Water temp gauge t-shooting, advice needed (3 Viewers)

[procomsignathid]

Hello all- I am t-shooting a high temp gauge after replacing my radiator. During that process, lots of things changed:

• New radiator
• New hoses
• New temp sender (used the one from City Racer, I also used teflon tape which I now know is a no-no)
• Added a thermostat (was none there before), 180-degree
• Added the stock heater back in (stock routing, no rear heater)
• Flushed fluid and changed from green to red (this stuff)

For context, this is a 1978 FJ40.

Despite all this, my temp gauge has been consistently on the 3/4 mark of the gauge. I got ahold of a laser thermometer and it looks like the output pipe from the block never gets above around 185 degrees. Temp on the gauge seems to cycle between just above the 3/4 mark to just below, so I believe the system is functioning in terms of the thermostat opening at the right temp. Haven't boiled anything and no leaks.

When I ohm out the current temp sender to ground (when the truck is at operating temp), I get anywhere from 19 to 65ohms, jumping all over the place. If I set the multimeter to continuity, it beeps occasionally. I was thinking the issue may be with the teflon tape I used to seal the threads, but if anything I would expect that to increase resistance, not lower it. Additional searching on that point suggests that the tape will end up sitting in the valleys of the threads and likely won't cause a continuity issue, I also don't see any resistance between the temp sender body and the block. That said, I'll use threadlocker next time.

So, I ordered a new temp sender (Toyota OEM part # 83420-20011), but haven't installed it yet. I'm suspecting that my current temp sender is faulty or I did something stupid when I installed it. Before I do all that work, I figured I would test the actual gauge in the cluster to make sure it's calibrated. I set up a breadboard with different combinations of resistors to get me the values called out in the table in the FSM, turned the truck on and validated the gauge ended up where it ought to. Everything looks good except the high-setting. 25 ohms barely pushes it over the 3/4 mark. Doing some testing, I found that the gauge needs about 15 ohms resistance to hit the max setting.

I'm not sure I can fix the gauge for the highest setting (maybe it's fine, eh). But I would like to fix the temp sender. So my next steps are to drain a bit of the coolant, swap out the temp sender for the OEM part (with thread sealer), and see whats up next. For those experienced with this stuff, am I skipping anything important? Anything you'd do differently? I want to get the stock gauge working as best as possible ideally.

Here's some pics of the testing setup:

 

[Mark Jennings]

Do you have a 14 PSI radiator cap? At 14 PSI you would be good to 254 degrees. This temperature is based on the cap, not what the engine will run reliably at. If you’re testing by driving the rig, the stock radiator is basically designed to keep the engine 100 degrees F above ambient with the T-Stat fully open. So on cold days like below 50, the T-Stat will partially close to keep the coolant system at 180. On a 90 to 100 degree day, under load, you should see 200 (maybe 210) degree coolant temperature…..and when you cruise into town, with stop and go ‘after’ your ‘under load’ run, you’ll see temperature increase, many as high as 220 (due to lower airflow) before you see a temperature decrease. However, you should see a decrease….after a while.

 

[procomsignathid]

Ill check the cap, got a new OEM one from SOR.

 

[procomsignathid]

I checked my receipt from SOR, and the cap itself, aside from trusting that it is a new OEM one for my year (looks like its the same from 75-84), I can't find much in the way of identification on the cap itself, aside from a "0.9" stamp on it. No idea what PSI its rated for. Here's some pics for those who are experts at radiator cap identification

I realize I made a small error with my resistance testing on the gauge, I strung together two 10-ohm resistors and a 4.7, should have done 10+10+1.5+1.5 (according to the FSM its 23 ohm, not 25). That said, it still wouldn't have moved the needle all the way to the right, as that required around 15 ohms.

So what do y'all think? My plan is to swap the sending unit next, unless Im skipping a step here. Has anyone had luck in calibrating their gauges?

 

[charliemeyer007]

My 72's temp gauge reads between 27 ohm pic and the 50 ohm pic. I think I put in a new sender in back in the late 80's. In the winter I block of the lower half of the rad with a curtain so it will heat up. I built a fluid heat riser or the intake to solve carb icing on foggy days. My gas gauge reads past the full peg for a long time - E is really empty 1/2 is sort of correct. I think there is some sort of voltage regulator in the amp meter that regulates power to like 7 V to the other gauges. I put in a direct read oil pressure gauge because I got tired of the wonky oil pressure even after putting in a new sender. Speedo lies and now my odometer quit working - it was my best gas gauge. I'll see if I can fix the odo after I do the carb and dizzy.

