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pick up sensor bypass please Pages: 12

osvaldo516 on Wed May 05, 2010 10:30 AM User is offline

Year: 1990
Make: honda
Model: prelude
Engine Size: 2.2
Refrigerant Type: 134
Country of Origin: United States

I have swapped a H22A1 engine into my 90 4ws prelude and I would like information on how to bypass the a/c pickup sensor that came with my car in order to use the new compressor that came with the h22a1 engine

h22 swapped 90 4ws prelude owner

HVargas on Wed May 05, 2010 12:07 PM User is offlineView users profile

Can you be more specific on what you mean by pickup sensor? Where on the car is it located, how many wires and what colors?


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osvaldo516 on Thu May 06, 2010 1:50 AM User is offline

it's located in the compressor but controlled by a compressor control unit located on the passenger side underneath the glove compartment.
If you want I can send you all the diagrams; just give me an email address to send it to.

h22 swapped 90 4ws prelude owner

osvaldo516 on Thu May 06, 2010 2:56 AM User is offline

Adam here is a link for the prelude a/c system click on the 1990 supplement

h22 swapped 90 4ws prelude owner

mk378 on Thu May 06, 2010 3:42 PM User is offline

So, basically the red/blue wire gets pulled to ground (through diode) to demand A/C. The control unit detects that and as long as the rpms are proper, pulls in the compressor relay(s) to run the compressor.

I'd just remove the control unit entirely. Lift the grounded end of the coil of clutch relay A and connect it to the yellow/black at the relay. Bypass the other compressor clutch relay (if equipped) by connecting the gray wire to the red wire. Where the control box was, connect red/blue wire to red/yellow wire. Now relay A is driven from the diode and will pull in whenever fan switch, A/C button, thermostat, and pressure switch are all OK for the compressor to run.

osvaldo516 on Fri May 07, 2010 1:45 AM User is offline

mk378 thanks for the info here is my current email address so that I can send you the diagrams I've been working on: (currently deployed in afghanistan until mid Jul)

h22 swapped 90 4ws prelude owner

NickD on Fri May 07, 2010 4:52 AM User is offline

So what you are saying is that your vehicle is equipped with the differential speed detector circuit and your new compressor does not the speed sensor. Always thought this was a bit crazy as some vehicles had this detector, others did not, function is to disable the clutch coil in the event the compressor should seize if the pulses coming from the compressor speed sensor are lower than the ignition pulses. Circuit wasn't worth a darn anyway, as the compressor could slip by as much as 20% and still be enabled, belt squeal would be so loud at that point, would even wake up the driver. And if the driver had any brains, would switch off the AC before the drive belt burns up and breaks.

And this circuit sure doesn't protect against a seized idler pulley bearing that is far more likely to happen, besides on these cars, a separate belt drove the compressor, so if it broke, still had the water pump and the alternator so you wouldn't be stranded like on these single belt engines. And the manual of course does not show the circuit, just the wires going to it, must really be top secret for a poor circuit to start with.

It got a bit messy for me after I pulled the circuit and traced out the components, noticed the input resistor was different for the compressor speed detector as the tach pulses, so I changed that resistor to that of the tach pulses and applied the tach pulses to both inputs. That way they would always be the same, no need for that compressor speed sensor. Really don't feel the engineer that designed this thought this problem completely through.

osvaldo516 on Sat May 08, 2010 10:10 AM User is offline

NickD says:So what you are saying is that your vehicle is equipped with the differential speed detector circuit and your new compressor does not the speed sensor.

Osvaldo reply yes this is correct

h22 swapped 90 4ws prelude owner

mk378 on Sat May 08, 2010 11:16 AM User is offline

Yes Nick is saying he has the same situation as you. His idea to feed the tach pulses instead of the compressor speed sensor is a good one. That would fool the compressor control unit into thinking the compressor is turning even with no sensor. But that's unnecessarily complicated here. Since your Prelude's compressor control unit doesn't do anything else though you don't have to fool it, you can just remove it entirely.

If your new engine's ECU has A/C in and out ports you should use them. In later models the ECU would get the signal from the diode and increase the idle speed, only after completing that would the ECU engage the compressor relay. You would still need to rewire one of the compressor relays to be low-active (ECU drives one end of the coil to ground, the other end is always live). And you don't need two compressor relays in any case.

