fuel/temp gauge resistor ohms

[docflower]

I just purchased a 72 c20 totally stock, original owner, 100k miles. Love the truck. Temp gauge and Fuel gauge do not work.

Temp gauge: Sending unit is functioning properly. Grounds seem to be good. My guess is the ceramic resistor on the back of the panel.

Fuel gauge: Bad sending unit (Full tank with 10ohms).

I have taken out the instrument panel and checked the resistance on fuel/temp gauge resistors.
Referring to this post...

http://67-72chevytrucks.com/vboard/s...d.php?t=360120

Observations: I am getting 2-3x the ohm readings on all posts (fuel/temp gauges) except the ammeter which is correct according to the post above.

Does a higher than expected ohm reading indicate a bad resistor? Also, what are the resistor ohm spec's for a stock setup so I can replace them to rule out a variable? I am having a hard time finding the ceramic block resistors. Can I use more modern resistors from say radio shack?

Thanks in advance.

[panhandler62]

Assuming that you have good, clean contacts; it probably does. I don't see any reason why you couldn't replace a resistor with a new one from the shack.

With much higher impdance numbers -- you might have burned up resistors and are actualy measuring the path back through the gauges. You can verify that by lifing one side of the resistor and seeing if you get infinite impedance then.

[docflower]

Funny you say that because I took measurements with the resistors off and they seemed to be open. I wasn't sure if I was just not getting a good reading, but maybe they are just burnt out. Thanks for the response.

Any suggestions as to what size of resistors should I look at getting?

[docflower]

Original ceramic resistors can be purchased here....

http://www.scribd.com/full/5722345?a...jcbazlklxtunnw

pg34 - 10$ a piece.

Not a give away, but I would rather go with a quality stock item any day over a rigged setup.

[VetteVet]

Test the gauges per this ----http://www.6066gmcguy.org/Electrical-02.htm---before you invest in new resistors.

[docflower]

OK so an update and some more questions....

Fuel gauge needed new float. Done and working great.

Temp gauge... I followed the information given by vettevet and it seems I have short to ground.

What doesnt work...
Temp gauge and two idiot lights (brake/temp). Not sure if this is the "idiot gauge panel" has the gauges for brake pres and temp, but also has two lights with functioning bulbs.

The connections on the printed circuit are connected to the temp gauge, idiot lights, and gas gauge. I pulled the panel and tested the connections are good on all of these and so are the bulbs.

I cannot find a short. I tested ohms from temp sender at post and at gauge connection (with gauge disconnected) I get a high of hundreds of ohms lowest I have seen is 140 or something (havent tested with motor fully hot, just after it has set for a few minutes).
I have tried hooking a spare temp gauge to the sender/ground directly and it didnt do anything, not sure if it would just thought Id give it a try.

Also, maybe a ground in the prop valve switch? I dont know where that would be so Ill look for that. Could that have be the short to ground? And thus have an impact on the Temp gauge?

Ignition switch ground issue?

Is it possible that the sending unit has is shorted to ground? I also get 12v to the panel.

Im going to get a 12v tester and light. Will report with results. Any help with the other issues would be great. Especially the idiot light part... seems like that would be indicative of something.

[docflower]

Probably should have said also the temp gauge is pegged to right. When I disconnect green wire from sender it mellows a bit but still at high end of gauge. Also when truck is off gauge settles a bit but still at H. Have used ohm meter and am getting what seems to be normal range for sender.

[docflower]

Ok. So the sending unit is functioning properly. Used a test light. No internal grounding.

When the gauge is out of dash it reads near middle of gauge (I have 2 gauges both act same). When I put in the dash it pegs after I turn on the power.

I have two dash lights temp/brakes. When I turn over the motor the brake light briefly lights up. Nothing happens with the temp light.

I have a short to ground somewhere. I assume. How could I isolate the short? When I have the sending unit connected to wire I tested the wire connecting the gauge and got a good ohms reading (measured ohms at sender connected wire, measured end of wire in cab same number both locations). With this continuity does that mean there is no short to ground in this wire?

[docflower]

So to isolate the short I can remove both ends of the sender/temp gauge wire and ground either end through the voltmeter and listen for tone. Sound right? If i get a tone the line is grounded somewhere.

