Crazy temp gauge

[Rob]

Two things happened to the 40 on the Yankee Hill run Sunday. When I got to the meeting spot near Central City, after an hour and a half on the road, my radiator overflow tube spewed a bit.

Later, on the run, I noticed the temp gauge needle was at high, then surpassed the high mark. The 40 never overheated, though.

When I got in it today to drive to work (which I do sometimes because the 40s just too cool not to drive once in a while), the needle went straight to high as soon as I turned the key.

AirRandy suggested on Sunday that the gauge might be out of whack. Anyone else agree? If it is the gauge, can it be repaired (by a rookie wrencher)? Should I put in a new thermostat? I happen to have one on hand and was planning to replace it this winter anyway. But if it's the gauge, that won't fix the problem.

All opinions and suggestions are welcome.

[RockyMtnHigh]

I would say try the thermostat first, which would be the easiest less evasive place to start.

[Hypoid]

Later, on the run, I noticed the temp gauge needle was at high, then surpassed the high mark. The 40 never overheated, though.

When I got in it today to drive to work (which I do sometimes because the 40s just too cool not to drive once in a while), the needle went straight to high as soon as I turned the key.
Find the temperature sender on the engine. Is the wire connected? If it is, unplug the wire and clean the connection. Plug it back in.

Turn the key on and see if the needle goes up again.

[Rob]

Find the temperature sender on the engine. Is the wire connected? If it is, unplug the wire and clean the connection. Plug it back in.

Turn the key on and see if the needle goes up again.

Thanks for the suggestion.

Found the sensor (I think). It was plugged in. I wiped off the connection on the sender. Didn't help. The wire, however, doesn't seem to be securely fastened to the connector. Bad connection?

[Haku]

Sounds like its getting too much voltage or something along those lines. Could be as simple as the grounds may not be connecting well or something along those lines too. If the connection to the sensor seems solid, then maybe check the grounds, and a all the wires you can find (shouldn't be many on that engine). Other then that, all I can say is maybe find another FJ40 and ask if you can borrow their gauge to test in your rig. If that one does the same thing, then that narrows down where the problem is. Rig looks like its in good shape, but its not unreasonable to expect a gauge to fail in a rig that old.

JH

[Hypoid]

I just looked up a wiring diagram. As I suspected, voltage feeds through the gauge to the temp sensor in the engine. The sensor has a variable resistance that is dependent on it's temperature. The gauge reflects that resistive value.

Without any hands-on diagnostic, I'm taking a leap and saying that the sender has failed. I am thinking that the gauge and wiring are OK simply because the needle moves.

Maybe one of the yota gurus can chime in and tell me if the resistance increases or drops with temperature increase.

Just for giggles, leave the wire unhooked and turn the key on, see if anything happens.

[Brody]

Judging from what you said that the gauge pegged just as you started it up. I am going to say that the temperature sending unit is on it's way out, not the gauge or the thermostat. The electrical connection could also be at fault, but the wire would almost have to be falling off the sender.

Gauges seldom bite the bullet, but sending units, electrical connections and thermostats do. Since you already have the T stat, it sure isn't going to hurt to replace it. The sending unit is very close to the T stat, so you might as well replace that, too. I bet your problems will go away.

BTW, you will probably need a Yota specific sending unit if you have the Yota gauges. Due to the varying voltages/resistance, etc. the brand of the gauges have to match the brand of the sending unit.

[Rob]

I just looked up a wiring diagram. As I suspected, voltage feeds through the gauge to the temp sensor in the engine. The sensor has a variable resistance that is dependent on it's temperature. The gauge reflects that resistive value.

Without any hands-on diagnostic, I'm taking a leap and saying that the sender has failed. I am thinking that the gauge and wiring are OK simply because the needle moves.

Maybe one of the yota gurus can chime in and tell me if the resistance increases or drops with temperature increase.

Just for giggles, leave the wire unhooked and turn the key on, see if anything happens.


Judging from what you said that the gauge pegged just as you started it up. I am going to say that the temperature sending unit is on it's way out, not the gauge or the thermostat. The electrical connection could also be at fault, but the wire would almost have to be falling off the sender.

Gauges seldom bite the bullet, but sending units, electrical connections and thermostats do. Since you already have the T stat, it sure isn't going to hurt to replace it. The sending unit is very close to the T stat, so you might as well replace that, too. I bet your problems will go away.

