ZewahTenao

Hey everyone,

Idiot box is giving me engine air thermosensor fault code. The part number is n326-18-845, but unfortunately everywhere I look online seems to indicate that it's been discontinued and out of stock.
The car is a 90 GXL NA. It runs fine but does have some minor, intermittent idle hunting, particularly when it's cold or damp outside.
TPS and idle are set, I don't have any vacuum leaks at the top of the intake but I'd like to replace the manifold gaskets if I'm going to access the thermosensor.
Does anyone out there have this part?
Looks like there are no wires on the part itself, just prongs. I've read that in the SA/ FBs those units do have wires that can snap over time with heat and vibration and can be soldered back together. I'm just looking to plan things out a little before taking the manifold apart. FSM says to test and replace if necessary but if I'm going there anyway it sure would be nice to have a spare on the off chance I end up not needing it if it can be cleaned/ re-secured. Lord knows parts are hard to come by these days.
Thanks!

ZewahTenao

And yes, I do realize I'm kind barking up a dead tree, as this thread from last year dealt with the same issue. Just taking another shot in the dark lol.

Air temp sensor? - RX7Club.com - Mazda RX7 Forum

Hot_Dog

There's been a lot of discussion on this issue in the past.

https://www.rx7club.com/single-turbo...ensor-1137070/
https://www.rx7club.com/power-fc-for...sensor-987045/
https://www.rx7club.com/single-turbo...ation-1144689/

Some folks have suggested using a Triumph "Fast Reacting" IAT Sensor.

ZewahTenao

Oh, from reading one or two prior threads, I thought that the Triumph one only worked on FDs? Worth looking into a little more, maybe I misread something. Obviously we're pretty deep into getting creative/ fabricating solutions for odds and ends. I'll go back over those threads.
I guess I should just be satisfied that my TPS is still good at this point, it's kind of scary that even things like vented rear calipers are basically impossible to find nowadays.
I just hate looking at a damn CEL, right now I've got the idiot box unplugged, checking codes once in a while just to see if something has changed. In the meantime I've been slowly hoarding parts, the thermosensor is one I just kind of want to keep scouring for just in case.
Cheers, thanks for the suggestion. Didn't mean to be irritating beating a dead horse.

Hot_Dog

Sorry I can't be more helpful. I've owned two RX7s, an '86 and a '91, and only had one of these sensors go bad on me in the last 36 years. They appear to have a low failure rate. That's probably why Mazda discontinued selling them. Maybe you can find one on Ebay? But, you may need to buy the entire intake just for the sensor. I agree that parts are getting more difficult to find for these cars as they are on the verge of extinction. Certainly, one will need to become more creative to keep them running. Good luck!

Hot_Dog

I've got a little bit more information to contribute to this topic.

After searching related threads regarding thread size:

Aaron Cake wrote that the IAT thread size is M10x1.0, which is the same as the oil pressure sending unit and the coolant temperature sending unit.

However, threads on oil pressure sending unit thread size were generally agreed on as 1/8" BSP (British Standard Pipe).

RotaryEvolution wrote that the IAT thread size is 1/8" BSP. The diameter of 1/8" BSP is 9.7 mm with a thread pitch of 28TPI, which translates to thread spacing of 0.91 mm.

So, who's right? I don't know, but thread size and pitch are so close that you can probably interchange devices between the two thread sizes with doing a great deal of damage if the threads don't match up.

Regarding the electrical characteristics of IAT sensor:

The S4 FSM gives these specs for the IAT resistance:
20C (68F) 41.5 +/- 4.15 kohm
50C (122F) 11.85 +/- 1.19 kohm
85C (185F) 3.5 +/- 0.35 kohms

The S5 FSM gives these specs for the IAT resistance:
25C (77F) 33 +/- 4 kohms
85C (185F) 3.5 +/- 0.4 kohms

Most likely, Mazda used an Amphenol NTC Type MS type RL0503-17.56K-96-MS thermistor for the S5 IAT sensor (https://www.amphenol-sensors.com/hub...032114-web.pdf). This thermistor has Ro@ 77 °F (25 °C) (ohms) of 30k and uses a F9.61 materials system. You can buy these thermistors through Digi-Key for about $3 each.

I have no information on an appropriate S4 IAT thermistor. It most likely wasn't made by Amphenol as they only have thermistors in the 30 and 50 kohm ranges.

So, since the Mazda S5 IAT (N326-18-845) is NLA, you can buy a 1/8" BSP plug and thermistor listed above and fabricate a suitable IAT sensor yourself, if you so desire.

If you're lucky, you may be able to find a similar replacement from another make/model car. There were some listed for other Mazda models (i.e., 323, 626, etc.), but they were different thread size M10x1.5 and IDK if they were electrically compatible withe the S5. I'm sure there is at least one out there somewhere that is compatible both mechanically and electrically. Most of them that I came across use either a 3/8" or 1/2" NPT or a flange.

Alternatively, there were a few aftermarket brand IAT sensors like AEM (35-2014) and Link (IAT1-8) IAT available in 1/8" BSP, but the AEM had resistance values that were about an order of magnitude lower that that of the S5. The FD IAT sensor typically uses resistance values about an order of magnitude lower that that of the S5. I wasn't able to find any electrical specs on the Link IAT. Ballenger Motorsports and Summit Racing have available a variety of IAT sensors in 1/8" BSP, but not sure the resistance is compatible with the RX7 IAT sensors.

j9fd3s

iTrader: (3)

i think Hot Dog is right, there are kind of two type of sensor, and the FD is the other one. unfortunately the tool i have doesn't list the sensor type, but it can sort by some of the other attributes
https://www.opticatonline.com/search...a_90=_Screw+In

ZewahTenao

This is fantastic information, thank you! I very much appreciate you taking the time, it gives me a good base to build on if it really comes to fabricating my own thermosensor, which I'm REALLY disinclined to do because of the aforementioned thread size/ pitch question. However, having that starting point is important.

I've found out that Mazda used the same part in contemporary B2200 pickups, 929 sedans and MPV minivans. The likelihood of finding an S5 (or 929 for that matter) in a junkyard seems pretty low, but coming across one of those trucks or minivans could be my saving grace.

I'll keep hunting, thanks again!