some help to someone. :D :D

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 Postby firstlude »Thu Oct 12, 2017 8:45 pm

good little write up and thank you for the advice about flux and solvent , ordering some now for some future soldering jobs not sure i need a litre of solvent :D
success is the ability to go from one failure to the next without any loss of enthusiasm :D :D

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 Postby dwallis92 »Thu Oct 12, 2017 9:07 pm

firstlude wrote:good little write up and thank you for the advice about flux and solvent , ordering some now for some future soldering jobs not sure i need a litre of solvent :D


Thanks mate :) Solvent is just for cleaning really, seems a bit silly but it is important. Especially after 'cause you don't want sticky flux everywhere (even more so when you consider it's highly toxic). I've used all sorts and in my experience nothing cleans like MEK.

Careful what you use it on though, it doesn't exactly like plastic or rubber lol. Glad I could be of help hope that flux serves you as well as it does me haha

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 Postby firstlude »Sat Oct 14, 2017 9:57 pm

can you believe i ordered the flux on friday morning and it arrived this morning now thats service :D
success is the ability to go from one failure to the next without any loss of enthusiasm :D :D

alinton
 
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 Postby alinton »Mon Oct 16, 2017 10:49 am

Mine failed a couple of years ago - I reflowed the solder joints as some looked a bit dodgy.

However it still didn't work. I suspect the coil went o/s.
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'96 BB4.

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 Postby dwallis92 »Fri Oct 20, 2017 7:47 am

alinton wrote:Mine failed a couple of years ago - I reflowed the solder joints as some looked a bit dodgy.

However it still didn't work. I suspect the coil went o/s.


Yea, non-starting and cutting out can be difficult problems to diagnose. If it's not the Main Relay or something else simple (like a fuse) then you have to start by deducing whether it's a fuel or spark issue. Start with the simplest/easiest things and gradually work your way through each possible cause until you find and fix the problem.

That being said though reflowing isn't always guaranteed to fix your Main Relay. Components in the Relay may be damaged, or you may damage them while attempting the repair. Still it's worth checking at the very least, cold solder joints are usually pretty noticeable and are easy to fix with the right stuff. And if it works it saves you a fair bit of money. :D


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