Is there a common location that these lines are shorted to ground?

[VetteVet]

Too many questions for one post, my responses in red.

Originally Posted by docflower

OK so an update and some more questions....

Fuel gauge needed new float. Done and working great. This kinda rules out the resistor as the culprit

Temp gauge... I followed the information given by vettevet and it seems I have short to ground. Good plan LOL
Yes you do have a short and it sounds like it's in the sender wire.

What doesnt work...
Temp gauge and two idiot lights (brake/temp). Not sure if this is the "idiot gauge panel" has the gauges for brake pres and temp, but also has two lights with functioning bulbs. If you have a temperature gauge and an ammeter along with an oil pressure gauge then you do not have an idiot light panel. There is no brake pressure gauge that I know of unless you mean oil pressure.
Is this your panel?


The connections on the printed circuit are connected to the temp gauge, idiot lights, and gas gauge. I pulled the panel and tested the connections are good on all of these and so are the bulbs.

I cannot find a short. I tested ohms from temp sender at post and at gauge connection (with gauge disconnected) I get a high of hundreds of ohms lowest I have seen is 140 or something (havn't tested with motor fully hot, just after it has set for a few minutes). Sounds right
I have tried hooking a spare temp gauge to the sender/ground directly and it didnt do anything, not sure if it would just thought Id give it a try. You need 12 volts on the gauge and the gauge has to be grounded to the cluster or the battery negative post. Then the sender has to heat up to lower resistance so the gauge needle will move.

Also, maybe a ground in the prop valve switch? I dont know where that would be so Ill look for that. Could that have be the short to ground? And thus have an impact on the Temp gauge? If you had a short in the prop valve your brake light would come on whenever the key was on and stay on.
It would have no affect on the temperature gauge or a temperature light.


Ignition switch ground issue? When you turn the ignition switch to start, the brake wire and the temperature light wire (if you have one), are grounded to test the bulbs in the dash. It may be possible for the ignition switch to be grounded at these points but the light would be on as soon as the key was turned on.

Is it possible that the sending unit has is shorted to ground? I also get 12v to the panel. If that was the case the temperature gauge would return to cold when you took the wire off the sender. Since it doesn't then I believe the ground is in the sender wire. I would take the sender wire off at both ends and sub in a new wire and see what happens.



Im going to get a 12v tester and light. Will report with results. Any help with the other issues would be great. Especially the idiot light part... seems like that would be indicative of something.See the above picture.

[VetteVet]

Originally Posted by docflower

Probably should have said also the temp gauge is pegged to right. When I disconnect green wire from sender it mellows a bit but still at high end of gauge. Also when truck is off gauge settles a bit but still at H. Have used ohm meter and am getting what seems to be normal range for sender.

This is a clue that the sender wire is grounded. Look for a chafed or pinched wire from the sender to the key and then to the gauge cluster.

[VetteVet]

Originally Posted by docflower

Ok. So the sending unit is functioning properly. Used a test light. No internal grounding. Probably not a good test, The sender has too much internal resistance to light a test light. The ohm meter is the best.


When the gauge is out of dash it reads near middle of gauge (I have 2 gauges both act same). When I put in the dash it pegs after I turn on the power. The gauge needle deflection coils are usually equal when there is no outside influence on the gauge so the needle will rest at center.


I have two dash lights temp/brakes. When I turn over the motor the brake light briefly lights up. Nothing happens with the temp light. Normal on the brake, The temp light may not be connected if it's a seven gauge panel.


I have a short to ground somewhere. I assume. How could I isolate the short? When I have the sending unit connected to wire I tested the wire connecting the gauge and got a good ohms reading (measured ohms at sender connected wire, measured end of wire in cab same number both locations). With this continuity does that mean there is no short to ground in this wire?

I'm a little fuzzy on this as to what you did, but with the sending unit wire off at the gauge you should get a very high resistance between the gauge end of the wire and the engine block. It should match the resistance of the sending unit. If you disconnect bote ends of the sending unit wire and check between each end of the wire you should get zero ohms or continuity. If you check between either end of the wire and the truck ground you should get an infinity reading or no ground continuity.