BTW, you will probably need a Yota specific sending unit if you have the Yota gauges. Due to the varying voltages/resistance, etc. the brand of the gauges have to match the brand of the sending unit.

Thanks, guys. Looks like sending units are only about $12 or so. I'll go ahead and order one, and I'll replace the thermostat this weekend.

Pete, do you know if the unit just unbolts and bolts in? Does it sound like a job for PB blaster?

[RicardoJM]

You have some good input Rob. I'll add my experience in this area. Recently I was experiencing this and noticed my connector was not tight on the sending unit. Sometimes it gave a good reading and sometimes it would bump up to high and every now and then it would shoot straight up to hot. When it did the later I would pull over and check the connection - then the connector separated from the wire. I spliced on a blade/splade (the female flat connector) on the wire and have been getting consistent readings on the gauge.

The ohm ratings on sender/gauge changed a bit over the years. I am pretty sure your truck is still OEM but you really don't know on a 21 year old vehicle. Just be aware, in the event your new sender seems to be "bad".

The replacement should be very straight forward. PB Blast the sender for a couple of days in advance if you can. Also, if the wire to connector seems to be loose and getting close to failing, you might want to consider removing the connector and putting a new one on. If you go this route, get a good measurement on the width. The connector I put on was just a tad narrow and it took some pick work to open it up just a bit.

Regarding the OEM temperature and oil pressure gauges, they do not provide precise measurements and many FJ40 owners put in new gauges so that they know the precise measurements. The F and 2F engines have a second port that would allow you to run both the OEM temperature gauge and a new modern gauge. For the oil pressure, you could install a T fitting that would allow you to run both.

As you replace the thermostat, make sure you get a new o-ring as well and that you put it in correctly (http://risingsun4x4club.org/forum2/showthread.php?p=13483&highlight=thermostat#post13483).

[Rob]

You have some good input Rob. I'll add my experience in this area. Recently I was experiencing this and noticed my connector was not tight on the sending unit. Sometimes it gave a good reading and sometimes it would bump up to high and every now and then it would shoot straight up to hot. When it did the later I would pull over and check the connection - then the connector separated from the wire. I spliced on a blade/splade (the female flat connector) on the wire and have been getting consistent readings on the gauge.

The ohm ratings on sender/gauge changed a bit over the years. I am pretty sure your truck is still OEM but you really don't know on a 21 year old vehicle. Just be aware, in the event your new sender seems to be "bad".

The replacement should be very straight forward. PB Blast the sender for a couple of days in advance if you can. Also, if the wire to connector seems to be loose and getting close to failing, you might want to consider removing the connector and putting a new one on. If you go this route, get a good measurement on the width. The connector I put on was just a tad narrow and it took some pick work to open it up just a bit.

Regarding the OEM temperature and oil pressure gauges, they do not provide precise measurements and many FJ40 owners put in new gauges so that they know the precise measurements. The F and 2F engines have a second port that would allow you to run both the OEM temperature gauge and a new modern gauge. For the oil pressure, you could install a T fitting that would allow you to run both.

As you replace the thermostat, make sure you get a new o-ring as well and that you put it in correctly (http://risingsun4x4club.org/forum2/showthread.php?p=13483&highlight=thermostat#post13483).

Thanks for the input, Ricardo. New gauges aren't on the wish list just yet.

I have the gasket and O ring for the thermostat. I changed one on my old 40 about 25 years ago, so it couldn't be too difficult. Of course, I'm not as a smart as I was back then.

I think I'll try a new connector before getting a new sender. The wire is kind of loose. If that doesn't cure it, I'll get the new sender and make sure I get it for the appropriate year.

I imagine I'll be replacing many parts over the next few years, and I look forward to learning how to do them all. I'm sort of a late bloomer when it comes to wrenching and knowing about vehicles and engines and such.

Surprisingly, though, for a 27-year-old rig, it seems to be in pretty good shape, at least under the hood.

[Roostercruiser]

i have the same problem in my burb

[Brody]

Pete, do you know if the unit just unbolts and bolts in? Does it sound like a job for PB blaster?

Yes, it just unscrews. And yes, it is something you want to hose down with PB Blaster or another rust busting product. You really don't want to break this off where it is mounted as it will quadruple the amount of work involved. Keep nailing it with PB like Ricardo suggested until a small amount of force is need to loosen it.

