Piggybacking stock ecu for A/C

[cvc9216]

I'm not convinced the RB ecu can run the A/C safely. So I'm going to piggyback the stock KA ecu. I figure it won't be too hard to wire it now, while I'm doing the rest of the wiring. My question is what signals does the KA ecu need to operate the A/C? Here's my list so far, anything else needed? TPS, tach, etc? Also, will the KA ecu accept the signals from the RB's coolant sensor and speedometer? Do I even need to hook up these signals?

ignition power.....ecu #38battery power.....ecu #70 & 113eccs relay............ecu #4 to eccs relay 1...........................ecu #56 & 61 to eccs relay 3...........................eccs relay 2 & 5 to battery powergrounds..............ecu #10, 19, 106, 112, 118, 39, 43, 111A/C signals..........ecu #21 thermo control amp............................ecu #9 thermo control amp............................ecu #15 triple pressure switch............................ecu #12 triple pressure switch............................ecu #57 ambient switch............................ecu #114 recirculation relaycoolant temp........ecu #50 & 51speedometer.......ecu #26defogger..............ecu #55

*some of the pin numbers may be wrong*

[Wulfgang]

You should not need the speed sensor. Also, it is not a good idea to try to run both ECU's on the same coolant sensor, since each ECU will try to regulate current through the sensor. They will almost certainly fight with each other, and you may ruin an ECU.

So after you do all of this, you should put a voltmeter on the thermo control amp signal and another on the output from the ECU (thru the triple pressure switch). When the compressor cycles, I believe you will find that the signals match each other exactly (i.e. they will switch in unison).

..... and that should prove to everyone that the ECU does not cycle the comrpessor - the thermo control amp does, which just makes running the extra ECU a waste of time.

But anyway, it'll be cool to have someone with a working A/C. Just don't count on your A/C switching off for things like hard acceleration and overheating.

Which compressor are you using, and in what car?

[Imissmyturbo]

I am about to hook up the AC in my RB25. It's in an S13. The RB25 has an AC cutout for WOT/acceleration. I will get some pics up of how it is done.

[Wulfgang]

All ECUs have the AC cutout for WOT and high coolant temperatures. But when cvc9216 runs a second (KA) ECU, how will it know when the engine is at WOT or getting too hot?

Imissmyturbo, did you use the RB25 compressor with S13 thermo control amp?

[Imissmyturbo]

I haven't used anything yet. I plan on using the RB25 compressor with a custom line going from the comprssor to the condensor and then the stock lines from the condensor to the evaportator and then a custom line going back to the compressor from the evap. I will be doing some more wiring research to figure out how to get it to work.

[AREITU]

Isn't there an RB20 S13 on Ebay with AC working?

[FAST-DATSUN]

S13 ac will work as the controler is in the body harness!!!!!!!

[goofynick6]

I wish there was a nice s14 a/c writeup

Nick

[AREITU]

I wish there was a nice s14 a/c writeup

Nick

From what I've heard, there isn't because every year of 240SX or something like that, is all wrong, and that the FSM is half the time. You can get the engine to start--no problem, but everything else is guess and check with a multimeter.

[AREITU]

S13 ac will work as the controler is in the body harness!!!!!!!

Do you happen to know about S14s?

By the way, a friend of mine (we're actually not far from you guys, and I keep asking them if they want to swing by your shop...) wants to do an RB25 *gasp* DE swap! Do the DET mounts work or is Nissan twisted enough to make it different?

[cvc9216]

I have an S14 with an RB25, using the RB compressor. I'd prefer to just use the RB ecu, but I guess it's a trade-off either way. It's kind of hard to understand the role of the ecu. What control does the KA ecu have over the rear defogger and recirculation relay? If I use the RB ecu, can I just ground out those relays, or switch them to ground?? Wulfgang, following the directions you posted, won't the triple pressure switch just be bypassed? And can someone explain to me what the thermo control amp does?

[cvc9216]

P.S., Mack, I was also wondering if you could answer a quick question about ABS. From what I can tell, the only signal the ABS control unit needs from the ecu is the tachometer? Will it operate with the RB's tachometer signal? thanks

[Wulfgang]

The KA ECU does not have "control" over the defogger. The defogger has control over the ECU. I believe this is an idle-up signal for the ECU, just like the power steering switch.

As for the recirc relay, usually recirc automatically comes on whenever the defrost vent is on. It looks like the ECU may just switch the vents to recirc whenever defrost is switched on. In the S13, this was just done in the push button panel. Anyway, whatever it does, I do not think it is critical.

No, the triple pressure switch will not be bypassed. You only need the dual pressure side (over pressure and under pressure) for the RB. The third part of the switch can be left unconnected. The dual pressure part is still in line with the A/C relay, so that it will turn off if the pressure gets too high or too low.

The thermo control amp has one simple function: when the evaporator air gets too cold, it shuts off the A/C; when the evaporator air warms back up, it turns the A/C back on again. This is what cycles your A/C, not the ECU. The ECU only intercepts a few things to make sure that you can get full acceleration when you want it, to be sure the engine doesn't overheat because of the A/C, and, only in the S14, maybe it also does the recirc function.

[cvc9216]

ok, what does the other side of the triple pressure switch do? Looks like it goes to ground when the switch is activated, why? The FSM says it controls the fan also, is that a function within the ecu? Is it also sufficient to hook up only the one wire from the thermo control amp? As for recirculation, I guess I can deal without that.

Also, any chance I can tie in the defogger to the rb's extra p/s idle-up signal? I'm not sure if it works the same way as the defogger switch (switched to +12V vs. switched to ground as in the case of the p/s pressure switch)

[Wulfgang]

1. Yes, the other side of the triple pressure switch is for fan control. You've only got electric fans now and are probably either using the thermo switch from the RB radiator or have your fans come on with ignition, so you need no ECU fan control scheme. The KA ECU controls the 2-speed electric fan using inputs from engine temperature, A/C load (the triple pressure switch), defogger, etc. If you really want that kind of sophistication for your fan control, you can buy the Perma-Cool variable speed control unit and program that.

2. The thermo control amp has 3 wires: 12 V supply, ground, and the switched signal (that goes to the ECU). I believe the 12 V supply is wired to ignition, and the ground goes through the push button control unit (which just grounds that wire whenever you turn the A/C on). The signal switches with air temperature and tells the ECU when the air is warm enough to turn on the A/C compressor. If the air gets too cold in the evaporator core, the water vapor will freeze, icing up the core and blocking air flow. That is the reason, the ONLY reason, you have a thermo control amp and need to cycle the A/C.

3. I would not worry about the defogger idle-up. The defogger really doesn't draw that much juice, so your RB ECU should be able to control idle fine without knowing about it explicitly. Many people don't even hook up the PS switch. A/C, however, really sucks the horsepower, which is why there is usually an FICD air bypass for A/C instead of just an ECU-controlled idle-up.

[goofynick6]

I'm decent with electronics..but this sounds like WAY too much of a pain for an s14. I was really wanting to actually keep a/c in this car, but this sounds like a mess to deal with.

Nick

[FAST-DATSUN]

tHE ABS WILL WORK THE SIGNAL IS ok...

[cvc9216]

tHE ABS WILL WORK THE SIGNAL IS ok...

great, thanks. Wulfgang I guess you've convinced me. As long as it works I'll be happy. It doesn't have to be 100% functional, I'm not going to use it all the time. But I have the wiring done, in a few weeks we'll find out if it works.