A few months ago I decided to take another look a very minor issue that that had persisted for several years with my 1990 USDM Civic Si. My brake warning light remained dimly lit most of the time. Sometimes it would go out for a while other times it was fully illuminated. Of course I checked the brake fluid level and it was fine. I figured out with the help of some folks here on Honda-Tech that the problem was with the ICU (the little grey box on the back of the fuse panel under the steering wheel ) The circuit board inside my ICU looked burnt or corroded so I went to three local junkyards, got lucky and found one. I plugged it in and the problem was gone. The brake warning light turned on and off just as it was supposed to. But a couple minutes later I realized the front corner lights were not functioning correctly. So over the course of the next month I ordered and received four more ICUs from various junkyards around the country. Each one had a different problem or combination of problems, brake warning light, intermittent windshield wipers, front corner lights. When the fifth one came I opened it up and once again it looked burnt or corroded but I gave it a try and it worked. I was relieved but now I had five bad ICUs and not much confidence that the sixth one was going to last and I didn’t want to go through that again.





I decided to make a replacement that I could rely on. While studying the old ICU and getting an understanding of exactly what its functions were I found a few things I did not expect. First, and perhaps this shouldn’t have surprised me, there are errors in the wire diagrams in the genuine Honda Service manual. Second my car works just fine with the ICU removed. There are a couple minor functions that I might need in order to pass a safety inspection, and my intermittent wipers don’t work but other than that the ICU is not actually very important. I also never noticed that when the headlights are off, the front corner lights flash with the turn signal lights in the front bumper but when the headlights are on, they flash opposite of the lights in the bumper. Perhaps most surprisingly I found several electrical connections coming into the ICU that the ICU doesn’t use at all, which means it is nowhere near as complicated as it could be. Once I understood exactly what each connection coming into the ICU was for, it was a relatively simple task to design a replacement. Design the circuit, test it on a breadboard, work out the bugs, design and order the PCB, solder it all together and plug it in. There were a couple frustrating hiccups along the way but that is pretty much it. Technology has changed a lot since the original part (38600-SH3-A11) was designed back in the late 1980s, I was able to reduce the component count from 77 down to 34.





I was also able to add some minor functionality enhancements to the ICU that I thought would be nice to have. The most significant is the ability to interactively alter the frequency of the intermittent wipers. The OEM ICU has a very brief pause between the wipes and the length of the pause cannot be altered. With my ICU, I can use the intermittent wipers just like before or, if I want a longer pause I turn them on and immediately turn them off, then wait for the amount of time I want the pause to be then turn them back on going forward it uses that period of time as the delay. If I leave them off for more than one minute it goes back to the default delay. If I turn it back on in less time than the default delay it resets to the default delay.

The second feature I added is pretty useless but I was looking for features to add. Normally if the door is open and the key is in the ignition but not in the on position, it will beep incessantly to remind me not to forget my keys. I added the ability to make it stop beeping. Just press the door switch three times in rapid succession, it will confirm my request with one continuous two-second beep and then stop beeping until the next time I put the key in the ignition.

As I mentioned before there are connections coming to the ICU that the OEM ICU does not make use of.



The missing wire with the red arrow is easily explained, the wire comes from the seat belt latch. So it is present on 1988 & 89 USDM models but 1990 (like this one) and 1991 models had a separate seat belt module so this wire is no longer needed. By making this change external to the ICU Honda was able to use the same ICU for the entire 4th generation. The wire with the green arrow comes from the switch for the windshield washer pump that sprays washer fluid on the windshield. As pictured below, I added that ninth pin to my ICU so I could put that existing wire to use. I would like to know if everyone has that wire on their car.



Adding the ninth pin enabled me to add a feature that is common on many modern cars; when you spray washer fluid on the windshield it automatically turns on the windshield wipers for a few wipes. I designed it so that as soon as I start the windshield washer pump it will begin wiping the window. After I release the windshield washer switch it will do two more wipes then wait a couple seconds and then do one final wipe. I am looking for more features to add so if you have any ideas I would love to hear them!



There are five more unused electrical connections where the ICU plugs into the back of the fuse box that may provide the ability to add some even more interesting features but so far I have not been able to figure out what they are connected to or why Honda made them available but didn’t use them. The next picture shows how the five unused connections flow through the fuse block but that is as far as I have gotten. Unfortunately the fuse box assembly (38200-SH3-A01 or A02) for 1988-89 is not the same as the 1990-91 part (38200-SH3-A51) so if I find any useful connections I will need to make sure the same connections exist in the other part as well.



I now have a working prototype installed in my 1990 USDM Civic Si. I have tested everything except the seatbelt buzzer and light because those functions are not performed by the ICU in a 1990 or 91 USDM Civic. I need to find a friend with an 88 or 89. To the extent possible I used AEC-Q200 certified parts which means they are designed for automotive use and should hold up. Next step is to run it in my car and make sure it doesn’t crap out in a month

 

Good job! And thanks for posting this helpful info 👍

 

 

How cool is this?! Hope you decide to share the project on GitHub or something so it can be expanded to other generations too.