Also keep nailing the steering adjuster on your 40, too as we will be tightening that up when we do the rod ends and linkage replacement!

[Rob]

Yes, it just unscrews. And yes, it is something you want to hose down with PB Blaster or another rust busting product. You really don't want to break this off where it is mounted as it will quadruple the amount of work involved. Keep nailing it with PB like Ricardo suggested until a small amount of force is need to loosen it.

Also keep nailing the steering adjuster on your 40, too as we will be tightening that up when we do the rod ends and linkage replacement!

Thanks, Pete. Been doing just that. I'll likely wait to order the rods until you and LaDawn get back from paradise.

By the way, I noticed on the two runs I've taken the 40 on that it steers better when I'm driving home. The key is I didn't air back up either time after the run. So I'm going to run them at about 26 psi for a while. It doesn't jump around as much on the road when I'm running at that pressure.

[Brody]

The key is I didn't air back up either time after the run. So I'm going to run them at about 26 psi for a while. It doesn't jump around as much on the road when I'm running at that pressure.

How are you determining the tire pressure that is going to be right for the 40? Just because the manufacturer has some numbers stamped on the side of the tire doesn't mean that is what you should run for tire pressure....Here is a very quick and easy way to do this:

Take a piece of regular chalk and, with your rig loaded with what you normally take with you, make a good mark all the way across your tire. Drive around the block. Look at the chalk line. If it is worn off in the center of the tire, you are over inflated. If it is worn off on the sides, you are under inflated. If it is even and you can't see the mark, you are properly inflated. Chances are that tire pressure is going to be different between the front and rears.

[Rob]

Take a piece of regular chalk and, with your rig loaded with what you normally take with you, make a good mark all the way across your tire. Drive around the block. Look at the chalk line. If it is worn off in the center of the tire, you are over inflated. If it is worn off on the sides, you are under inflated. If it is even and you can't see the mark, you are properly inflated. Chances are that tire pressure is going to be different between the front and rears.

I'm gonna try that trick as soon as I can find some chalk. I guess I could use my chalk line. Bright blue is such a nice color.

[Pathrat]

Take a piece of regular chalk and, with your rig loaded with what you normally take with you, make a good mark all the way across your tire. Drive around the block. Look at the chalk line. If it is worn off in the center of the tire, you are over inflated. If it is worn off on the sides, you are under inflated. If it is even and you can't see the mark, you are properly inflated. Chances are that tire pressure is going to be different between the front and rears.

Cool! The Boy and I just learned a simple and easy way to check tire pressure. Thanks!

[Rob]

Cool! The Boy and I just learned a simple and easy way to check tire pressure. Thanks!

Hint: Keep an eye on the dumb guy's posts. You'll learn all sorts of cool stuff from that smart guys who take pity on him and tell him how to do things. :thunb:

[Rob]

Update: I have the week off from work, so today I decided to put a new thermostat in the FJ40. Went ahead and drained all the coolant and flushed out the system as well as I could.

The new thermostat didn't cure the gauge problem.

But after I had everything reconnected and all the drains closed and refilled the radiator with coolant and water, I decided I needed another gallon of coolant. So I popped over to the Car Quest store to get some. The young woman at the counter asked if I needed anything else and I said only if you have a temp sending unit for an '82 Land Cruiser. Damned if she didn't find one and have it in stock.

I went home, took the old one out and put the new one in, then went for a drive around the block. The temp needle nudged itself a couple of notches above cold (the engine was already warm from the trip the Car Quest).

Thanks again for the advice, guys. It was, indeed, the temp sending unit.

[Brody]

Good to hear! You will be surprised what you DO NOT have to pay through the nose for at Toyota. It is a popular truck/car so most places stock a large amount of Toyota parts for all makes and models.

[Rob]

I didn't even consider calling a dealership. But I didn't think a small store in town would have the unit for a 27-year-old rig. I'll be checking in with them first on parts like this before I go to one of the online places that specialize in FJ parts.

I also picked up a V belt for the air pump today at NAPA. The one I got from MAF looks like it's too narrow. I also got a belt from MAF for the alternator and PS pump, but it looks like the right one. I won't know for sure, though, until I put them on.

The larger belt looks straightfoward enough to install, but I'm still trying to figure out the air pump belt. It looks like the tensioner is on the air pump, but it looks hard to get to without either putting it on a lift or taking parts off above it. Maybe I'll wait till you get back from the island.