Water temperature sensor: How to tap and install
 

Disclaimer: This will be overly detailed so as to try to give people more data with less questions as opposed to the opposite. The job is not hard, regardless of the detail.

Every aftermarket temperature gauge on the market requires a temperature sender to get it's readings. They work by using a variable resistor inside the sender that changes resistance based on water temperature. By sending voltage through the sender and comparing the returned voltage to the sent voltage can determine the actual water temp as the water temp varying the resistance results in a different output voltage. This is precisely how the gauge or control unit works to display the proper temperature (it has been calibrated to the response of the sensor already).

There are multiple places to mount the temperature sender such as the throttle body coolant lines, upper radiator hose, engine block, or thermostat housing. The goal is to obtain a location where one can have accurate readings but more importantly readings which are capable of a high rate of change. Ideally the best place would be in the water pump housing or in a block coolant passage right after the thermostat. Unfortunately due to physical design issues of the thermosensor and clearances, the water pump and/or block are a bad idea.

Almost all of the sensors out there are 1/8 NPT size threaded probes with the electrical harness on the external portion.

Pros/Cons of locations:

1. Throttle Body coolant lines.

Pros: Easy to do. You can use a freely available tee-line with a 1/8 NPT tap in the tee to install the sensor. Reasonably accurate block coolant temperatures regardless of the thermostat state.

Cons: Slow rate of change. Less room for installation when the TB coolant lines are commonly bypassed. No advantage over the other locations except for ease of installation. Due to the flow of coolant through the block, this is really only showing you temperatures indicative of coolant passing through about half the block, post-radiator cooled. Grounding issues.

2. Upper radiator hose.

Pros: High rate of change when the thermostat is open. Harder to find a tee-line adapter for the size of the radiator hose compared to the throttle body.

Cons: Somewhat useless when the thermostat is closed as the readings are more stagnant. This can be an issue when one has a stuck thermostat. In addition we are really interested in the sum of block AND post-radiator temperatures which this location isn't best for. Grounding issues.

3. Thermostat housing.

Pros: High rate of change when thermostat is open. Medium rate of change when thermostat is closed. Since the upper radiator hose is the one delivering coolant to the radiator, as regulated by the thermostat, changes in block coolant temps. are fairly immediate. No grounding issues.

Cons: Requires removal, drilling, and tapping. Stuck thermostat is still an issue, but less so due to the proximity of the sensor to the thermostat itself - basically you'll know when things are wrong before you warp the block.

Since the goal is to do things the right way and monitor coolant temps with the most accuracy and highest rate of change one should mount the thermo-sensor in the thermostat housing. It's a good position as it lets you gauge the rate of change in coolant temps _exiting_ the block, but pre-radiator. If you are seeing a fast rise from 185 to 220F, you should know fairly quickly that there is a problem and to reduce the load on the engine or go full heater/fans and shut it down.

---

Now I know the concept of "drilling and tapping" is scary to some, but it's really not that hard at all. When it comes down to it you're basically just drilling a hole and threading it so you can screw something into it; in our case, the thermo-sensor.

All of the tools required are available at any place selling halfway decent tools. I got mine at Sears as Home Depot and Lowe's have fairly bad selection when it comes to individual taps.

You will need:

1. 1/8-27 NPT pipe tap.
2. 11/32" drill bit.
3. Tap wrench.
4. Some kind of cutting lubricant (or kerosene).
5. Teflon tape.
6. Liquid gasket (I used some Hondabond I had sitting around).

All together it's about 15$.

http://www.anodized.com/~clayne/fd3s/tools_s.jpg

A good place I found to try a practice tap was in my stock Mazda scissor jack. Since it's a solid piece of aluminum you can tap the center of the bottom to do a decent test run. It won't harm the integrity of the jack.

Actual directions:

1. Drain a good amount of coolant from the radiator.

2. Remove thermostat housing by removing bolts. One bolt is by the airpump, so if you want to do this easily, loosen the alternator belt slack and use a long extension to get to the bolt. Otherwise you're stuck taking off the airpump which is an unneeded hassle. The gasket may make the housing stick to the block in which case some gentle prying or rocking back and forth will break the seal loose.

3. Remove thermostat and drill a hole using the 11/32" bit through the flattened out area (you can see it in the picture) right in front of the lower bolt location. Aluminum is soft and drilling through it is easy. Make sure to only drill through that one side and not all the way through both sides of the housing. We only want a tap, not a skewer mount for the t-stat housing. :)

4. Using the tap and the tap-wrench, being rotating the tap into the hole we just drilled, making sure to put cutting lubricant on it as you go. Initially the tapping will be easy, but as it gets harder back the tap out every hard turn about a turn or so to remove the metal shavings from the tip. Keep going until you have about 10% of the tap left showing. This is enough.