 

I ran the prototype in my car all day today. The main thing I needed to confirm was that it would not generate too much heat. In the past some folks have indicated that they thought the OEM ICU ran hot and perhaps that contributed to the ICU failures. Someone even said they drilled holes in the plastic case for ventilation. I left the fuse box loose so that I could pull the ICU out quickly after I got home. When I opened up the plastic case I would say the circuit board was just barely warmer than ambient temperature. I tried touching each of the components to see if any of them felt warmer than the others and perhaps the primary IC was a bit warmer but it was barely noticeable and certainly nothing I am concerned about, I would say it was about the same temperature as the IC in the OEM unit when I performed the same test with it.

There was a pretty significant cost associated with this project. Partly because I didn't know what I was doing and I had to start over a few times. While I continue testing the prototype, I am modifying the design so that it can more easily be manufactured. When I place that order it will be another large expense but, it takes too long to assemble and solder these together by hand, and my time has value. I will be able to provide them at a lower cost if I don't have to do so much work on each one. I don't expect that I will ever recoup what I have invested in this project, but the personal satisfaction I get helping someone keep another EF on the road is worth quite a bit to me.

 

Here is the next version. I had twenty of them made. They are made with AEC-Q200 qualified components. I still had to add the connectors myself. I need to run this version in my car for a few weeks. The surface mount components make it look ridiculously simple when compared to the original.

 

wow.. that is awesome! I am always worried about mine going bad. Awesome to have a source for a new one.

 

Great work! I am one of the crowd that drilled holes in my plastic cover, on one of my replacement ICU's that I have sourced (I have a few spares).

So this would work on a 1991 CRX?

 

These are compatible with any car that originally used part numbers 38600-SH3-A11 or 38600-SH3-A01. I think that includes all USDM Civics and CRXs 1988-1991. I also believe that I will have a version to replace 38600-SH3-C12. (Canadian Version) The Canadian version is actually less complicated. I am trying to make this thing bullet proof. I eliminated all but one electrolytic capacitor. I am using metal film resistors instead of carbon film resistors. I am specifically adding protections to areas of the circuit that have failed in the past. I used optocouplers to electrically isolate some problematic inputs. My design takes into account the possibility that some folks may have replaced the original light bulbs with LEDs. LEDs can put additional stress on transistors that were expecting a bit more current. If anyone has old ICUs that have failed but not been thrown away please hold on to them. I need the connectors but I don't want to take them off working parts. I am looking into getting the connectors reproduced but not having any luck. Because I am adding a pin to the connector to enable the extra feature I require nine sets of connectors to make eight ICUs. If anyone has advice on how to source reproductions of the connectors I would love to hear it. I'm going to take a few months to test these before I try to offer the first batch for sale.

 

What exactly are you needing to use for the connector, the plastic part and the pins together? Or would it help if someone could make just the plastic part, maybe look into 3D printing as an option?

 

I need both complete connectors. The plastic parts do seem like they could be 3D printed. It turns out that outsourcing 3D printing is not as cheap as I thought it would be. I wouldn't mind having a 3D printer so I would be happy to go that route for the plastic parts. I tried a few places like Xometry.com for the metal parts and they said they could not make them. The connectors are surprisingly intricate. I think they could be simplified and still be compatible. Generating a proper CAD file to send to a manufacture for a quote is not something I know how to do. The other problem is tooling is expensive and I assume the volume of parts I would need is very low. If I ever ended up making close to 100 of these I would be very surprised. I was thinking of just charging a core fee so people would send me their old ICU. The problem with that is, if the ICU I sold them fails I can give them a refund but I wouldn't be able to give them back their old ICU and that seems less than ideal. I'm thinking that most of the connectors will need to be sourced from junkyards and it will probably add significantly to the cost of building each ICU, further reducing the number of ICUs I will ever end up building. I could sell them without connectors and let folks find and attach their own connectors but that would make it problematic for me to provide any sort of warrantee. Each person would need to acquire two ICUs if they wanted to source the one additional pin. I have an STL file for one of the plastic parts and a STEP file for the associated metal pins but the quotes I got were ridiculous. I wish I could just find the company that made them originally. I think I can source the connectors from ICUs in Civics thru at least 2000. Looking at pictures online the connectors look the same.

 

Having people source their own connectors from old ICU's is probably your best bet. You would just need a small inventory of them to get started, could build them to order and then have people supply theirs as a core to replace what you've used. I already have a couple of failed ones in my garage as I know others do as well, and would be happy to send the extra connectors.

However, I think it needs to be a 1:1 replacement and having that extra pin may not make it as feasible.