NickD on Sat May 08, 2010 3:56 PM User is offline

Module I had was the central point for the AC system, climate control, dual function switch inputs, compressor and other outputs. All I had the the shop manual module wiring and was loaded with solid state and relays. So I just located the tach and pickup inputs, cut the pickup wire from the connector to the PCB and jumper that to the tach and traced the circuit enough to realize the pickup input resistor was different. Removing the tach wire did not work, call it the lazy mans' way of fixing something, had too many other things to do. Could have been something to bypass as the compressor relay would not energized, didn't care, and didn't trace that out. Was a quick fix that lasted until the car rusted away.

All cars rust away.

osvaldo516 on Sun May 09, 2010 10:22 AM User is offline

guys thanks for all the help and comments, however I'm more of a hands on learner type of person. If you guys were kind enough to send me an email to my government computer at I would then be able to show you the diagrams I have made based on me reading the manuals from 88-91 and 92-96 preludes, I traced all the diagrams and made one, i just need to know if it will work.

h22 swapped 90 4ws prelude owner

osvaldo516 on Tue May 11, 2010 1:57 AM User is offline

alright I finally figured out how to upload images here, so please take a look at the diagrams I've uploaded 88 91 prelude diagram.JPG engine 92 96 prelude ac.JPG ac diagram.JPG

h22 swapped 90 4ws prelude owner

NickD on Tue May 11, 2010 11:27 AM User is offline

Just a bunch of lines connecting blocks, no pin numbers nor pin identification, not easy to work with, that's why I have to pull the modules and study them. See you have two clutch relays in series, could be on is speed, the other is normal gas control. Also have the tach and the pickup coil going into the compressor control unit, so you have a couple of possibilities. Japanese were very notorious for making constant changes, very well possible those diagrams are not even accurate, have to dig into the vehicle.

mk378 on Tue May 11, 2010 1:23 PM User is offline

Remove the compressor control box from the car entirely. If you have two clutch relays, bypass one of them. You only need one. Rewire the other relay so one end of the coil is connected to a source of +12 volts (the yellow black wire). The other end of the relay coil goes either to the diode, or to the ECU if it has A/C control.

Passing the signal through the ECU will give better control of idle speed as the compressor cycles on and off. The later model ECUs have this feature. The two wires are usually called "ACS" (A/C Switch) which is an input from the diode. The output is called "ACC" and is an output to the relay. These signals to/from the ECU are negative logic. When a wire is near ground voltage it is active. When near battery voltage it is inactive. You will have to pull a diagram of the car that the engine / ECU came from to find the pin numbers.

osvaldo516 on Wed May 12, 2010 3:45 AM User is offline

h22 swapped 90 4ws prelude owner

Edited: Wed May 12, 2010 at 4:31 AM by osvaldo516

osvaldo516 on Wed May 12, 2010 5:44 AM User is offline car compressor control system.JPG engine ECU AC pin out.JPG engine ECU AC pin out part 2.JPG control override thru pin connector.JPG

the last pic here is an idea i thought of

h22 swapped 90 4ws prelude owner

1Funryd on Sun May 16, 2010 12:54 AM User is offlineView users profile

Year: 1988
Make: Honda
Model: Prelude
Engine Size: 2.3
Engine: H23a Swap (not original motor)
Refrigerant Type: R-12

Hello mk378 and NickD and my man Osvaldo.

I appreciate the effort here. Osvaldo and I have been working toward finishing the AC issue for the H22/23 motor swap into the 3rd gen Honda Preludes.

I am currently working with my manufacturere in Arizona out of Mesa AZ, to produce the AC compressor bracket to make this swap a complete swap.

Osvaldo and myself have decided to move forward since my manufacturer has too many projects on the table right now, so we both have decided to make both AC systems work, instead of wait for the AC compressor bracket, that is why we are both here on this forum, trying to learn more about ac systems.

I have been working with our local AC guy, and today we cut the 4-wire harness off of the old B20a ac compressor and cut the green and white wires and tucked them, then kept the factory diode that is in the wiring harness off of the compressor.
We then soldered the Red and Brown wires off of the OEM compressor harness and spliced and but connected those two wires to the Red single wire on the newer Sanden H23 AC compressor.

System was vaccumed, and we changed out the drier, and new o-rings at the drier connections. We turned on the system, but to no avail, the compressor did not kick on. I need to add, that we when we hotwired the Red single wire from the compressor, the it did kick on and started working. So we thought the Relays could have gone bad.
We tested the power at the spliced Red and Brown wire, to the single Red wire from the Sanden Compressor, and we lose voltage. This is where we are at as of this post.

I will be heading back to his place tomorrow to check the power coming from the CCU box. I will tell him of your suggestions, which we were thinking to do as a last resort. It seems after reading this thread that we will have to by-pass the entire control box to make the system work again.

I have replaced the two AC clutch relays with brandnew ones today. We thought this could be an issue causing the power loss when the system is turned on, but this did not work.

