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Discussion Starter · #1 ·
A few weeks back, I posted a question about the temperature guage in my 84 CJ 258. Jaffer and some other guys gave me some great advice. It's a Stewart Warner replacement guage and it was running a little high, not pegged, but always at "H". Well, anyway, what prompted the post was that the original temp guage was working fine, but I replaced it when I replaced the busted fuel guage. The original guage sat right where it should in the middle of the green band.

So, I replaced the sending unit with a new Mopar sender and same thing. To make sure that the new SW guage wasn't improperly calibrated I tested the new sender once the engine was warmed up, and I only got 5-6 ohms resistance. This explains why the guage reads so high.

I can't bear to think that my engine is suddenly running that much hotter. Do you think the old guage was bad since it said my temp was OK? Did I get the wrong sender? Is it possible to install the sender incorrectly, either too deep or too shallow?

Please help me nail this down. The last thing I want to do is replace the thermostat if it's actually some stupid electrical problem.

TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition
 

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usually with aftter market temp guages.. you have to replace the sender.. to what goes with that guage.. has to do with the resistance in them.. and what the guage is setup to read..
this was the case with my autometer temp guage...

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Probably just smoke that I'm blowin' here ..but it'll trigger something either in your head or someone elses. Obviously you've got some mis-matched wiring/sender/supply voltage type thingie goin' on here.

Where did you grab your supply voltage for the gauge? Doesn't the CJ dash have some type of voltage reducing thingie....so if you grabbed your 12v source from the normal gauge "hot" ...you would not be getting 12v? Then again ..I think todays gauges use Zener diodes that regulate the voltage ???? Anyway ...with a reduced voltage (if that's the case) would give you a lower reading I would reason given that all electrical meters are current meters ...with reduced voltage ..you got reduced current hence the needle shouldn't go as far toward FSD (full scale deflection).

What is the resistance of the sender when its cold? That is, does the SW ever read cold? Is that an OEM sender (which I think you said it was) ..and is that sender matched to a reduced voltage gauge?

Rereading your post ...is this a SW SPECIFIC replacement for the OEM gauge? ..or just a SW gauge that you picked out to replace your OEM gauge. That is, does SW make a gauge specific to your application? If that were the case ..and you, in fact, had one of those voltage reducing circuits ..but you wired it to true 12v ..then I could see the high reading. Then again I get back to the zener diodes......


GeeAea




Now that I've quit smoking my doctor was pleased to inform me that I'll die in much better health!!!
 

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Quote:
"To make sure that the new SW guage wasn't improperly calibrated I tested the new sender once the engine was warmed up, and I only got 5-6 ohms resistance."

Yep, sounds like the new S&W gauge is OK, and depending on your defination of "warmed up", that's THE WRONG SENDER!

I do have a set of new replacment S&Ws and a sender bought from FWDHardware.
I also have an AutoMeter head temp gauge.
When the (dry steel) head temp gauge is reading 185-190*F my S&W OEM replacment gauge and FWDH sender regesters just a little ways into the band of the S&W gauge.
Put your sender in a pot water then test the resistance from post to sender body using my article's tests with the water at increasing stages of warmth up to boiling...
With an ohmmeter check the resistance between the post and ground.
It should be:
* 73 ohms Cold - Test when engine is slightly warm. A stone cold engine will read a much higher resistance.
* 36 ohms Beginning of Band
* 13 ohms End of Band
* 9 ohms Hot

Note: COLD water will give a VERY high resistance reading ... that's why I say "slightly warm" for the cold reading.

Ya got to methodically do the WHOLE batch of tests for that particular circuitry to narrow down the problem.
My CJ Gauge article presents the entire batch of diagnostics for OEM gauges and senders* ... excluding voodoo, black magic, witch's curses and hexes...

*Post to post resistance tests on Mopar Sewart Warner replacement sets do no apply but the sender resistance parameters should conform to OEM specs and make these S&W gauges read properly.



JAF
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If you want to get rid of your guage problems for good, do what I did and forget those crummy electric guages and go to MANUAL guages.
There's no wires to cross or resistence to measure, just ONE temp wire from the engine to the guage.
Problem solved.

