93-97 Coolant Temperature Gauge Modification

[Mr.Bryan]

Yup, I did it a few weeks ago. It's a cool mod! thanks for the write up, it only took me about 35 min. to do.

My temp gauge has been moving all over the place. I've noticed with ambiant temps around 80-85 out, ac on and at about 65-70mph i'm about 2-3 needls widths away from red, but if I kick the ac off it drops down a bit. I'm going to try flushing out my system in about 2 weeks and I'll see if that makes a diffrence. Actually I think I'm going to tear the whole front end down and flush everything out. I think i've got mud between my condenser and the radiator. Damn Ih8mud!

[RavenTai]

Cool let us know if that helps.

[Gauge]

Shamefully, I am only getting around to doing this mod now after purchasing the resistors - what seemed like a year ago. Completed it this weekend actually without a hitch.

First off I want to give a BIG THANKS to Raven Tai for doing all the work on this mod. I remember when he was doing the testing and reading his every post, as his progress developed - most of it electrical mumbo jumbo, at least to me anyway. His instructions were great and saved me much time, effort, and most likely a broken dash if I had to figure out how to dissamble the thing myself.

I have not driven my rig much after the install, but I have noticed that the gauge is MUCH more responsive. With the AC on and driving agressively, the temp gauge reached the 3/4 mark - something I have neve seen before in my years of driving. Actually it was quite disappointing to see how easily I could make this engine "hot" with little more than the AC on and some agressive driving on a cool spring day in May. I am afraid to see what will happen in August. No wonder Toyota installed the "zener diode". Probably cheaper than installing a larger radiator.

The 80 series engine has an iron block and an aluminum head. These metals expand and contract at different temperatures and the head gasket has to work especially hard to try and seal together two surfaces that are expanding and contracting at different rates. In my opinion, this is why these engines are prone to head gasket failure. Having a properly working temp gauge is a MUST to monitor temperatures and adjust your driving BEFORE temperatures reach their maximum and the head gasket has blown.

Raven Tai we all owe you BIG TIME for this. If you are ever in Philadelphia, look me up - dinner and drinks are on me.

Cheers!

[semlin]

Originally Posted by Gauge

I have not driven my rig much after the install, but I have noticed that the gauge is MUCH more responsive. With the AC on and driving agressively, the temp gauge reached the 3/4 mark - something I have neve seen before in my years of driving. Actually it was quite disappointing to see how easily I could make this engine "hot" with little more than the AC on and some agressive driving on a cool spring day in May.

just think of the engine temperature as being highly responsive to environmental t stimuli. it sounds better. funny that landtank does not get this kind of fluctuation. what is his secret i wonder? you might think he lives in aplace that is mostly flat as a board with no a/c use until late spring

[Mr.Bryan]

well, I got all the crap out of my condenser and radiatior, and I had my whole system power flushed and new toyota red put in and it runs a tad bit cooler. Not too much diffrence but the needle doesnt get more then 3 needle widths from the bottom of the red w/ ac on and in traffic so I'm happy. Thanks again ravan tai

[RavenTai]

Originally Posted by Gauge

Raven Tai we all owe you BIG TIME for this. If you are ever in Philadelphia, look me up - dinner and drinks are on me.

Cheers!

Thanks that sounds like a good time,



I have learned a lot about my beloved cruiser from everyone here, this was something I wanted for my truck & I had a good time figuring it out I knew others would be interested also. feels good to give something back

it is interesting that many trucks pop up in temp so easily, mine does the same but as we have moved into warmer weather mien has not popped up any higher that it did in cooler weather, it does go up with AC use when idling or in traffic but mien seams that once it hits a certain point (~205) it never goes any further. But it has been an unusually cool spring here, rarely in to the 90's, we will see what it does in lower triple digits with 90%+ humidity. If it goes up much more I am going to look into cooling system improvements,

I am moving to a hotter more humid environment in a few months, I will really need it then.

[ashooter]

Has anybody figured out anything on this mod with regards to a 91-92 truck with a 3F-E yet?

If this temp gauge problem is a (the?) cause of the FZJ's head gasket failures, might it be reasonable to assume that FJ's temp gauge works better, since they don't apparently have the same HG problem? I admit I am almost a complete mechanical idgit, and I know even less about electrickery, so I may be reaching with that guess.

