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I am a new owner of this 1993 rx7. It has some odd electrical problems. The first we want to tackle is that the horn beeps when the car is off, its constant,
When the car is on the horn behaves normally.
The previous owner took out the horn relay.
I have the 1994 RX7 wiring diagram book and have been studying the diagrams and reading this forum.
I am stumped.
Yesterday we disconnected all 3 connectors of CPU2, I was expecting this to be alarm related and the horn would stop beeping when the car is off and behave normally.
It did not.
The wiring diagram also shows the diagnostic module can set off the horn, but I want to avoid messing with that.
I guess start with the basics, a horn usually operates when connected to a ground. If the relay has been removed by the PO, then there's definitely some wiring hackery going on.
Do you have an after market steering wheel and boss hub?
Does it have (or did it have) an after market alarm system?
If the alarm is going off, the horn will HONK-HONK-HONK, like 1 per second, not a continuous HOOOOOONK. If it's continuous something else is weird. Also when the alarm goes off the headlights flash on and off too.
You say when the car is off - so if you get in the car, horn is blaring, put the key in and turn to On does it stop?
The horn circuit is pretty basic. There's most likely some sort of aftermarket wiring that's messed up. If the car is lowered check the driver's side fender liner to see if the tire has eaten through the fender liner into the wiring harness - this is fairly common and it can tear up and short wires there giving some VERY weird electrical problems. You have to jack up the car and THOROUGHLY look up into the fender, just a quick look with. the car on the ground won't cut it.
Agree if it's only going off when the car is turned off there has to be something up with the wiring. Nothing really changes that much with the OEM horn circuit based on the ignition being on or off.
If unplugging CPU 2 didn't cut it out and it only does it with the motor off that sure sounds like there is some aftermarket wiring somewhere doing something wrong, especially since you also mentioned the OEM horn relay being removed.
It sounds like you are testing a relay controlling the horns though. Did you reinstall the oem horn relay? Or did you find one that the PO routed the wiring through? If its one that came with the car you could try replacing it with a new relay...perhaps the one the PO used isn't working quite right and that low voltage you detected is intermittently engaging it.
I will have to test exactly when the horn starts working right, with the key just in, or acc on, or does the car need to be running.
Will get back to the forum on that.
The car has been lowered. We checked that wiring harness 3 times before buying. We didnt jack up and remove the wheel, but we will do that.
Headlights are not flashing, so thats a clue.
We did re-install the horn relay, thats how we found when the car is running it works normally.
The air pump relay is right next to the horn relay and he same. We tried using that for the horn relay with the same result.
There are some other strange things electrical things also:
The rear hatch lock is electrically released. We can shut that hatch and its locked.
When the car on ON, it seems to release.
The wiper motors go really slow, like there is a current limit somewhere.
And the radio was stuck on, we have had it in the shop for a good mechanical going over and they removed the radio,
The shop said this was pretty common.
And the speedometer doesnt work, but the shop said this was due to the engine harness, which we are replacing.
The car has been lowered. We checked that wiring harness 3 times before buying. We didnt jack up and remove the wheel, but we will do that.
To check the wire harness Dale is referring too, you probably need to remove the wheel liner to get a good look. If there is no wheel liner and you were able to see the harnesses just by looking under the car....well that's not ideal and I'd definitely take a much closer look at those harnesses.
Headlights are not flashing, so thats a clue. We did re-install the horn relay, thats how we found when the car is running it works normally. The air pump relay is right next to the horn relay and he same. We tried using that for the horn relay with the same result.
What a dick. Sounds like he couldn't figure it out either and just disabled the horn. CPU 2 fails in strange ways and the lights wouldn't necessarly flash if it was an issue with the OEM alarm....that said, if you unplugged it then you already ruled CPU 2 out, just a fun fact. If there is an aftermarket alarm or something hiding somewhere and is the cause of the issue then same might apply though...ie the headlights wouldn't necessarily blink.