 

And I think they used the same connectors on ICUs up till at least 2000 in many if not all Civics. The ICUs are different but the connectors look the same. see 38600-SR1-A01

 

That's freaking awesome!

So when are you offering these for sale? I have 2-3 bad ICUs for my 91 Wagon.

 

I don't want to inconvenience folks by selling them something that has not been thoroughly tested. I have one in my car and I also have someone else testing one. I don't want to get a reputation for selling junk. I figure a couple months is really the bare minimum test duration. Perhaps I can make them available just in time for the holidays. I thought of another feature I want to add so I am doing a small tweak to the design. I am adding an audible alert when the door is open, the key is out of the ignition and the headlights or running lights are still on.

 

Your humbleness belies the greatness of what you've done. Awesome!

 

This is awesome! I am always worried my ICU is on borrowed time.

 

That's really awesome you are doing all this! I haven't had intermittent wipers for that past couple years I have owned the car. My side markers have never flashed either, didn't know they were supposed too. The car has been a labor or love, have enjoyed working on it and figuring out all the small issues. But I am in no way electrical savvy. I have had to re-pin the ECU as it had the wrong one PS9 (4 Door Sedan, Auto) I have a 90 Manual DX Hatch. Fixed the engine harness cause it was a pile of spaghetti from the previous owner. I feel the previous owner took a lot of stuff from this car and hid everything very well. Not sure how it made it to my house to be honest. Anyway, I realized he other day taking a closer look at me ICU that it only has the one wire on the plug in the center to what you describe as the "seat belt" sensor, no other wires. My ICU box has a "D" on it with the serial number #38600-SH3-A121-M1. It has 9 pins as shown in your pictures, but the plug only has the seat belt wire you specify taking out. Where do the other two center wires go too? If you could shed some light on where the wires connect or what they all do. Would really help. Mine may be fried as well.

 

Also, as an idea. Would you be able to produce and program the boards and just sell them as is? and anyone who has their own extra or fried boards could maybe take the pin and prong housing from theirs and install them on the new boards themselves? would that help so that you don't have to find more boards yourself unless to obtain the plug and prong housings? I would gladly buy a board from you and install the plug and prongs from my the original if you left instructions as to where the pins are to be placed etc.

 

I don't know where you are located but your connectors look normal for a Civic built for the Canadian market in which case the corner lights are not supposed to flash. I don't think I have ever seen an ICU with the part number you provided (38600-SH3-A121) If the A was a C it would confirm that you have a Canadian ICU it still wouldn't tell me what market your car was built for. Based on your description of the condition of your cars electrical system and the part number you provided I would not assume that the ICU you have is the correct ICU for your car. USDM 4th gen Civics only used two different ICUs as far as I know 38600-SH3-A11 or 38600-SH3-A01. If you determine that you need/want an ICU configured for a car built for the Canadian market just let me know and I can build one for you after I confirm that I have solved one little issue with how the seat belt light works on the Canadian Civics.

 

You may be right, the picture I took with the serial number cuts off at the #121-M1...so it must be #C121-M1. Thank you for clarifying my side signals aren't supposed to work. I do live in Canada near the Chilliwack area. If you are able to make a board that would be fantastic, Honestly your saving us Honda lovers with your amazing skills, appreciate your efforts immensely.

 

@DaRk_Precurser I need a detailed description of exactly how the seatbelt light in a Canadian Civic is supposed to behave. I have a partial description but the seatbelt latch switch in the car that was tested to get the information was not functioning correctly so the description I have could be incomplete.

Here is the test procedure and questions:

1. Get in the car close the door, do not buckle the seat belt, insert the key and turn it to the run position (just before the start position) What does the seat belt light do? The description I have is that the light turns on for ½ second and then off for ½ second and it repeats five times then the light goes off and stays off even if you do not buckle the seat belt.
2. Repeat the same test procedure except this time buckle the seat belt while the light is still blinking. Does the blinking stop immediately when the seat belt is buckled?
3. After the seat belt light stops blinking, what happens if you unbuckle the seat belt?
4. Does the car have an audible alert or is the seatbelt light the only notification it provides?

 

Once you turn it to the run position. The light turns on for ½ second and then off for ½ second and it repeats five times then the light goes off and stays off even if you do not buckle the seat belt. As you described. As it’s flashing it is audible for the 5 flashes. When you insert seatbelt as it’s flashing, it still proceeds to flash but the audible beep stops. Hope this helps.

 

That was very helpful! Based on your answer, I suspect that the car that I got my previous information from the seat belt buckle switch failed in the buckled position.

After the seat belt light stops blinking, what happens if you unbuckle the seat belt?
If you don't buckle the seat belt at all does the light remain on while you are driving?
If you buckle the seatbelt before turning the car on does the light still blink 5 times?

 

This is soo good. I'm not even worried about additional features I'd be happy to have just a standard version of this with modern electronics in there. keep this up and I will 100% buy one once complete for my 1991 Hatch.