When I put in my aux tank in 1984 I was able to use some low ohm 2% resistors in line with the Datsun B210 tank sender. Flick the switch on the dash and the aux tank would read on the dash gauge from E which was empty to 1/2 which was the full 12. 5 gallon the tank held. The aux tank now leaks so I pulled it. I might try and fix it or even better make a new 22 gal tank to fit behind the rear axle.

 

[procomsignathid]

My 72's temp gauge reads between 27 ohm pic and the 50 ohm pic. I think I put in a new sender in back in the late 80's. In the winter I block of the lower half of the rad with a curtain so it will heat up. I built a fluid heat riser or the intake to solve carb icing on foggy days. My gas gauge reads past the full peg for a long time - E is really empty 1/2 is sort of correct. I think there is some sort of voltage regulator in the amp meter that regulates power to like 7 V to the other gauges. I put in a direct read oil pressure gauge because I got tired of the wonky oil pressure even after putting in a new sender. Speedo lies and now my odometer quit working - it was my best gas gauge. I'll see if I can fix the odo after I do the carb and dizzy.

Charlie you just turned a light bulb on in my head... I skipped a step in the FSM and didnt get for proper voltage / resistance on the back of the cluster. I've also been hearing buzzing from the voltage regulator... Might be one of my culprits. My fuel gauge never goes all the way to full, ha.

 

[charliemeyer007]

If you look around there are some threads about the regulator in the amp meter.
All the gauges IMHO are sort of basic as in they read in the vicinity. The J3 Cub I learned to fly, gas gauge was a cork float with like a coat hanger poking threw the cap. Lots of coat showing was lots of gas, If the 90 degree bend in the rod was getting near the cap you were low on fuel. Pro Tip if you think you might need carb heat turn it on, because if you wait until you are sure you need carb heat it might be too late to work

 

[pb4ugo]

Someone posted this yrs ago.

I run an sbc and use the stk gauges in the cluster. My oil gauge seemed to run low so i temporarily installed a mechanical oil gauge to compare the two. I should do the same for the temp gauge. It's not a priority because there's no indication that it's even close to running hot.

 

[Mark Jennings]

The 0.9 stamped on the cap is 0.9 Bar or close the 1 atmosphere, and that = approximately 14 PSI. So you have a 14 PSI cap.

 

[procomsignathid]

The 0.9 stamped on the cap is 0.9 Bar or close the 1 atmosphere, and that = approximately 14 PSI. So you have a 14 PSI cap.

Ah, that's good to know. So re-reading your post above about how I should see a decrease after awhile, it really only ever gets as low as the the pic I showed at 27 ohms (maybe a smidge under the 3/4 mark, let's call it 5/8). So I think next I'll go through the steps of checking the back of the instrument cluster on the points the FSM recommends to check voltage / resistance, and if that checks out I think Im left w/ changing out the sender.

 

[procomsignathid]

I went through the FSM steps (2.1, 2.2 and 2.3), and found the following:

• Step 2.1 (check battery voltage at terminal A): pass.
• Step 2.2 (check for varying voltage between 2v and 7v at terminal B): fail. I do see varying voltage, but its from 2v to battery (~11.5v)
• Step 2.3 (check resistance is 25 ohms between terminals B and C): fail. Resistance is around 25ohms, but occilates between that and open circuit.

Unfortunately, the FSM does not say what to do if these things don't pass the check. Does that mean the voltage regulator is bad?

EDIT: This aftermarket regulator couldn't possibly be the problem, right!?

 

[Mark Jennings]

The Motocraft solid state regulator won’t have the same values as the ‘coil wound’ regulator…..uses a transistor for switching, not contacts.

 

[procomsignathid]

The Motocraft solid state regulator won’t have the same values as the ‘coil wound’ regulator…..uses a transistor for switching, not contacts.

Not sure Im following… would this result in what Im seeing or am I chasing a red herring? (Not sure what the regulator does)

 

[GA Architect]

Someone posted this yrs ago.
View attachment 3935536

I run an sbc and use the stk gauges in the cluster. My oil gauge seemed to run low so i temporarily installed a mechanical oil gauge to compare the two. I should do the same for the temp gauge. It's not a priority because there's no indication that it's even close to running hot.