[semlin]

91-92 has a cast iron head so much less chance of blown head gasket. the temp gauge does appear to be "numbed" from my experience. it has a different part # and a different sender part number so someone would have to pull one and post photos. the 93-94 had a different part no. to the 95-97 gauge but was virtually identical.

[ashooter]

semlin,

Not likely to blow a HG, but it could still be getting plenty hot and not show anything exciting on the gauge, huh? Hmm... I will keep and eye out for somebody more technically savvy than me to dig into a 91/92 and see what they find out.

[semlin]

if you just pull the gauge and photograph it then put it back, there is a good chance we can tell you if the mod will work. pulling the gauge is a 1/2 banana job that takes 20 minutes each way.

[RCFloyd]

Hey folks,

The last time that I read through this thread, no one had performed this mod on a 93-94 rig. Has anything changed, or do I need to be the guinea pig?

Robert

[semlin]

i have it in my 93. Read raventai's posts for the temp readings. It seems to be slightly cooler at the red than the 95-97 with the same mod.

[Biff]

Anybody want to sell an extra pair of resistors, damm shipping is to expensive at the store.

[semlin]

i have a set. i can mail them from canada if you pm me your address. they will arrive in about ten days unless your border guards blow them up. you will have to thank raventai for them

[RCFloyd]

Parts ordered... Got an extra set for someone out my way...

Robert

[ashooter]

Originally Posted by semlin

if you just pull the gauge and photograph it then put it back, there is a good chance we can tell you if the mod will work. pulling the gauge is a 1/2 banana job that takes 20 minutes each way.

I have a long list of to-do's already, but when I get time, I will pull it and take some pics if nobody else gets around to it first.

[7P's]

There always has to be one and I'm it this time. I've got a '95 cruiser and read and adhered to Rav's directions and just completed the mod as directed and here's what happened. From startup to approximately one mile - city driving - the gauge remained on the cold peg - then all of a sudden it shot up to 2/5 toward the red (not quite half way) back to the cold peg, back up to 2/5, back to cold peg and back again to the 2/5 mark all in about 10/15 seconds after which it remained at the 2/5 mark for 5 or 6 minutes and then it went back down to the cold peg and remained there until I returned home, which was approximately 10 minutes later. Any words of wisdom out there as to what might have went wrong?

[cary]

My guess is you didn't get a good solder connection. Go back and resolder the board connections. You didn't get the board really hot by accident did you? 50 watt soldering gun or something like that? If you did, could be the board got overheated. Someone with better solder/electronic skills will likely chime in with an answer soon.

[Mr.Bryan]

loose connection maybe? Not soldered good enough? (not knocking you or anything) resistors grounding out on something else? Bad resistor?

[RavenTai]

It does sound like a bad connection, I am afraid you are going to have to go back in. fortunatly it is much quicker the second time,

check your solder points, they should look shinny and smooth, a crackeled or modeld or flat looking surface indicates a cold solder joint, one tip on soldering you want to get the iron as hot as it will get, heat the joint area and then take the heat away in a smooth short ammount of time, as you are melting solder your iron is getting colder and the board hotter, you want to get the joint hot before ecerything arroudn it gets overheated, the quicker you are the better,


and also check the screws that mount the gauge to the cluster, also inspect the plugs into the cluster, and generally look everything over for good contact

if you are good with electronics you could test it outside of the cluster using voltage from your battery and 2 watt potensiometer as a synthetic temperature sensor,

testing is what the calibration resistors were all about, I never have gotten to that part as it has been good for everyone so far and it would have been an added expense for everyone, I should have known that there would be troubleshooting to do eventually and I have not prepared, sorry by bad,

If you cannot get it figured out let me know, do you have a meter?

[7P's]

I was guessing a poor solder joint as well but when I soldered everything they looked damn good. Just went back in and took everything out - took 10 minutes this time. Everything looks good. I've got an old craftsman meter but I think it grew legs, as I couldn't find it earlier this afternoon. I'll resoldered everything again and see what happens. I got an extra pair of resistors so if this doesn't do it I'll throw the other set in tomorrow.

[7P's]

I resoldered using a lot of flux - slapped her in and the needle jumped right up to 2/5 at startup (engine is still hot) so I think it'll work.