5. Throughly clean the housing in a sink and blow air through it to remove any metal shavings/fragments left.

6. Wrap one-half of the sensor or sensor mounting fixture (if DEFI) with teflon tape, leaving some uncovered metal for proper grounding. Screw it in to your new threaded hole and tighten until it's very snug but don't over do it as it is aluminum. If it's a DEFI sensor you should put teflon tape on one-half of the actual sensor itself as well, since this screws into an adapter which screws into the t-stat housing.

7. Apply liquid gasket to the mating surfaces of the housing and the mating surface around the rubber gasket of the thermostat. Secure the t-stat back into the housing and bolt the housing back up to the block. Wipe off liquid gasket bleed and let it dry for about 15 minutes.

8. Attach thermo-sensor to gauge or control-unit (if DEFI).

9. Replace lost coolant.

10. Check for leaks.

11. Start up car and blow out your o-rings. Just kidding. Do standard cooling system bleeding procedure.

http://www.anodized.com/~clayne/fd3s/sender_s.jpg

You now have a gauge which will show cooling system effeciency under average driving and track conditions. In addition, it can also save your engine's ass from overheating.

You should see around 180-185F (t-stat limited) while cruising and increasing temps under heavy load. The key here is to be proactive about rate of change in temperature. Don't do something when you see the temperature hit 250F. Do something when you see the temperatures rising quickly past 210F BEFORE they hit 250F.

just coppied this to the archives. nice post. ;)

Re: Water temperature sensor: How to tap and install
 

That completely disagrees with my findings. I can see water temps climb 20 degrees in a matter of seconds everytime I launch at an autox. They also drop just as rapidly when the fans come on or when I spray water on the radiator between runs. Temp readings on the gauge itself prove to be accurate and completely in step with all the stock ecu coolant temp sensors as my aftermarket gauge temp readings are always perfectly in step with the fan speeds.

Certainly there is less coolant flowing through there than the radiator hoses, but that does not automatically mean there is not enough flow for the temp sender to read properly. My findings shows the slow rate of change idea in the throttle body coolant line to be completely false.

As for only showing temps in "half the block" what location will show temps anywhere other than the point where the sensor is?

FWIW my opinion is that I wish to know coolant temp inside the block. I don't care if the radiator, thermostat housing or hoses are 500 degrees and on fire; if the block is cool than the engine is safe. For the same reason I don't care how hot/cool the water in the radiator hoses is either. The best location is in the block where the stock gauge sensor is as that doesn't require extra "plumbing" like the tb lines do, but I am not ready to remove it at this time.

Great info here!

The t-stat housing location is at the end of the coolant cycle coming out of the block and will give you most indication of overall coolant heat content exiting the block. The hottest temps (excluding localized hot-spots) will be happening here, as this is the last point before being sent to the upper tank of the radiator for cooling. There is no single location that is the best since the concept of just a single temp. sensor is a compromise in itself. The best is two locations, one post-block, and one post-radiator. But if one had to choose any single "best" location, I think the one which will read the highest overall block heat and still have fast deltas is the way to go - which is the t-stat housing.

I used the same place.

Only had to remove the IC, drain fluid, and pull off the hose going to the top of the rad. Any tapping fillings fall into this pipe. All I had to do was flush from above.

This was a lot easier than pulling the housing.

I too use the throttle body coolant line and find the "rate of change" is comparable to the stock sensor.

its a 15 minute job to install in the throttle body coolant line and just as accurate.

either way, exceptional write up.

My only issue with the t-stat housing is that everyone uses the front of the housing which is after the t-stat. If the thermostat fails you're screwed.

I would love to tap the rear of the thermostat housing (behind the thermostat) where the stock coolant fan switch is, but I don't see room :(

You're not exactly screwed. If the thermostat fails I guarantee you you'll see the temperature start increasing with enough lead time to do something about it - due to the proximity of the sensor as mounted in the t-stat housing.

But here's an example of why, in my opinion, the TB coolant lines are a bad idea:

1. I'm not 100% sure of this as I do not have my FSM on me right now, but I believe the TB lines are routed seperately to and from the block utilizing additional block orifices - which is not main coolant flow. Now under water pump pressure, coolant is being cycled through these lines, but if you think about it, it's sort of iffy as you're almost getting water temp readings which are guilty by association with the main coolant heat levels :).

But a more important reason would be that this location is not at the end of this closed-loop cooling cycle. Let's pretend for a second that we had a larger than average hot-spot that changed with engine load. Let's also pretend that larger than average hot-spot is located directly on the exhaust side of the block. You probably see where I'm going with this. Since (and this will be verified) the TB coolant lines are a: not connected in serial to the overall system, b: most likely receiving their coolant before this "hot" point in the coolant flow path you end up seeing readings which may not be the most accurate, even if their rate of change is comparable. Even worse is that this coolant may also be hitting the TB section right after being cooled by the radiator.