'79 CJ-5 258
 

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Discussion Starter · #6 ·
Thanks guys, I'll check the sender out of the vehicle and hopefully get this thing figured out.

TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition
 

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Discussion Starter · #9 ·
LEVE,

Grounding problem, where? As far as I know (trust me that's not much!), the SW temp guage gets 12V from the fuel guage and and is grounded to the speedometer housing via a small brass fitting on the back. The only other connection that I see is the wire to the sender.

Do you mean the sender's not grounded?

TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition
 

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Texas, I was waiting for that question. The sender is grounded at the block. But the ground path for the electron return is:

Sender to block.
Block to battery cable
Battery cable to negative battery post,

Or,

Sender to engine block
Engine block to transmission case/grounding braid
Braid to firewall
Firewall to dash
Dash to gauge.

If any of the paths above are tarnished or perhaps corroded then resistance builds up. It's just like you put a resistor in series with the gauge. It depends on how the current seeks it's ground path as to how the gauge acts.

Here's my reasoning, and perhaps a "Ground" would not be the best description. Remember if you know what it ain't you have have the battle taken care of. So one question I'd as is did you verify the engine's NOT running hot with another gauge? It may be running a little hot.

Drop that sensor into a boiling (212*F) pan of water, pull it out and quickly read it's ohm value. That's going to be pretty close to a full reading. Also use a candy thermoter to watch the water cool. When it reaches 200*F then take another reading... This reading would most likely the "normal" ohm value for the sender.

Here's my logic...

1. You know how the original gauge worked:
a. It did work, showing prgressive temp changes.
b. The gauge would settle "high", what you considered above normal.
2. You felt this was a bad gauge.
3. You replaced the gauge with an gauge that met OEM specificaitons.
4. If the gauge were to be faulty then the new gauge should have shown a different (not necessarily right..) temp.
5. The new gauge showed the same condition.
6. You say you tested the temp sender, and it was 5 - 6 ohms when hot, which is only about 4 ohms lower that spec.

What's the reason for this difference?

Have you subed in a known good 10 ohm resistor in place of the sender? That's the first thing I'd do. I'd test it in the following manner:

1. From the sender harness to engine block ground. Note the temp gauge movement.
2. Removing the harness from the gauge and using leads clip the 10 ohm resistor from DASH Ground to the gage. Note the gauge movement.

If there is NO problem with return grounds then:
a. The movement of the meter should reflect a more "Normal" range of movement.
b. The movement of the meter should be the same in each test.

If the movement is NOT the same then there is only one of two problems...
a. A build up resistance in the ground path.
b. A build up of resistance in the harness and connections.

Elimination of ground path problems could easily be located by running a wire from the dash ground to the body of the temp sender.

Elimintion of harness problems could be easily located by running a jumper wire from the gauge directly to the sender.

In each case if a difference in meter movement is noticed... then you've found the problem.

Now one other problem that could be causing a higer reading is a high voltage feeding the gauge. Remember that the movement is a function of additive resistance. In both cases the gauges act the same.... This tells me that you've got more resistance in the circuit to make the gauge go high, or you've got more voltage feeding the temp gauge than is necessary. Measure the voltage and see what it's at. If it's higher than about 7.5 volts, well, you're gonna read high.

So, try getting a 6 VDC lantern battery and feeding this voltage to the gauge voltage input and hook up a 10 ohm resistor like the tests above. I'd bet you'll see the gauge movement right in line with what you're expecting.

So now if this is true, then the regulator strap is dying. You'll have to replace the strap, by a solid state gauge regulator (about $18 from JCWHitney) or build your own regulator to feed the guage the amout of voltage that will move the meter to what you feel comfortable with...

 

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Discussion Starter · #11 ·
LEVE,

Thanks a lot for the GREAT explanation. I'll go through these steps and with your advice, I'm sure I'll nail this down.

Just to clear up one point, the SW replacement guages don't use a regulated jumper strap. The temp guage gets 12V from the fuel guage, so the guage is getting the right voltage. The problem is definitely on the sender circuit.

Thanks for everybody's help.

TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition
 
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