Help please...70 convertible. I've owned this car many years and can't remember temp gauge ever working. read every post for many years tried many things. Gave up and took to my mechanic who's good with vettes. Replaced temp gauge with new from corvette central.
Replaced thermostat which he said was stuck open. Replaced the sending unit which he said didn't match temp gauge. Changed the resistor 'cause he said wasn't measuring the engine temp correctly. Sounds like he covered all the bases and tells me I'm good to go.I picked it up at closing time and noticed that the temp never moved from 100*. I stopped for a long time and just let it idle and the gauge moved slightly. I have a gauge in the thermostat housing that reads at 180* Where do I go from here? I would be willing to replace the whole system for aftermarket if that is what I have to do. What should I tell my mechanic to check when I take it back? Thanks.

 

Did they use teflon tape on the threads of the sender? I have heard that can be a problem.

Frank

 

I have talked to them several times about this and they are of the opinion that the threads cut through the tape enough to make contact. I don't know what they did but I will ask them again. thanks

 

Well, at this point you are better off doing your own work. Your mechanic hasn't got a clue. A [good] mechanic only has three things to check, once the gauge cluster is opened up. (note: there are some simple things that can be checked before the cluster should be accessed; these could rule out the sender and the thermostat.)

1) the resistance between the sending unit lead (removed from the gauge) and ground. This will determine if that wire is open and/or if the [cold] sending unit is set at an appropriate resistance value for a cold engine.

2) throw a low voltage 'spike' into the meter to see if it works at all. If it works at all, the problem is very unlikely to be the gauge.

3) run engine till warm and perform the same measurement as in #1 (assuming that test #1 did not find an 'open' wire). See if change in sender resistance is appropriate for the sending unit at operating temps.

Depending on what results you get from the above tests, the 'real' cause for your problem can be determined.

Your mechanic just changes something and looks for the problem to be 'fixed'. He continues that process until a solution is reached. Unfortunately, every "try" costs you money, whether he guesses right or not. I'd say he's not a very good "guesser".

 

Your mechanic is right on this one. Jump to the bottom of this page for an actual test:
http://www.lbfun.com/warehouse/tech_...eCF-Thread.pdf

 

7dshark ,
with the ign key on pull the green wire off the sender-needle should go full COLD
now ground that green sender wire to the back of alternator case-needle should go full HOT
Let us know what these 2 tests show
next test will require a trip to Radio shack so you can spend around $3
I suspect your sender is like many senders on the market,not "sending" the correct signal to the gauge so it will read accurately.

 

I will do the test when the car comes back from the radio installer. The biggest problem is that I am MR. BLACKWRENCH..I brake everything! That's why I have my mechanic do all the behind the dash stuff. I wish it wasn't that way. At 51 yrs old my patience is declining rapidly. Is it crazy to think that a correcly funtioning temp gauge system will function the same as in a new vehicle?

 

Once the bugs are worked out they do real well.

 

Here's the procedure to diagnose temp gage problems:

Tests
A. Disconnect the green wire at the sending unit and turn on the ignition switch. The gauge hand should stay against the left side stop pin.

B.Ground the green wire disconnected from the sending unit. With the ignition switch still "ON," the gauge hand should swing across the dial to the right stop pin.

Corrective Measures
If the gauge hand does not stay to the left as in test "A", either the wire is grounded between the dash unit and the engine unit or the dash unit is defective.

Test further by disconnecting the dark green sending unit wire at the gauge. Turn on the ignition switch. If the gauge hand stays on the left hand stop pin, replace the disconnected wire. But, if the gauge hand still moves, replace the dash gauge.

If the gauge hand does not swing across the dial as in test "B", there is an open circuit in the wire between the sending unit and gauge, the gauge is defective, or current is not reaching the dash gauge.

Test further by grounding the sending unit terminal (dark green wire) of the dash gauge and turn "ON" the ignition switch. If the gauge hand now moves, replace the disconnected wire. If the gauge hand still does not move, connect a test lamp to a ground and to the power (pink wire) terminal of dash unit. If the lamp does not light, test the wire between the ignition switch and the dash unit by connecting a test lamp to the "Accessory" terminal at the ignition switch and to a ground. The test lamp should burn.

If the gauge hand operates correctly as in tests "A" and "B" but the gauge does not indicate temperature correctly, either the sending unit is defective or the dash gauge is out of calibration. Replace sending unit one of known accuracy. If gauge reading is still incorrect, replace dash unit.