One knows for a fact that the t-stat housing point is going to catch the coolant with the highest heat content resulting in the highest coolant temperatures visible.

Nice write up.

I installed my sensor in the upper radiator hose. I used a Greddy 38mm radiator hose adapter. It can be purchased from any Greddy dealer for around $30.


My VDO sender fitting did not match the Greddy hose adapter, so I had to drill and tap it. Not all senders have the same size fittings.

Movin back up as I just used this because I was having a hard time finding the right tap for my Defi D Series gauge. The tap worked perfect and the install was a breeze once I figured that out.

Ok, this is an old thread but I have a question relating to it about the water sensor shown on the install. I have a "Stock" late 94 and it has this sensor on it at this location, slightly different model sensor. I do not have another guage other than the factory unit. Was this added to later model 7s or did someone add this and the guage is missing? I am 1000 miles from the car so can't check anything and don't know where that wire goes until I check it when I get home.

Thanks.

The only factory 7 that had anything in that location was the automatic. If you have any kind of of a sender it is from an aftermarket setup.

Man, bringin' threads back from the dead! :)

To throw my 2 cents in - I am VERY opposed to installing the temperature probe in the thermostat housing. Throttle body coolant line is the way to go, guys. Fast rate of change, fully accurate readings, easy to install, easy to reverse.

I have personally tested both locations. If you want to look like a dummy that can't hook up a gauge with a water temp gauge that reads nothing until the thermostat opens, then the gauge pops up, go ahead. If you have a faulty thermostat, you might not know it until too late. And, it's not reversible, requires FAR more work, etc. etc. etc - PLEASE do not drill/tap the thermostat housing! It's a POOR idea all the way across the board.

Dale

Yes, I have an automatic and it is late 94. So Do I have a factory sensor and is it suppose to go to something and be hooked to something, and what? It looks just like the add a guage post here, and it is suppose to be "stock" and it was broken and I fixed it. What was it suppose to connect to? if it is suppose to be there? When it was unhooked it did not seem to make a difference after I fixed it, and hooked it back up(To whatever). It is difficult to see, where it is located, so I would think many people can not even see if it is there. I personally wouldn't mind hooking up a good guage to it(though if it is wrong I would install a sensor in the TB, but not if it has another function.

Being an automactic this is probably not a high interest area but I would like to know.

Thanks,

That's a temp sensor for the tranny. I'm not 100% sure of the exact function, but I think it changes how the tranny shifts with coolant temps. It's a Good Thing to have.

Hit up the shop manual for more information.

Dale

Thanks Dale,

I just ordered one from Ray, we'll see if it makes a difference in whatever.

Like Dale said it's for the tranny ECU (I believe the automatics have there own ECU for the transmission and it should be hooked up.

What kind of adapter did you use in the TB coolant line to hold the temp sending unit? I've been looking online and I can't seem to find an inline adapter small enough to fit that hose.

---->https://www.rx7club.com/showthread.p...highlight=temp
Post #36 has the specifics and pics.

Thanks Sarge, but I now I'm wondering if I have bigger probs...

Hello all,

After I have read this tread, I have a question, my mechanic installed my new temp. sensor to my t-stat housing the other, making it ready for installation of the wires etc. but on the t-stat housing itself there is 2 plugs, one on top of the housing and one in the side of the housing, he has removed the plug in the side and screwed the new sensor in to that thread, is this completely wrong ?? as I can see from the post/pictures all choose to drill into the front of the housing, make new thread, and install the sensor there, is this the only place it should be put on the t-stat housing ?? thank you very much.

Rene
DENMARK

My Greddy temp sender fits the stock location on the rear iron near the oil filter pedestal.

I know this is a SUPER old thread, but I wanted to show where I just put my aftermarket coolant temp sensor. I have Spa Techniques gauges and these sensors are small enough I found a place to mount it on the rear of the water pump housing (near the two other sensors) and still have space.

Last night, using an extra WP housing to figure out if it will work...
http://i44.photobucket.com/albums/f9...3/2a202c64.jpg
http://i44.photobucket.com/albums/f9...3/2dc0fcd9.jpg
http://i44.photobucket.com/albums/f9...3/c696abf1.jpg

Where the sensor would sit on the current WP housing...
http://i44.photobucket.com/albums/f9...3/3d262410.jpg

Tonight I drilled and tapped the extra housing...
http://i44.photobucket.com/albums/f9...3/635c8967.jpg
http://i44.photobucket.com/albums/f9...3/f7aeb032.jpg

Worked pretty well, so I drilled and tapped the actual housing and then test fitted it...
http://i44.photobucket.com/albums/f9...3/0a0a9698.jpg
http://i44.photobucket.com/albums/f9...3/e0cc17e4.jpg

Another good part about this is that I don't have to ground the sensor with an extra wire.