I'd check all of the typical aftermarket hiding places inside the car. Drivers foot well, behind the driver kick panel, behind/above where the crotch vent panel goes, inside the two side popouts on the dash, in/behind the glovebox, passenger footwell, passenger kick panel, behind/around the ECU location, inside the radio cavity, and I'd slip my hand between the transmission tunnel and the dash (like how the factory antenna connections run up into the radio cavity...sometimes people squeeze things in there.
There's other spots...but that covers where I find things 99% of the time.
If you tried multiple relays with the same outcome then its not an issue with the relay, but I would try to track down that trace voltage if its on the control pins of the relay. Do the other relays have that 0.2v when the car is off?
The rear hatch lock is electrically released. We can shut that hatch and its locked.
Yeah that's not factory...at least for US spec cars AFAIK. I would take a closer look and see what was added to electronically release the hatch. Maybe I'm just an idiot but I'm 99.9% sure mine is just a pull cable like the hood release is (ie it works without the battery connected)
The wiper motors go really slow, like there is a current limit somewhere.
Not necessarily unusual, especially if the engine isn't running. This is a common complaint and I'm not sure anyone has found the silver bullet to fix it. You may want to check the condition of your wiper linkage when you have a chance and make sure it isn't filthy/dry/rusted.
And the radio was stuck on, we have had it in the shop for a good mechanical going over and they removed the radio,
Yeah the factory radio can get stuck in there pretty good, but I'm not sure if its due to age or just people smashing it in there wrong. It seemed like mine was forced in incorrectly which led to one of the metal retaining wings being unable to disengage like it should...it was a PITA to get it out. I've never had an aftermarket radio get stuck though.
And the speedometer doesnt work, but the shop said this was due to the engine harness, which we are replacing.
Dunno what shop you are using but that sounds suspicious to me. The tach/speedo circuits on these cars fail all the time due to age, and I'm not sure I've seen a thread where it ended up having anything to do with the engine harness. Generally the cluster needs to go to a shop that can perform the needed repairs to the electronics. I know there is a dude on facebook that people on this forum use with good success but I can't recall his name right now. I am sure Dale knows.
Going to dig into the wheel well to check the harness again, there is a liner but its easy to move. I have a snake camera I can use to look around.
Will check all those spots for some after market work.
Good to know the rear latch is not electrical and we can just replace it.
Would like to find out the dude on face book who can rebuilt the cluster.
We may give a try is the shop doesnt get it fixed with the new ecu.....
We have fixed them before, hardest part was getting it out without breaking any plastic.
One other problem we found was the reverse lights dont work, but that is probably just a switch we need to replace.
I was hoping all these odd electrical problems would lead to the same answer and only 1 thing needs to be fixed.
For the reverse light, what side of the trany is the sensor on?
I spent about 30min looking at the harness under the wheel well, I did not remove the wheel cause the car is too low for our floor jack, we will have to use the stock jack and will do that.
I did see where there was some rubbing, but the electrical tape was barely scrapped, so, so far I have not found anything that looks like a short under the wheel well.
We will get the car raised an inch or 2.
I verified the rear hatch in mechanical, so maybe some grease or a new latch, this is a relief.
I verified when the car is off the relay input is ~0.2v, when acc 0.2. when on 0.2 volts and the horn will beep. When the car is running the input is 12v is something and the horn works normally.
Interesting after that cycle the relay input is a solid 0v or gnd.
I am still looking for some after market electronics, nothing yet.
I did find some frayed wires in the engine bay. At the drivers side engine in the corner where the firewall and fender meet.
I have attached some photos in case someone can tell me what thy do. I think the wires are frayed, not broken or shorted but need to look more.
The fray looking towards the firewall Show the location of the fray looking across the firewall from the drivers side
I forget which side its on for the FD. You should see two, the reverse switch and the neutral switch. You might be able to search on this forum and find pictures/threads on it.
If its that low and you see rubbing definitely take a closer look.
Hard to tell from the photo but it looks like at least one of those wires might have some damage on the insulator. I see some rough/jagged looking edges, but it could just be from zooming in. Wouldn't hurt to take a look for any exposed copper. If you see some...just reinsulate it with something (ie electrical tape). If you
So I think the FD01/CPU2 or body cpu is bad. But maybe not the cause of the beeping horn.
We are replacing the speakers in the front doors and fixing the mirrors so they work.