Thank you for your suggestions.


Edited: Sun May 16, 2010 at 1:00 AM by 1Funryd

osvaldo516 on Wed May 19, 2010 12:59 AM User is offline

1Funryd I guess we are on our own, yet again.

h22 swapped 90 4ws prelude owner

1Funryd on Wed May 19, 2010 3:46 AM User is offlineView users profile

No we are not, we have figured it out thanks to you and mk378 and nickd. We took this information and applied it, it took us all day and the next but we finally got the system to work off of the stock P14 H23 non-vtec ecu, which will still work for the P13 H22 vtec ECU as well.

We got it to work and bypassed the control box. We bypassed the Relay B and I posted it up on preludpower on my build thread with some pictures and instructions, go and check out my build thread.

It was much simpler than we initially thought. Thanks guys you helped us get out of the rutt we have been in for three years.

My new issue is now I just lost cooling today after work, but I believe it to be my clutch has gone out on my new used compressor. Teaches me a lesson to not waste good money on core components. LoL!!! But the AC issue has officiall been resolved. No need for the AC bracket at all.

NickD on Wed May 19, 2010 7:14 AM User is offline

I offered all I could short of working in electronics for over 50 years. Even working with a Honda electrical manual, tell you how to change the pickup coil, but don't tell you anything about how the system works and only offer a block wiring diagram. I would never blindly change wires to the connector unless I know what I am doing.

This requires removing the PCB, scanning both sides of the board, reversing the bottom image, super imposing those two images together on auto cad and making a schematic diagram showing the components and how they are wired. That drawing is a mess as it is a PCB layout, rearrange it to make sense of a working schematic with the inputs on the left and outputs on the right. That I can make sense of, know how the circuit works, and determine what has to be done to modify it for my application.

Changing engines and compressors is not in the book, blindly changing connectors in best case can blow a fuse, worse case, end up in smoke.

How about buying a used motorhome with a GM climate control with 19 unidentified spade lug terminal and a previous owner that randomly switched wires around? Pace Arrow had no circuit diagram nor records as to the pin identification, said they purchased the unit that way from Chevy, couldn't find any circuit diagram anywhere even going back ten years on GM climate control pin identification. Had to make my own schematic, I did identify all the wires and how they were left by the previous owner. To save time, did all hand drawings, what a mess that guy made.

Honda manual is so bad in the AC section, could never find the exact oil requirements for the AC system when redoing a complete rebuild, say when you purchase a replacement, the exact amount of oil is already in there, that is incredibly stupid.

I am putting a car radio in a boat now, and finding errors in this car radio output schematic pinouts making me experiment, shouldn't have to do that. Use a current limited lab type supply so I won't burn anything up, that happens when working with a battery capable of delivering 1,500 amperes. Who in the helll checks these drawings out!

I did simulate that tach versus compressor pickup coil PCB on my bench to learn there could be as high as a 20% slippage before it would switch out the compressor. Whoever designed this circuit is a complete idiot in my book. Simulated using digital output pulse generators, nice to have all this gear or I would be dead lost. Need both the schematic and the actual vehicle, otherwise just guessing.

In other applications, could talk to a tech for hours on the phone and can't seem to locate a problem, had to hop on a plane and go there, to find just one misplaced wire in a multi-million dollar industry control system, he swore the wiring was correct.

If your pickup doesn't work, according to the Honda manual, should replace that or the pickup, check the wiring, replace the entire vehicle, or find an identical vehicle that works and replace one component at a time. Automotive schematics are the worse in the world, what idiot said, "solid state, do not measure"? Another thing I hate, is always hot, when you wake up to a dead battery. A zillion always hot! And where is the ground? Never thought I would locate a fuel pump ground under the drivers seat, but that is where that wire led me to.

mk378 on Wed May 19, 2010 7:40 AM User is offline

Funryd, you could look at adjusting the clutch gap before condemning the compressor. If the clutch coil is getting power but the plate doesn't pull in and engage, it could just be that the gap between the plate and pulley is too wide. That can be adjusted by taking the bolt or nut out of the center shaft, remove plate, and remove some shim washers from the shaft behind the plate. Proper gap is about 0.020 inches, it can be even closer as long as the plate doesn't drag on the pulley when the clutch is off.