I'd say that temp gauge pic @pb4ugo posted is accurate, see below for my '77 FJ40's OEM TEMP gauge & Sniper's temp gauge. Like yesterday, with the outdoor heat index over 100F and over 20 minutes of driving, the highest my Sniper read was 186F.

 

[Mark Jennings]

I'd say that temp gauge pic @pb4ugo posted is accurate, see below for my '77 FJ40's OEM TEMP gauge & Sniper's temp gauge. Like yesterday, with the outdoor heat index over 100F and over 20 minutes of driving, the highest my Sniper read was 186F.

View attachment 3935866

Not being familiar with the Sniper, where dose it pick up water temperature. Is it at the back of the head, as the original temperature sensor?

 

[GA Architect]

Not being familiar with the Sniper, where dose it pick up water temperature. Is it at the back of the head, as the original temperature sensor?

I have my Sniper's water temp sensor in the rear block drain location.

 

[procomsignathid]

My gut tells me that the buzzing I hear and the strange values Im seeing on the back of the gauge could suggest swapping out the regulator for an OEM one (looks like CityRacer sells the old 3-wire one I need, as mine is a 10/77 model). I figure I'll swap the temp sender first since I have that, and then maybe tackle the regulator after the fact. Based on y'alls experience, would you suggest another direction? I don't really want to add any additional gauges.

 

[Mark Jennings]

My gut tells me that the buzzing I hear and the strange values Im seeing on the back of the gauge could suggest swapping out the regulator for an OEM one (looks like CityRacer sells the old 3-wire one I need, as mine is a 10/77 model). I figure I'll swap the temp sender first since I have that, and then maybe tackle the regulator after the fact. Based on y'alls experience, would you suggest another direction? I don't really want to add any additional gauges.

Mr. Toyota seamed to have picked one of the I6 hot spots, back of the head between cylinders 5 & 6. Water through the block travels from bottom to top, and being open passage system, water near the front two cylinders and around the bottom would be cooler than the water at the top, and toward the rear of the engine. The thermostat housing would give you a good mixed coolant average. I think I would want the best average for FI, and the hottest point for the cluster gage. Although, the resolution of any automotive temperature gage and/or sender may not notice the difference. The accuracy is dubious without some sort of calibration. One of these days, one of us should stick a sensor in a cup of boiling water and see exactly where 212 reads on the gage face (at sea level of course).

However, if what you have works for you…Awesome. I love it!

 

[procomsignathid]

Mr. Toyota seamed to have picked one of the I6 hot spots, back of the head between cylinders 5 & 6. Water through the block travels from bottom to top, and being open passage system, water near the front two cylinders and around the bottom would be cooler than the water at the top, and toward the rear of the engine. The thermostat housing would give you a good mixed coolant average. I think I would want the best average for FI, and the hottest point for the cluster gage. Although, the resolution of any automotive temperature gage and/or sender may not notice the difference. The accuracy is dubious without some sort of calibration. One of these days, one of us should stick a sensor in a cup of boiling water and see exactly where 212 reads on the gage face (at sea level of course).

However, if what you have works for you…Awesome. I love it!

Im half-inclined to do the boiling water test, however Im around 5000' of elevation. I have the old sensor, the cityracer one, and the new OEM one. would be an interesting test...

 

[firemanj92]

Ah, that's good to know. So re-reading your post above about how I should see a decrease after awhile, it really only ever gets as low as the the pic I showed at 27 ohms (maybe a smidge under the 3/4 mark, let's call it 5/8). So I think next I'll go through the steps of checking the back of the instrument cluster on the points the FSM recommends to check voltage / resistance, and if that checks out I think Im left w/ changing out the sender.

The 0.9 stamped on the cap is 0.9 Bar or close the 1 atmosphere, and that = approximately 14 PSI. So you have a 14 PSI cap.

Actually .9 bar converts to 13 psi, .97 bar converts to 14 psi. The significates of psi is a higher boiling rate so for every psi (3 degrees for every 1 psi) which in turn can help prevent overheating. If the cap is too high (depending on condition of cooling system) it can lead to overflow, leaks, or damage

Numbers are with 50/50 coolant
3 x 13 psi = 39* above 220 for a sum of 259*
3 x 14* +42* .....262*

Done nerding out with Hazmat/O-chem stuff.