[Rich]

Raven,

You posted the data below in a previous thread. These are tested temps for the unmodified factory gauges?

                     93   97  

Pegged hot           269  269  
Top of red           253  259
Bottom of red        242  248

Did you by chance test or calculate resistor combinations that would put needle at bottom of red zone with temperature being around 240° F?

[RavenTai]

Originally Posted by Rich

Raven,

You posted the data below in a previous thread. These are tested temps for the unmodified factory gauges?

                     93   97  

Pegged hot           269  269  
Top of red           253  259
Bottom of red        242  248

Did you by chance test or calculate resistor combinations that would put needle at bottom of red zone with temperature being around 240° F?

those are for unmodified,

I did not calculate but I can, question is how accurate they would be without comparing them on a known good gauge, there is slight variation between what the thermistor curve said it should do and what it actually did but for something like this just need to get in the ball park, there is also some slight variation depending on weather it is running or not (12.x vs 14.x volt source)

34.4 ohms should get you to top of red on a modified gauge
44.5 ohms bottom of red
75 ohms just below center

240F would be 30 ohms for either gauge,

[Rich]

Raven, I may not have been clear on my second question. Are you able to suggest what pair of resistors I could use to modify my '97 gauge to remove the dead zone and have needle at the bottom of the read zone at approx 240°F? Thanks.

[landtank]

Originally Posted by Rich

Raven, I may not have been clear on my second question. Are you able to suggest what pair of resistors I could use to modify my '97 gauge to remove the dead zone and have needle at the bottom of the read zone at approx 240°F? Thanks.

So you're thinking that anything under 240* is realistic?

I personally like it very much where it is right now.

I look at the AC cutoff mark as the "Oh shit we've got an issue here" mark and lets start taking systems off line before it hits the fan.

[RavenTai]

ahh I though you were asking about calibration check resistors.

To answer no, not with any accuracy, what the gauge did on paper was always noticeably off from testing.

At the end of the development thread Darwood did some calculations with some resistors, calculating for anything outside of the center (balanced Wheatstone bridge) requires working with thevenin equivalents something I could not master without sinking a lot of time in it. Darwood apparently did work it out, I have not had a chance to test any of his suggestions

you would want to raise the value of the diode replacement resistor to widen the range, changing resistor R1 changes the center point, changes in one have a slight effect on the other, I have not been able to figure it out on paper alone, trial and error was required but you may have better luck.

Personally I would have no use for a 240F red but that is your truck

[Rich]

Originally Posted by landtank

So you're thinking that anything under 240* is realistic?...

I would like the needle hitting the red zone to indicate the need to stop immediately and let the engine cool down.

It all depends on the context, in general, if the engine were running at say a steady 220° F, in a situation where running hot is to be expected, I.E. heavy loads, steep mountains in the summer desert, then I think that is acceptable. A spike to 230° F near the top of a steep pass wouldn't likely cause me to stop.

In contrast, seeing 220° F on a cool fall evening driving the flats lightly loaded would merit serious attention, and that's why I mention context matters.

I'd like have the gauge setup such that having the needle near the red should be concerning and having the needle in the red should demand action, and for me, 216° F seems too low to demand stopping the vehicle when operating with high loads in very hot ambient conditions.

In general, I think that the red zone should be used to indicate that the danger of boil over is being approached, and having 240° F at the beginning of the red leaves a large margin of over 20° for that.

[RCFloyd]

Ye-haaw! Done. Works like a champ, in-that, it moves! I haven't put it to any test yet, but I'm sure that it's A-OK.

Also replace my new 180 degree Stant T-stat, with OEM 195 degree. I feel that it's important to maintain the 'hot' side of things; the way that the factory intended...

Schucks, and I do mean shucks... I order my Hayden Fan Clutch from them for $65, but when it wasn't there when it was supposed to be, had a friend swing by to pick the pre-paid unit up for me. But, wait, they wouldn't give it to him, even though I'd called ahead and given his name... they needed the credit card that had already been nicked for the part! He offered cash, CC, his first born... all to no avail. Not very nice considering that it is a 130 mile round trip for me. Weird world, eh? Order stuff all day long w/o a physical card, and then this...

Thanks to all who helped in this venture

Robert