truckntractorhick

ok so i was driving my jeep and the temp gauge when from normal 210 up to 250 in like 5 seconds! so to be safe i pulled over and shut it down just to be safe...... when i started it up it said 210 again.... so what would cause this crazy temp gauge? p.s. no CEL

rich

P.M.S?

CodeXJ

I'm trying to be witty and make this an ancronym for 'Bad Sensing Unit', but not getting anywhere. So I'll just say, "Bad sending unit"

Or a poultrygiest. Beware the ghost chicken!

jorget

Thermostat going bad? Have u replaced it yet?

truckntractorhick

yea i replaced the thermostat this past winter.... i know thats no it im gonna try and put a new water pump and bypass pipe in to see what that does

CodeXJ

Before throwing money at parts and time at labor, I'd really consider replacing the sending unit. It should be like $12-15 and takes 10 minutes. The kind of temperature rise your describing isn't likely to happen outside of a nuclear reactor, remember you've got several gallons of water and coolant in there, to get it to change temperature that quickly you have to dump a lot of heat into it. Even if the coolant stopped circulating, I think it'd take more than 5 seconds to increase the coolant temperature that much.

truckntractorhick

yea haha thats true..... but i need to get a new one anyways. so where is the sending unit located?

beeman98663

While we are talking about temp gauges, I have a 94 Grand Cherokee Laredo that the temp gauge has not been working correctly since I bought it. I am stumpped at this point. It sits there at zero. When applying the brake it shows a very slight negative fluctuation of about a 64th. You need to look real close.
Heres what I did.

Changed out the temp sensor located at the firewall, top of the engine block-no change
Changed out the temp sender unit located at the front by the thermostat- No change
Swapped out the gauges from another instrument cluster - No change
Swapped out the instrument unit. - No change
The only thing left is the wire in the harness going thru the firewall to the instrument panel.
Am I overlooking something? Please help

CodeXJ

With the hood up, standing facing the motor at the front, look to the back driver's side of the head. There should be a little spot at the back corner of the valve cover where the temperature sender screws into the head. At least, thats where it is on the pre-97 heads, not sure if they changed it after that.

Oh, and unlike me, be CAREFUL screwing the new one in. The sensor portion that screws in is really just a hollow brass tube. I broke mine off flush with the top of the head.

CodeXJ

I'm at a loss with that one, I suppose you could get a continuity tester and start checking connections, but that sounds like a real pain in the butt. It might be easier to just mount an aftermarket gauge.

BTW, the temp sensor on the back of the head is the one for the gauge. The one on the front of the motor is for the ECM/fan. Why two? Guess Chrysler Corp had extras.

beeman98663

Thanks for the quick response, I was going to the light tester next. Another option would be to run a new lead on the temp sensor and connect it directly to the positive screw on the back of the gauge. Please correct me if I'm wrong, I'm not an electrician, since there is only one lead coming from the sensor it must be hot coming from the instrument panel,going thru the sensor and to ground going into the head. In other words, its a hot coming from the gauge?

TrollHammer

The tidbit of info I didn't get with this one is that it's under the fuel wiring harness box back there. Hides it completely on mine.

The thing I'm wondering is why they add the front one when they don't add a fan on models without A/C (my '91 was that way). Had the front sensor, but no fan, fan relay, or fuse. Not that I'm complaining, since it made it easier to add the fan later, just confusing that they would add a $20 sensor on all models even if they don't need it, but they don't put a $5 switch for the trans power/comfort selection on all, even though the rest of that system is all there anyway!

On the aftermarket guage: where would you mount it? I bought one to put in and other than figuring out where the huge hole in the firewall is going to go (non-electronic), I'm not sure where it will thread in, as the only spot that seems big enough for it is the front sensor, which is being used for the fan....

I'm not going to say it can't work, but my experience with mid-90s XJs is the guage isn't exactly like an old fasioned voltmeter. Mine are connected through my computer, not through the sensor, according to the haynes manual schematics. I'm chasng the same prob.

CodeXJ

Nah, my '93 and '94 both have AC and both sensors in them. When I broke the one off at the back of the head on the '93, the electric fan continued to work, I just didn't have any temperature gauge reading. I think they did away with it at some point in the later model years, but I'm not sure when. What I thought was odd is that the '94 doesn't have gauges, it's got an idiot light, but it still has the same sensor at the back of the head.

beeman98663

Its not that hard to get under the dash to access the instrument panel. Unscrew the 4 screws that hold the clear plastic and the gauges can be removed. Seems the existing space can accommodate an after market. Haynes manual describes pretty good how to take the dash off. You can do it in <10 minutes by removing 12 screws. Hope this helps.

Below the gauge has a reasonably deep well

TrollHammer

Swapped my sensor from the '89 into the '91, and it's working now. The engine, btw, is a '98, so for those keeping score, at least all i6 engines from 87-98 are the same as far as this sensor goes (the one in the back of the head for the guage).

Only thing I would have done different is I forgot to put some teflon tape on it when I put it in, I've not checked for leaks yet, as I just did it, but that would have been better piece of mind.