fcd / pressure sensor weirdness
 

Ok, the situation.
australian delivered 1988 s4 rx7 turbo.
Voltage at pin 2b (pressure sensor) of ecu, permanently 5v, no matter *what* I do. Disconnect pressure sensor, drive on boost, etc etc.
Pressure sensor functions and outputs perfectly.
Car still drives normally, and just tested today that fuel cut comes on at about 3.65v on pressure sensor. Had to run wire from pressure sensor into cabin...
standard boost gauge works fine.

Anyone got a clue what's up?? I can't work out how the ecu is cutting fuel, if it's not getting any readings from the pressure sensor...

nobody? :(

is the factory boost gauge reporting correctly?

You saying the stock boost gauge reads good. Then if you are really on 2B, the wiring must be miswired b/t the ECU and pressure sensor but is good b/t the boost sensor and the boost gauge.. That's the only reasonable explanation.

What color is the wire at the ECU? At 2B?

Well, there could be another reason. I notice on the series five ECU 2B isn't the boost sensor, it's 2H.

What is the number on top of your ECU? N332? N333? N???

n318...
I'm checking the right pin. it's a differnt colour to any wires coming off boost sensor. boost gauge reads correctly, and ecu still fuel cuts when boost sensor reaches 3.65ish volts.

makes no sense at all to me.

How about disconnecting all three plugs at the ECU and ringing wires from the boost sensor. As in....the brown/red wire at the boost sensor to every wire on the ECU plugs. Ones bound to ring out.

You do have a brown/red wire on the boost sensor??

ringing... that'd be continuity testing I assume? :p:
Will give that a try. Pretty sure there's a brown and red wire, but I think a green and white wire is the signal line. Will have a look later today.

Your harness must be different than the USA models. The signal wire on a 87/88 turboII is a brown/red wire. It splices off the boost sensor and goes to the boost gauge in the combination cluster and also goes to the ECU at the pin/socket 2B.

See: https://www.rx7club.com/forum/attach...&postid=601909

Yeah. Continuity testing. I'd sure try to find out where the signal wire is going on the ECU.

Green/white? Hmmmm. 88???? That does not match the 87turboii fsm that I attached nor the 88fsm that is online at http://www.iluvmyrx7.com

Dang furrin car you got thar.https://www.rx7club.com/forum/attach...&postid=601909

ok, more info.
I started off continuity testing, but I was getting beeps for all sorts of weird things, so just looked for any paths of low resistance...

5v map sensor - low resistance path to 2a (5v reference)
signal map sensor - no low resistance path to anything

wire colours:
signal at map sensor - green and white
5v at map sensor - brown and white
signal at ecu (2b) - brown and red + blue stripes
5v ecu (2a) - brown and white

voltages:
signal at map sensor - 2.33 ignition on, varies with boost
5v at map sensor - 5v
signal at ecu (2b) - 5v
5v at ecu (2a) - 5v

It's got me stuffed, there's no way I can trace the wires as far as I can see. I'm not expecting much to come of it, but if you've got any more ideas Hailers, I'd love to hear them :confused:

*cough*
last try :D

Oh. I see what happened. The output of the boost sensor has a splice to the boost gauge, which in turn is grounded. Perhaps that's why you got all the beeping a going on. If the plug was pulled off the back of the combination meter that feeds the boost sensor, then you should only get one wire that beeps when checking continuity for the output of the boost sensor.

The N318 makes me wary. IF it was a N332, I'd say splice a wire from the output of the pressure sensor to the ECU at pin 2B.

So how about this. You know the output of the presusre sensor when the car is idling. Soooo....unbolt the ECU so you can get at the back of the plugs while they are all attached, and idle the engine. Now look at each pin on the ECU, for 2.33 or so volts while its idling. Maybe you can find it that way. Cough, cough, hack, hack.

2.33v doesn't exist at the ecu side of things, just ran through them all again :confused:
if the fuel cut wasn't working properly, I'd run a wire straight from the pressure sensor to the ecu. But I'm unable to fathom how the fuel cut is working at the moment.

I really can't help. Is it possible you have a F-Con or the like and the wires are of a different color. I've never seen a F-Con so this may be a reach.

Do any of the wire colors on your car match those seen on the 88fsm that is online? Ah! I give up. Too different for me to figure out without seeing it.

Considering everything but a mild exhaust was stock when I got it, I'd have trouble beliving they had a fuel controller rigged up somewhere in the tangled mess.

Any thoughts on possible repurcussions if I scrap the factory wire and run a temp one from the sensor to the ecu?