I'm wanting to install an aftermarket water temp gauge in the 86. I'm wanting to use the water temp sensor on the back of the water pump housing that goes to the ECU. But cannot find the pitch/thread size for an aftermarket unit. So shouldn't I be able to cut the wires on the factory sensor and wire it up to a 2 wire aftermarket gauge? I mean, a temp sensor is a temp sensor right? If it was good enough for OEM specs then should be good enough for aftermarket?

I'm not using an ECU so no worries about that.

Or if anybody knows the thread/pitch of either factory water temp sensor ports that would be awesome as well. I read that the factory sensor under the oil filter is close to a 1/8 NPT thread but is off by line one thread and "could" work but will damage the one thread. I really am trying not to drill and tap into anything, but really want a more accurate gauge than factory.

 

Don't believe that sensor works like a normal temp sensor (if it was it would be identical to the one which runs to the factory temp gauge). The one located in the engine housing is a different story.

 

What he said. Most aftermarket sensors don't have the same wiring in the sensor as our factory sensors. You could get a replacement sensor and if you wanted also install a aftermarket sensor hooked up to a different gauge

Also I know a quick search shows people using the oem hole for an aftermarket gauge but from my learnings that's not true for most cars. Unless you can remove the engine and retap it then I'd leave the stock one in there

 

The one in the block under the oil filter is a tapered thread (british pipe thread) in my car. Some have a straight thread with a crush washer (old style).

See below info from other threads on this forum:


The 81+ FB and Early S4 cars use the sender w/crush washer you needed. For some reason, Mazda chose to use a weird thread pitch on this, M8x0.75. Partway through I believe 1987 is when they changed to the 1/8" NPT version with the same bullet connector. Not exactly sure when the change was made, but my 87 GXL (built on 11/11/86) had the same M8x0.75 sender that your car has. (CORRECTION - NOT NPT, IT'S BSPT, which is 1 thread different)

S5 & FDs use a 1/8" NPT gauge sender with a 0.250" quick disconnect type connector here.

The two-wire sensor is the Coolant Temperature Sensor, mounted on the rear of the water pump housing. Thread pitch is M12x1.5. It feeds temperature information to the ECU. On top of the water pump housing filler neck is the A/C Fan Thermoswitch that uses one 0.250" Quick Disconnect type connector. This is M16x1.5 and breaks continuity at 195 degrees (207 for S4 TIIs). S5 cars use a M16x1.5 thermoswitch that behaves normally, turning on at 207. FDs use a higher temperature version of the S5 switch sharing its connector as well.

Connectors for the S5/FD fan thermoswitches seem to be difficult to find due to desirability AFAIK. If/when I find out who the manufacturer is along with a part number, it will be added to the Second Generation FAQ.


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There are five temperature instruments on the engine:

Water temperature sending unit (meter) - located on back iron next to oil pressure sensor on drivers side. This sensor only operates the temperature gauge in the instrument cluster. It supplies a ground bias to the gauge.

Intake Air temperature sensor- located on the underside of the upper intake manifold. This sensor feeds a ground bias to pin 3L of the ECM and is used for engine fueling, idle, and spark controls.

Fuel thermosensor - located on the primary injector rail under the upper intake manifold. This sensor feeds a ground bias to pin 1U of the ECM and is used for operating the fuel pump speed relay amongst other engine fueling controls

Engine coolant temperature sensor - located on the engine coolant fill housing on the back side below the water thermoswitch sensor. This sensor feeds a ground bias to pin 3E of the ECM and is used for cooling fan normal speed operation (fan relays 2 and 4) and engine fueling / idle / and spark controls.

NOTE Fuel thermosensor and Engine coolant temperature sensor are the same part numbers and are interchangeable.

Water thermoswitch sensor - located on engine coolant fill housing on the back side above the engine coolant temperature sensor. This sensor feeds battery ground to fan relay 3 to set fan speed medium or high depending if AC is on or not. It is also a backup normal speed fan switch should a failure occur in the normal fan operating circuitry.

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https://www.rx7club.com/2nd-generati...-senor-945416/

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And from another forum:
It would be a one in a million chance of finding a gauge that would have the same voltage to use the OE sender and still be accurate. The threads on the stock sender are BSPT (British Standard Pipe Thread). Most common gauges use National Pipe Thread, so you cant even use that location.

 

Just a minor correction to PR's excellent data dump above:

Mazda did NOT switch to NPT, they went with BSPT. The difference is one thread per inch (27 v 28) and I have sucessfully interchanged them (although not proud of it).

If you install an aftermarket gauge and want to keep the stock sending unit in it's original location, there's a nice 16mm plugged hole in the lower front iron that works perfectly.

 

Yeah that tidbit wasn't right - I corrected in the lower part of it to the right info I just bought one, so researched literally this same thing.

 

Yeah, I found all of that info earlier. Was just thinking I could possibly simplify it by possibly using an OEM sensor on an aftermarket gauge. But I now know this is not the case, thanks for posting all of that though.

 

I've decided to use the stock water temp sensor under the oil filter stand. I forgot to mention while my car is an 86 model, my engine is from an 88. So I just bought a set of ProSport Evo gauges with the 1/8" NPT-27 sensor and bought and adapter that takes it from 1/8" NPT-27 to 1/8" BSP-28.

Now, one thing I did get from all of this was the thread/pitch size of the temp sensor on the back of the water pump housing. Being a M12x1.5 I've decided to use that spot for my e-fan switch, that should work pretty good don't you all think?

 

Yes, that spot will be fine.

 

It also depends on what resistance value the sensor is valued at for a specific temperature. Not all sensors are created equal and many sensors have differing resistance values along with response rates.

Some sensors read 2500ohms at 68 degrees and some will read 4000 ohms. Use the sensor that the gauge came with to remove any possible inaccuracies. The gauges and sensors are matched by the manufacturer for a reason.

 

the '88 1/8" BSPT is very rare, you should check your iron first for it before assuming you have it, because you won't get anything to fit into the previously smaller metric hole on the 87 and earlier.

i run an S5 iron on the rear for the purpose of using a NPT sensor, the 1/8" NPT fits fine, as long as it's brass and you don't crank it down like a gorilla. an adapter will take the sensor out of the path of coolant and make it sluggish to respond to rapidly increasing temps(when you really need it most).

 

the change was actually 9/1/1988, so it happens very late in the 1988 model year.

the wrinkle is that every iron made after this, gets the new thread, so if your iron was replaced with a new one, then you have the new thread

Mazda actually offers a sender for the new thread with the old temp range, and electrical connector, G607-18-510


the FC does not use the fuel thermosensor. the FC uses a different sensor than the FD does. additionally while the FD fuel and coolant temp sensors do interchange, they are not identical and have different part numbers.

 

its rather easy to tell without pulling it out, the BSPT sensor doesn't sit flush and does not use the aluminum crush washer underneath. all the early metric sensors use the aluminum crush washer and have no gap to the iron.

 

This kinda goes with all pipe threads. Severe damage can occur with any overtightened pipe thread. Even on stuff that is not cars.

 

yep, the brass pretty much conforms to the BSPT but i wouldn't take it out and put it in an NPT afterwards.

 

mods delete my post

 

Revival. Too late I saw it.

 

Dang it