1Funryd on Wed May 19, 2010 10:59 AM User is offlineView users profile

Originally posted by: mk378
Funryd, you could look at adjusting the clutch gap before condemning the compressor. If the clutch coil is getting power but the plate doesn't pull in and engage, it could just be that the gap between the plate and pulley is too wide. That can be adjusted by taking the bolt or nut out of the center shaft, remove plate, and remove some shim washers from the shaft behind the plate. Proper gap is about 0.020 inches, it can be even closer as long as the plate doesn't drag on the pulley when the clutch is off.

mk378 that is exactly what my ac guy was asking me about yesterday when I told him I lost cooling. I will be heading back to his shop this weekend to try and close the gap first(if there is a gap) and then test it.

I appreciate all the help that both of you have given me and osvaldo.

osvaldo516 on Thu May 20, 2010 11:59 PM User is offline

oHHHH my GOSH!!!!!!.....Thanks everyone!!! God bless ya all!!! for real!!!

h22 swapped 90 4ws prelude owner

osvaldo516 on Mon May 24, 2010 3:44 AM User is offline

here is what 1Funryd posted also in our honda prelude community we hope this helps other people as well.

1. You need to bypass Clutch Relay B under the Battery try, there are two relays. Relay B has the solid colored wires so to do this you need too:
Cut the Red wire and Grey wire on Relay B Butt, connect them together with insulated butt connectors to be heat shrank by heat gun. For weather proofing. You have now bypassed Relay B.
Cut the Yellow/Blck wire on Relay A side wiring, this wire is a hot wire that needs to be connected to a 12v ignition switched connection or just run a jumper wire straight to the ignition harness under the dash in front of the fuse panel on the drivers left leg side. I just ran the jumper wire along the front of the battery along the side of the engine bay fuse panel zip tied it to the high pressure ac line that is there and the behind the Vaccum box along the fire wall and into the rubber gromet that is right behind the gas pedal and straight to the fuse panel and plugged it there.

The 1st accessory wire is the fat Yellow/Blck wire on the ignition wiring. There is no need to undo the dash this wire runs along the bottom of the fuse panel coming from the ignition. But if you need wiggle room then go ahead and drop the under dash panel. Tap that wire with those female electrical T-tap connectors, you will need the yellow T-taps on this wire because it is a thick guage wire.
Next we are going to connect all the hot wires together since they all need the 12v ignition switched power source. Cut the Red/Yellow wire on Relay A as well, then splice Yellow/Blck and the Red/Yellow together so they are all connected because they will be supplied by the same jumper wire to the 12v switched source.
When you butt connect all these wires together please get the insulated heat shrink butt connectors so they are weather proofed, you are not going to want to mess with these wires again.
2. Next we move on to the AC Compressor wiring.
First cut the harness off of the original B20/21 compressor, this is crucial because it already contains the diode that is going to be needed to reduce the electrical back surge to protect the ac clutch relay. Cut as close to the compressor as possible, leave about at least 2" left at the compressor for possible reconnection later should you choose to. The harness from the compressor will have a Red and Brown wire, and a Green and White wire, then the one wire left which is the Diode wiring. Connect the Red and Brown wire together, then cut the Green and White wire as close to the harness as possible and electrical tape it up, it is not going to be used. The diode wire will have a connector on it you will want to cut off the the bullet style male connector and connect it to the one single Red hot wire on the H22/23 compressor. This will keep the Diode inline of the hot wire on the H compressor.
Clean it all up and make sure everything is nice and taped up and insulated.

3. Now to move on to the ECU side. The H22/23 ECU's control the AC clutch, and Condensor Fans and the throttle response for adding fuel to the system once the compressor kicks on so that it does not stall.
First take off the glove box, and unplug the CCU unit, it was the control module that everyone always needed to re-solder due to corroded solder points and the weak link in the OEM AC system
Now cut the Red/Blue and Red/Yellow wires behind the harness plug and butt connect them together, and that will bypass the AC Control box or otherwise known as the CCU unit.
You are now electrically ready to plug it all up and turn it all on. I recommend you do not tape everything back up until you have tested your connections, or else you will have to take all apart again.

You will need to get custom AC lines made to route the lines properly to clear the motor should you need to pull the motor at a later date. Once make those lines and vaccum the system and the refill it, it will cost you a lot of money to refill it again should you pull the lines back off if you need to pull the motor out. So keep that in mind when having them made.

And make sure you have enough of the tubing coming out of the ac compressor so your local AC shop can have enough material to work with or they will charge you more for adding and welding in more aluminum ac tubing.

h22 swapped 90 4ws prelude owner

1Funryd on Tue May 22, 2012 11:05 PM User is offlineView users profile

Will this work for the OEM setup on the 88-91 Preludes? Can they get away with doing the same by pass on their system if their pickup sensor is gone bad and they do not want to pay for a new compressor when theirs is working minus the pickup sensor?

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