If the gauge hand is at maximum at all times and test "A" and "B" indicate that the wiring and the dash unit are in condition, the sending unit is bad and needs replacement.

If the gauge hand will not move, the dash unit is bad, or incorrectly installed. Correct the installation or replace the gauge unit

 

Hi Roger,
I'm curious about your Radio Shack mention.
Is there some kind of little tester that can be made?
I know there's one to test the fuel gauge... is there one for the temperature gauge too?
Regards,
Alan

 

I would get an infrared thermometer and see exactly where your at as far as the actual engine temp and go from there.

 

Hi Alan,
Yes , and its pretty simple and cheap. It's just a 68 ohm resistor and a 10 ohm in series for a total of 78 ohms which should make his 1970 gauge read aprox 210 degrees. Just clip the alligator to the alt and plug the green wire on the other end.Willcox has the ohms verses temp in his help section under temperature gauge.
Alan a $10 pot will test all your gauges,Willcox also has an outlet for the pots.

 

Is that some kind of Florida snake???????

Ok.. been reading this today.. have not had a chance to reply..

Please do as suggested on testing the gauge. This the first place you need to start. You need to establish if the gauge is working properly.

Second thing.. if the sender he replaced was the original one.. I'd go back to the mechanic and retrieve it asap! Working or not.. you should keep this.. (it is fixable, just ask Roger).

Doing the test that Roger posted is a quick way to establish if the gauge function is working.

I put no faith in the current sending units... None of them work... I have no problem saying.. NONE of them work properly and I've tested them all!

The fix for a sending unit not putting out the correct ohms is not resistance inline. You can do this but since the gauge is non linear it will only read correct at on position. All other points will not be correct! Yes, you an use a resistor to make the gauge read correctly at say 210 or 100 and some people elect to do this.

You can change the resistance to ground on the back of the gauge to tweak it to make a correction. (maybe we will talk about this more later, kind of hard to do but very do-able) Or you can repair the old sender.

Yes.. Teflon tape has been known to cause an issue or two. IBfun did a great write up with a good example of the threads cutting through the tape. But it is also possible for "Bubba" to go crazy with the tape and the sending unit not make a good ground, or for a person to not tighten up the sender enough to cut through the tape. Best thing to remember is to apply the Teflon tape to only the first three threads. The tape is not a sealant, but rather a lubricant to allow the threads to seat and mesh properly.

If you find the gauge works correctly then you have an input ohms problem to the gauge and we will move on from there... There are some great minds that have posted in here and between them we all can help you figure this out.

Post back..

Willcox

 

But what about the gauge.. you said it was new..
I don't think they used the 90 ohm resistor on the new imported gauges and if it's new it's imported... This might be the problem too.. Does anyone know about this..

Now.. if you get to Radio Shack.. you'll need to get resistors that are 50. 75 100 and 200 ohms..

Why... because for an original temp gauge to work properly.. it requires..

220 ohms to make 100 on the face.
109 ohms to make the first mark.
79 ohms to make 210 mark.
65 ohms to make the mark just after 210
51 ohms to make 250 mark..

So if you get 50-75-100 and 200 you will be able to get real close to these readings on the gauge. Or I can get you a link where you can order an adjustable pot that you can dial up..



Willcox

 

Alan.. Next time I order me a batch of the linear adjustable pots.. I'm gonna get you one! On me..

I think Roger will attest to the value of having a few of these laying around. And they simply replace the sending unit for testing. But with an ohms meter you can preset the output to what you desire.. The put it in line. So if you want 70 ohms resistance you just dial it up and then test.. then dial up a new measurement and test again.. Might have to send you a Willcox special tester.. http://forums.corvetteforum.com/c3-g...-me-today.html

 

Tester is still working great Ernie,Thanks
I think you should start selling them.

How's the weather up there , 91 here today ?

 

Oh the weather is all crappy.. if it rains any more.. I may have to go in the Ark Parts business..

 

The Willcox special tester sure made life much easier and a lot more fun.

 

I see you have it behind the flux capacitor testing electrodes.... Marty will love it when he stops by.. Love it..

Since I have a habit of cooking the pots.. I'm about due for a new load.

We can get shark going.. now we just need some info from him.

 

First test full cold. Second test (grounded) full hot. what next?