After we got everything back together we checked that the alarm would arm. It did.
Then the car would not even turn over. The overheat exhaust light is on, but the car has been sitting for hours.
So we pushed it into the garage and started thinking. Pulled out the connector at the top of the FD01 module and the car started right up.
We did most of the work with the battery disconnected so I dont think we zapped anything.
Guess we will take the module apart and rebuild it or send it out.
Got out the oscilloscope and looked at all the signal on CPU2, the starter cut relay pin is something arounf 5v, thats why the car wont start.
Also, after looking at everything I believe its the diagnotic (air bag) module causing the horn to constantly beep.
It probably went bad and the PO just took the horn relay out.
Has anyone had a this module go bad like the CPU2?
My car came with an after market steering wheel, so the only help I can provide is to maybe look at a thread for putting an after market wheel on.
The Airbag module can be bypassed by using some resistors in the connector. If it is the airbag diag module causing this problem, I'd worry that the airbag would be bad and may randomly deploy.
I've never heard of the airbag module causing the actual car horn to sound. It will sound a buzzer (the airbag diagnostic module has its own internal buzzer, AFAIK it does not signal the CPU2 buzzer) in the cabin if the airbag warning light is burnt out though.
Is your airbag light on? If you aren't getting the error light your module and airbag system should be fine.
Also, don't go poking around anything to do with the airbag circuit with a meter unless the service manual states to do it, otherwise that's asking for trouble.
Other warnings do trigger the CPU 2 buzzer however (low coolant, seatbelt, security system armed beep, etc)
The air bag lite is not on, not do I get any blinking warning lights..
But I am thinking the PO has done something.
No buzzing from the airbag module.
No plan to poke around with a scope on the module, I dont even want to be the one try removing it to see if it fixes the horn.
I am just hearing the horn a constant beep when the horn relay is plugged in and the key is out of the ignition, or the key is in the ignition and the car is not running.
When the car is running the horn acts normally.
We have pulled out the CPU2 connections, and the horn still is a constant beep when the car is not running.
If it still beeps when CPU 2 is totally disconnected then that's a head scratcher.
One thing you could potentially do to narrow down the location of the culprit is to start pulling fuses one at a time and see if the horn behaves (excluding the horn fuse, obviously)
If there is something aftermarket, or a weird short/failure going on....that might at least point you in the direction of some specific circuit(s) to look at that are putting noise on the horn circuit.
There's not a ton of OEM electronics I can think of that should be staying running/hot with the ignition off outside of CPU 2 though.
Still trying to figure this out.
Found 1 broken wire, fixed it. Horn still beeping when car is off.
Still looking for some after market stuff, cant find anything.
We have pulled the center console, the kick panel (waiting for our cpu 2 replacement module), and pulled the glove box.
Pulled most fuses but not all, still beeps.
A question for eveyone, can I just yank the fuses or should we disconnect the battery then yank the fuse then reconnect......
There are 3 things wired into the horn circuit, the switches on the steering wheel, CPU2 and the diagnostic module and perhaps some hacked wiring I have not found yet.
Somewhere these 3 control switches need to get connected together, does anybody know where those connectors are located?
It looks like on the wiring diagram the diagnostic module and cpu22 connects with X-09 and then the RH air bag module connects with s-10 (this is strange) and then
the steering wheel with s-09. then to the horn relay control pin.
I cannot find the physical location of the connectors in the car.
So finally got around to looking at this again. We pulled the instrument cluster and the air bag bulb had been removed.
Put one back in and now the air bag light is a constant.
The trouble shooting guide says check the connectors if good replace the module.
So we removed the orange and blue connectors, then removed the module and reconnected the orange and blue connectors
as I read the horn connection goes through these connectors.
Put the relay back in and now the horn works as expected.
So seems the diagnostic module is bad.
We will remove that then check the crash sensors and harness and open up the module case to see if the internal fuse is blown or there are leaky caps.
I suppose worse case we leave the module unconnected.
I read there are 2 pins you can jumper so the car thinks the diagnostic module is functioning normal, even though is been removed.
02-09-21, 01:03 PM
