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]

DrUnit previously posted:

[Rob]

Hypoid previously posted:

[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.

[Rob]

Hypoid previously posted:

[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.

[Rob]

RicardoJM previously posted:

[Rob]

Brody previously posted:

[Rob]

Brody previously posted:

[Pathrat]

Brody previously posted:

[Rob]

Pathrat previously posted:

[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.

[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.