Temp Gauge

If I might resurrect one...I recently picked up a 94 with 138K cheap, because the previous owner said it had an overheating problem. Stop and go traffic only brings the needle to 11:00 position. Getting up to highway speed is something else. Then the needle jumps up past H, then back to 12:00, real jerky and fluctuating wildly back and forth. If this were the gas gauge I'd think it was an electrical short.

I took out the thermostat which had no effect. I can see the coolant flowing in the radiator. There is no bubbling, no coolant smell, no boiling over...None of the normal signs I would associate with overheating in cars I've had in the past. I'm wondering if the temp sending unit or sensor might be responsible? Or gauge? Or is it actually overheating?

I'd bet it's a crappy radiator. Is it still the original??

Or do you have air in the system, is it loosing any coolant??

I'd take a look at the connection on to the sending unit...

it's on the back of the motor, driver's side, and a loose connection will make it go haywire. see if you can pull the connector, put a little dielectric grease on it, and put it back on.

I am not aware of anything wrong with my radiator, but I will have to replace it with an OEM unit because of SCCA stock rules. I really have no timetable for buying the new one other than before 2/2008, but if you think you might want to buy mine for cheap, then I can order a new one sooner rather than later and we can have a tech day to install them. Let me know if you are interested.

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Originally Posted by tailchaser

I'd take a look at the connection on to the sending unit...

it's on the back of the motor, driver's side, and a loose connection will make it go haywire. see if you can pull the connector, put a little dielectric grease on it, and put it back on.

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Sending unit connection is loose. Plus, wire is exposed and frayed at the spade connector. I can rotate the little plug that comes out of the sending unit, but not the sending unit itself, is that normal or should it stay in position?

Oh yeah, not losing any coolant that I can determine at this point. I don't think it's the original radiator, looking down inside it looks clean.

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Originally Posted by cam76034

Sending unit connection is loose. Plus, wire is exposed and frayed at the spade connector. I can rotate the little plug that comes out of the sending unit, but not the sending unit itself, is that normal or should it stay in position?

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Here is a thought, Fix the wire and see what you get. ;)

If ya still have a problem replace the sensor.

Or you could always do a 3Bean and pull the motor and put the car in a ::box:: for a year. ::Banana::

I take it you now own a '94 Miata??

Purchased for change he found in his sofa... ;)

The connector on the sensor should NOT turn. Replace it.

Aha! I didn't think so.

Thanks Raymond!

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Originally Posted by trickyrix

Purchased for change he found in his sofa... ;)

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It's a very big sofa that I bought at Sofa King!

sofa king cheap. ;-)

Only you folks that have lived on the east coast will understand that it's just not a Sofa King sofa if it wasn't delivered by Norfolk & Way Movers...

When your boss tells you that you've got to relocate, remember to say, "Norfolk & Way!"

Wow, what a difference after disconnecting that pesky wire to the temp sending unit! No more distracting bouncing needle action. Shoulda cut that wire 1st thing.

Seriously, I did pull the wire and somebody has definitely been down this road already; the wire had been stripped and rewrapped around the spade connector so it is loose. Not much room to work with but I'll try to solder it back on and see if that changes anything...if not then next step will be a new sending unit.

Don't solder.

If you can unplug enough items and unbolt the wire harness from the mount on that back corner you can pull it out to work on it.

Use a butt-connector (splice) to add a length of wire and another spade connector to it.

Use crimp connectors to do it so you don't have that point of failure where the wire will break in two from the vibration.

And if the connector itself on the sensor moves, replace it. It's bad.

RJ

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Originally Posted by channelmaniac

Don't solder.

If you can unplug enough items and unbolt the wire harness from the mount on that back corner you can pull it out to work on it.

Use a butt-connector (splice) to add a length of wire and another spade connector to it.

Use crimp connectors to do it so you don't have that point of failure where the wire will break in two from the vibration.

And if the connector itself on the sensor moves, replace it. It's bad.

RJ

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Yeah, I can rotate the little connector off the sensor 360 degrees. Thanks for the tip on that and the time I was about to waste soldering.

S.P.F.!!!
It's gotta be the gauge!
Replaced the temp sending unit and connections to it. On the way to the highway at 35 -40 mph the gauge sat at 11:00 the whole time. Wind it up on 121 to 70 and things are looking good...for about a mile, then the same fluttery needle going past H, then back to 11:00. Pull over on the side and no signs of overheating, everything seems normal. Take it up to speed and it happens again.

Any suggestions on how to test the temp gauge? Maybe get an extra and wire direct to it?

NO...

It doesn't gotta be nothin'...

You need to hook some long wires up and connect 'em to a voltmeter... Drive down the road and see what happens.

If it still overheats you should see the needle sit still if it's the gauge. if the voltmeter moves to indicate the car is getting hot then the car is getting hot and the gauge is correct.

If that's the case then you have a blockage somewhere. Maybe the radiator...

RJ

So run the hot wire from the voltmeter to the sending unit and ground to ground, right? Could I just tap into that line so to observe the gauge and the voltmeter at the same time?

I thought about blockage, but shouldn't the needle move in a more linear way, instead of bouncing all over the place?

Thanks for the voltmeter tip, I would not have known that.

Well, hmmm...

Hang on a sec... I'm thinking that won't work as the sensor is a path of variable resistance to ground. There has to be a voltage source somewhere and I don't have the miata wiring diagrams on this computer in the shop. What you'll end up reading is a jumping voltage source from somewhere up the wiring from that variable ground point.

Trace it back in the diagrams. You may just have a bad connection on the instrument cluster or a bad wire somewhere else. When the car warms up can you slap the dash and get it to jump all around?

RJ

Okay, I'm trying to read the electrical diagram, and I'm not too good at this. From the temp sensor is a blk/blu wire that leads to the temp gauge. Temp gauge shares a common ground with the tach, fuel gauge and oil gauge. Then there's a blk/yel wire that connects all these gauges to the fuse box.

So...I should be able to tap the blk/blu wire with my voltmeter, right? As the car heats up there would be current through that wire?

Oh yeah, beat the crap out of the dash with no results.