[VetteVet]

---

Originally Posted by docflower

So to isolate the short I can remove both ends of the sender/temp gauge wire and ground either end through the voltmeter and listen for tone. Sound right? If i get a tone the line is grounded somewhere. I think you have it.

Is there a common location that these lines are shorted to ground?
Not really.

[docflower]

Thank you for your responses.I do have the panel you have a pic of above, sort of. My w/s washer is top right and lights are top left. I have two idiot lights on fuel gauge, one is brake one is temp. Both bulbs work and I have continuity.

I hooked the multi meter to the sender end of wire with both ends (panel and sender) disconnected, no tone. Hooked the multi meter to the gauge end of the wire and got a tone. Not sure how one end is grounded .
Also from diagrams seems like the wire leaves the panel and goes through ignition. Is that correct? Mine does not. Seems to bypass ignition and go to rear brakes. Brakes are part of a trailer brake system that seems to be a mess.
I will open dash sat and report results.
.
I will find you ground! Grr
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[VetteVet]

-----

Originally Posted by docflower

Thank you for your responses.I do have the panel you have a pic of above, sort of. My w/s washer is top right and lights are top left. I have two idiot lights on fuel gauge, one is brake one is temp. Both bulbs work and I have continuity. Someone has done some rewiring if you have a temperature idiot light on this type gauge.

I hooked the multi meter to the sender end of wire with both ends (panel and sender) disconnected, no tone. Hooked the multi meter to the gauge end of the wire and got a tone. Not sure how one end is grounded . Sounds like the sender end of the wire is ok and the gauge end of the wire is grounded. Were you able to connect the multi-meter with one lead on the sender end and the other lead to the gauge end of the sender wire. You should get a tone if it's the same wire. If you're connecting one lead to ground, the negative one and the other lead to each end of the temp wire and You're not getting a tone on the sender end but you are on the gauge end, it means there are two different wires in play. The wire to the gauge is grounded but the wire to the sender is not.


Also from diagrams seems like the wire leaves the panel and goes through ignition. Is that correct? Mine does not. The idiot light dash does but yours does not have the idiot light wiring in the dash panel, unless some one wired it up to work. Does it come on when the key is turned to start?The dashed line that goes to pin 5 in the diagram is deleted for the temp light.




Seems to bypass ignition and go to rear brakes.NO way Brakes are part of a trailer brake system that seems to be a mess.Some where in this wire is where your ground is located.
Run a new wire from the temperature sender to pin 6 on your dash gauge plug and your temperature gauge will work.


I will open dash sat and report results.
.
I will find you ground! Grr
Posted via Mobile Device

[docflower]

If you have a temperature gauge and an ammeter along with an oil pressure gauge then you do not have an idiot light panel. There is no brake pressure gauge that I know of unless you mean oil pressure.---- I mean red brake light on fuel gauge. I havent seen an "idiot panel" so Im not sure what it is. Assumed this was an "idiot" light. I do have the gauge panel.

When you turn the ignition switch to start, the brake wire and the temperature light wire (if you have one), are grounded to test the bulbs in the dash. It may be possible for the ignition switch to be grounded at these points but the light would be on as soon as the key was turned on.---- I think I get what you are saying here, as I said above I do have two lights in the fuel gauge area and both have functioning light bulbs (tested with meter) in place. I have only seen the brake light turn on as I turn over the motor.

I have two dash lights temp/brakes. When I turn over the motor the brake light briefly lights up. Nothing happens with the temp light. Normal on the brake, The temp light may not be connected if it's a seven gauge panel.---- How would I know if the light is functioning? I have tested with my meter and it seems to be on the same circuit as the brake light and has available power.

This is a clue that the sender wire is grounded. Look for a chafed or pinched wire from the sender to the key and then to the gauge cluster. ---- I bet this is the problem. I will need some a half a day to go through the whole deal with dash panel removed.

Ok. So the sending unit is functioning properly. Used a test light. No internal grounding. Probably not a good test, The sender has too much internal resistance to light a test light. The ohm meter is the best.---- I used both light/meter to verify.