This sensor is 1/8NPT. The last drill bit I used was a 21/64". With the sensor fully tightened down, it puts the tip of the sensor in the flow of the coolant but not enough to block it any.

nice! Only problem with that is you have to remove the entire water pump housing but it makes for a cleaner install.

I'd like to bump this sentiment, installed the sensor as Dale recommends and cannot imagine installing otherwise! Just my two cents.

The only downside with the throttle body line is if you use a 1-wire temp sender that grounds through whatever it screws into. You can add a ground wire to the brass T and ground it, though.

This is a good option if you have the water pump housing off already and want to do it. The thermostat housing is still dumb any which way you look at it.

Also, I'm just not keen on 1-wire temp sensors, that's just a lazy design.

Dale

Clean setup on the waterpump housing, but is there any noticeable reading difference between the 2 location?

+1...on all points. And nice job Speedjunkie.
No difference between readings from the waterpump housing and the throttle-body coolant line. That line goes into the back of the waterpump housing. (see "from TB out to WP" below)

http://i1126.photobucket.com/albums/...antdiagram.jpg

From this diagram, it looks like the coolant flows in the upper drivers side of the engine, and out the lower passenger side. It appears that the coolant to the throttle body coolant line escapes the engine before being completely heated by the passenger side of the engine.

If I am seeing this correctly, this would explain why my TB coolant line sensor is always several degrees cooler then my PFC reading.

Yeah it's quite a bit more work to install it this way, but I'm doing other major installation things anyway so I figured why not. Thanks!

That's where I had it installed this whole time, but I had to ground this new sensor and I'm going to use that coolant routing for the new wastegates and figured this was just easier.

Yeah it's a pain to have to ground this thing, but I like that these gauges take up less space being dual gauges. When I first installed these, I didn't realize I had to ground it and I was having all kinds of problems with the gauge reading correctly. I grounded it with a wire like you mentioned and it still didn't fix anything. Turned out to be a bad sensor.

Thanks! I'll have to see once I run it, although with the variables of ambient temp and all that, I may never really know, especially if it's just a couple degrees difference. It's been warmer lately than it was when I put the car down a week ago.

Thanks!

That's how I'm seeing it too. I figured if flows as the diagram shows (and it should), I should see hotter temps at the back of the thermostat since it flows through the exhaust side after the TB line. We'll see I guess.

It flows to the throttle-body coolant line right after passing BOTH combustion chambers. It just doesn't pass through the exhaust side of the housings.
I bypassed the T-body some years back but still use the line for my gauge sensor. Your experience is opposite of mine. The gauge is consistently in complete agreement with the PFC. If anything the gauge (DEFI 'D' Series) reacts a hair quicker.

Jim,

Glad your temps correspond. Mine don't. The exhaust is the hottest portion of a rotary engine. It seems to me that the coolant would be heated even more on the exhaust side.

Adam

temp sender location
 

Since we are replacing our factory W/T gauge with something more accurate can I just use the location of the factory sending unit and screw in the new sending unit there?

Joe

Depends on the thread pitch. I think the stock one is M10x1.0 and sensors tend to be 1/8NPT. But you can use this...

https://www.pegasusautoracing.com/pr...asp?RecID=1832

Good observation the TB water is taped out of the block before real heating from the exhaust side of block. Makes sense, mazda wanted to pre-heat the TB for anti-icing not super heat the intake air.

Are we sure the exhaust side is hotter than the combustion side? I ask because I've always seen the modifications for cooling done on the plug side of the housings, not on the exhaust side. And after the combustion event, which side of the housing (referring to the diagram) has the most prolonged contact with the hot exhaust gases?
Intuitively I think it should be the exhaust side, and certainly it's last on the coolant flow, but....

Another factor is that the turbos are bolted to the exhaust side, which would tent to heat that side of the engine even more.

Water Temp gauge
 

I have been reading this thread and no one mentions installing the new temp sender in the old temp sender location. Since we are not using the old gauge why isn't this the easy way to accomplish hooking up the new gauge?

Joe

I'm still using my stock gauge as well. I don't look at it as much as the aftermarket one, but in case that one fails I can at least see something with the stock one, even though it's not great. That's why I ended up choosing not to put it there. I had thought about it though.

i just bought the prosport rad hose tap for $25. Nice write up im too chicken to attempt this.