# ECU confusion



## 86_300zxturbo (Jan 22, 2005)

I have read everything I can find on ECU diagnostics. I have a Chilton's book, but they don't even mention the ECU (DON'T buy a Chilton's book).

I have read at least three different procedures for doing the diagnostics, and they're not even really that close. I just did the simpliest procedure I found and got three codes: 14,23,31. 

My next problem is that I have found many different code listings that say different things also. For example, the cover of my ECU says that 31 is the air conditioner, but the sticky on this site says that 31 is an ECU malfunction. What gives?

And for the bonus question: what is a chts and what does it do. I have many messages about this thing, and have even seen a picture of where it is, but that's all I know about it. 

Thanks,

Rob


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## Crowbar60 (Jan 22, 2005)

CHTS is cylider head temp sensor. Its a thermistor that changes resistance with temp (resistance goes down as temp goes up). Its very important as far as getting a car to start and run correctly (and they fail often it seems).


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## Crowbar60 (Jan 22, 2005)

I remember now that theres a procedure to clear out that AC code. Top of my head, you need to have the car not running. Turn ign switch on so ecu lights up. get into diagnostic mode. You will see the AC code (trust the codes that are listed only on ecu and in a real field service manual). Push in AC button and turn on fan. Floor accelerator all the way. This should leave you with the only 'real' error codes about your car.


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## Crowbar60 (Jan 22, 2005)

http://zdriver.com/forum/showthread.php?t=6233

check out this thread. more guys that dont know about the AC thing.

The procedure should clear out the AC 3-1 and the throttle switch 2-3 (when you press the pedal, it checks that the switch is working). That should leave you with only the real error code 1-4. What is that on the ecu?


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## Zen31ZR (Mar 24, 2004)

You did not apparently read the first part of the sentence on the #31 code, it says _load signal circuit_. I beleive this has to do with the A/C. A lot of manufacturers list _ECU malfunction_ as an alternate to several codes, it just means that if many or all of those codes come up that it's possible the ECU has an internal problem.


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## Zen31ZR (Mar 24, 2004)

14 is _vehicle speed sensor_. I beleive thats the pulse converter mounted on the passenger side of cars equipped with the digital dash. There is a normal speedo cable that comes up from the transmission and hooks to that, from there wires run to the dash speedometer display. If that's gone, might as well convert to an analog dash, it's a horribly expensive part to buy, unless you can find a used one somewhere.

EDIT: Theres also a VSS on analog cars, but the ECU is the only one that uses it, mostly for the top speed limiter function. It is also used to display the trip and distance to empty miles if you have those functions.


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## Zen31ZR (Mar 24, 2004)

The VSS differs in both form and function between digital and analog dash equipped cars. In the analog one it's a pulse unit as well, but the function is much different. In the digital dash cars it's internal function is very much like a distributor crank angle sensor......

All of these questions could have been answered by refering to the FSM. That's one big reason why it's included in the stickies.  I would suggest downloading it and getting familiar.


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## Crowbar60 (Jan 22, 2005)

I would suggest ONLY using the codes on the ECU and the FSM from the year car you are trying to troubleshoot.


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## Crowbar60 (Jan 22, 2005)

http://pages.cthome.net/bbachinsky/nissan_ecm_codes.htm

Heres a list of comprehensive codes. Still, I would defer to the FSM FROM that year car. Do not apply all info from an 88 FSM to 84-86 cars.

Note: 31 1984 through 1986 EFI models; Problem in air conditioning system; all other models: ECU internal problem


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## Zen31ZR (Mar 24, 2004)

Crowbar60 said:


> http://pages.cthome.net/bbachinsky/nissan_ecm_codes.htm
> 
> Heres a list of comprehensive codes. Still, I would defer to the FSM FROM that year car. Do not apply all info from an 88 FSM to 84-86 cars.
> 
> Note: 31 1984 through 1986 EFI models; Problem in air conditioning system; all other models: ECU internal problem


 Read exactly what it says for code 31, for the turbo models.  The code for the A/C simply means the ECU has not received an ON/OFF signal from the A/C after the ignition switch has been turned on. This on the _ET (turbo) models only_, the NA cars are different........


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## Zen31ZR (Mar 24, 2004)

All other indications for code 31 as on the NA cars, is no signal received from the power steering pressure switch, the rear defroster, heater fan switch, electric radiator fan control, lighting system, _and_ the A/C system.


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## Zen31ZR (Mar 24, 2004)

So, if you can, avoid the Haynes manual too, they are the ones that specify an ECU malfunction for the 31 code.......

I wrote that sticky back when I had nothing but a Haynes manual, I'm going to have to rewrite it from the FSM now, as it appears the Haynes is in error in several areas.


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## Zen31ZR (Mar 24, 2004)

Crowbar60 said:


> http://pages.cthome.net/bbachinsky/nissan_ecm_codes.htm
> 
> Heres a list of comprehensive codes. Still, I would defer to the FSM FROM that year car. Do not apply all info from an 88 FSM to 84-86 cars.
> 
> Note: 31 1984 through 1986 EFI models; Problem in air conditioning system; all other models: ECU internal problem


 And actually the FSM doesn't indicate any ECU malfunction with that code, so we can strike that. Time to rewrite the code sticky.....


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