I'm a little fuzzy on this as to what you did, but with the sending unit wire off at the gauge you should get a very high resistance between the gauge end of the wire and the engine block. This was the case, same ohm reading at end of wire as at senderIt should match the resistance of the sending unit. If you disconnect bote ends of the sending unit wire and check between each end of the wire you should get zero ohms or continuity. If you check between either end of the wire and the truck ground you should get an infinity reading or no ground continuity.---- I didnt check the ohms with both ends disconnected. I will do that.

Sounds like the sender end of the wire is ok and the gauge end of the wire is grounded. Were you able to connect the multi-meter with one lead on the sender end and the other lead to the gauge end of the sender wire.No, but I did the ohm test with the sender connected and thought that would suffice. Does it? You should get a tone if it's the same wire. If you're connecting one lead to ground, the negative one and the other lead to each end of the temp wire and You're not getting a tone on the sender end but you are on the gauge end, it means there are two different wires in play. The wire to the gauge is grounded but the wire to the sender is not.---- This is correct. Seems to be a ground in the gauge end of the wire.

Also from diagrams seems like the wire leaves the panel and goes through ignition. Is that correct? Mine does not. The idiot light dash does but yours does not have the idiot light wiring in the dash panel, unless some one wired it up to work. Does it come on when the key is turned to start?The dashed line that goes to pin 5 in the diagram is deleted for the temp light.---- The diagram I was referring to (http://farm4.static.flickr.com/3571/...59575fe1_o.jpg) gives me the impression that the 20dg leaving the 6pin location goes to the ignition and then onward. You are obviously right, I just got confused as the diagram doesnt really clearly indicated where the non-idiot panel 20dg goes after 6pin.

Seems to bypass ignition and go to rear brakes. NO way Brakes are part of a trailer brake system that seems to be a mess. Some where in this wire is where your ground is located.
Run a new wire from the temperature sender to pin 6 on your dash gauge plug and your temperature gauge will work.---- And then the answer. I will try to find the ground by spending some time this weekend. If I cannot find the ground I will run a new wire. Everything is just so clean and nice up there under the dash, I dont want to make it not look good.

Thanks again for your help with this. Do you have a shop/site?

[VetteVet]

If your temperature light comes on in the panel then you must have a wire for it coming through the key switch to test it in the start position. You would also need a separate sending unit since the light and gauge use different resistances to function.

I have a nice shop but no website. I gave it some thought but figured it would be too boring so I just adopted this forum LOL.
I only wish I had a 12 foot ceiling so I could install a lift. Mine is 10 feet so not quite enough. Some day I may modify it and put one in.

[docflower]

3. Remove test lead from body of sending uint & connect lead to
***termanal of unit. If bulb lights, engine sending unit is
***internally short-circuited & should be replaced.


So when I used used the bulb (per above) above I had no light, but as you said the bulb method might not work. I hooked my meter to the battery and the sending unit ground and the sending unit teminal, both read 12.6v when motor is both hot and cold. Im I correct in assuming I have a bad sending unit? Seems like an internal short. Would this short make my gauge peg?
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[docflower]

Blah.

[docflower]

No problem with the sending unit. Paid 9 bucks to figure that out. Tried two gauges both act the same. They both settle until I turn the key then instant peg even with cold motor. Disconnect the sender tthey settle. Seems like it must be a short to ground I just can't find where.

unless.... Both gauges are bad. I get proper ohm reading to harness. I dont get any reading on either post (left/rt) to ground. And 87 over resistor. This was whil panel and everything was in place.
Posted via Mobile Device

[VetteVet]

Here are a couple of most excellent pictures by one of our fine members. OK three.

[docflower]

So if I am reading those pix right I should be able to see an ohm reading at the left teminal similar to what I would find at harness before the panel (450ohms cold). Correct? I am not getting any reading from the left teminal. On the right teminal I am getting the 12.6v expected. Would this be indicative of a bad gauge? How does a gauge go bad?

for 29 dollars I'll find out next week if the gauge is indeed bad when the new gauge comes in from lmc.

thanks for the guidance vet. I have learned a lot during this adventure!
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[eric8]

So what fixed it?

[docflower]

Bad gauge.

[eric8]

Cool thanks for the update! My truck is having similar symptoms.