Temp Gauge Sending Unit Issues (Page 1/3)
stark86 MAR 10, 06:44 PM
Hello Fiero Forum,

A couple weeks ago I picked up an '86 GT from a neighbor that's been sitting for years; Nevertheless, I got it running and it seems to be in good condition overall (apart from the moldy interior and seized up shifter assembly). Now one issue I've been fighting is the Temp Gauge--I realized that after idling for considerable amount of time, the gauge would barely move, although the radiator cap is warm to the touch (I already replaced the thermostat).

Not trusting the gauge, I tested its accuracy by connecting resistors to the wire, and all seemed to be good. So, I ordered a Temp Gauge Sending Unit from the Fiero Store and installed it--But now, instead of moving little, the temp gauge wont move at all. I took a multimeter and attached the negative probe to ground, and the positive probe to either pin on the Sending Unit, which displayed an Open Loop on by meter. Believing that the Teflon tape on the threads could be blocking the sensor from getting a good ground, I used tiny jumper cables to connect the gauge to the negative terminal--Still nothing.

I already (stupidly) threw away the old Sending Unit, so I can't cross-examine my testing with the old part. Am I doing something wrong? Could my sending unit be faulty? Please bare with me: I'm fairly new to wrenching on cars and VERY new to dealing with electric.

Thanks
Patrick MAR 10, 07:14 PM

Welcome to the forum... and to Fieros!

Electronics aren't my strong suit... but this chart may help you.
Spoon MAR 10, 07:18 PM
Easy test,

https://www.youtube.com/watch?v=pWBsq0J9McI

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

armos MAR 10, 07:38 PM
This is from the 86 service manual:

it agrees with you that one terminal of the gauge sender should be in contact with ground. However, that ground might be coming from the wire that plugs into it. Do you have ground contact on one of those 2 wires?
[EDIT]:what you said is probably correct - there's probably no ground at either wire or terminal, and it strictly grounds through the threads.[/EDIT]

I tried to check my car, but I can't figure out how to get the connector off without breaking it.


They also say there should be a resistance of 1365ohms across the terminals of the sender at 100F, and 55ohms at 260F.
So if you plug 68ohms into the connector the gauge should point almost at 260F.

There's a more complete chart of resistance values for the other temperature sensors (which connect to the ECM), but at least some info says that the gauge sender uses a different scale than those others.

[This message has been edited by armos (edited 03-10-2023).]

stark86 MAR 10, 08:06 PM

quote
Originally posted by armos:

This is from the 86 service manual:

it agrees with you that one terminal of the gauge sender should be in contact with ground. However, that ground might be coming from the wire that plugs into it. Do you have ground contact on one of those 2 wires?

I tried to check my car, but I can't figure out how to get the connector off without breaking it.


They also say there should be a resistance of 1365ohms across the terminals of the sender at 100F, and 55ohms at 260F.
So if you plug 68ohms into the connector the gauge should point almost at 260F.

There's a more complete chart of resistance values for the other temperature sensors (which connect to the ECM), but at least some info says that the gauge sender uses a different scale than those others.




If I'm testing this right, I don't think I have ground contact on either wire. But shouldn't the ground contact come from the threads on the sensor contacting the cylinder head?
Patrick MAR 10, 08:14 PM

quote
Originally posted by stark86:

...shouldn't the ground contact come from the threads on the sensor contacting the cylinder head?



If it had a single wire like the fan switch... yes, it would then definitely need to ground out through the base... but I admit I'm not 100% sure about this temp sender.

[EDIT] According to The Ogre's Cave, that temp sender does need to ground through its base.


quote

Resistance to ground for gauge.
Don't use Teflon thread sealing tape on grounded shell NPT sensors. It will likely interfere with getting a good electrical connection through the threads.


[This message has been edited by Patrick (edited 03-10-2023).]

stark86 MAR 10, 08:47 PM

quote
Originally posted by Patrick:

If it had a single wire like the fan switch... yes, it would then definitely need to ground out through the base... but I admit I'm not 100% sure about this temp sender.

[EDIT] According to The Ogre's Cave, that temp sender does need to ground through its base.




I see what appears to be ground symbols on the box containing the labels "Sender Resistance" & "Switch," so if I'm reading the diagram right, the ground connection would come from a source other than the two wires, which is what I'm assuming to be the base of the sending unit. I'm gonna be honest, this is my first time trying to decipher a wiring diagram, so I may be wrong.

EDIT: Just saw your edit -- That makes sense... I'm gonna take the teflon tape off (That was pre-applied) tomorrow and see if that does anything. Fingers crossed there's no leaks

[This message has been edited by stark86 (edited 03-10-2023).]

Patrick MAR 10, 08:49 PM

quote
Originally posted by stark86:

...if I'm reading the diagram right, the ground connection would come from a source other than the two wires, which is what I'm assuming to be the base of the sending unit.



Yes, correct. That's what I added above.
theogre MAR 10, 09:33 PM
Yes... Fiero and Thousands of others, Any Sender for Dash Gauges ground thru the base.
Teflon tape can cause problems with them.

ECM Temp sensors and many others have 5V and/or Ground wires + "Data" wire.

While many love Teflon Tape... Is a pain to clean if have to take apart again for any reason. (Hard to apply w/ one hand too.)
RectorSeal and other "pipe dope" that dries have same or worse problem. You often need Dremel w/ Wire brush tip etc to clean these.
Is why I use Non drying Teflon pipe dope on most things. Example: https://www.lowes.com/pd/Oa...hite-Sealant/4750813

Plus this stuff will last a long time after the can is open. I've been using same can for 15+ years.
Others can/will dry in the container after first use.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

sanderson231 MAR 11, 11:34 AM
Are you sure you bought the right sensor? There are two coolant temperature sensors. One is a combination sender and switch. The sender is for the instrument panel gauge. The sender is variable resistance and goes to ground via the threads. The switch is for the coolant temperature warning light. The switch also goes to ground via the threads.

The other sensor is a variable resistance sending unit connected to the ECM. The ground is in the ECM.

So check the resistance on the sensor you bought and see if there is continuity to the threads on the "A" wire (should be about 1400 ohms at room temperature) . If not, I think you have the wrong sending unit.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged