Diagnose transmission temp gauge

[opus]

I just replaced the sending unit.  How do I test the gauge to be sure it is in operating condition.  Its a 12v gauge, rear engine bus.  Never fiddled with electric gauges before.....

[DaveG]

Gernerally speaking, guages have power to them and the "sending unit" has varing amounts of resistance, completing the electrical circuit. In the old days, we would turn the key on and have someone "ground" the sending unit wire and the guage would go to maximum.

Different guage mfgs. use different amounts of resistance to actuate their guages, so a service manual may say something like "175 ohms at 100 degrees" and you would take a temperature reading (thermometer in radiator, heat gun oil pan) to get a rough idea of temperature and then use a DVOM to measure or read the resistance of the sender.

Hope this helps you basic understanding of electric guages.

[opus]

If I ground the wire, the gauge is pinned to the max, but I cant get any reading at the gauge driving down the road.  Do I measure the ohms at the gauge or sending unit??  I guess measuring ohms is something I am not sure of, unless it is something I can hold in my hand, with both ends accessible.

[JohnEd]

The sending unit and gauge must be compatible.  At 180 degrees a sending unit might have 100 ohms of resistance while another might have 10.  You have verified that the wire that runs to your meter is good and that your gauge is probably good.  Research your gauge and determine what sender unit it should have and install that.  The sender you have will have a chart that will show you what resistance you shoud measure at any temp.  Get a digital ohm meter and learn.  For now...get a friend to help that has a meter.  Any mechanic should....

John

[opus]

Gauge and unit are correct.  I'll get hold of the manufacture and get the specs on what it should read.

[Busted Knuckle]

OK just because I have personally done it and learned the hard way.
I have to ask. "Was teflon tape, paste, or other sealant used to install the sending unit?"

I learned not to use these the hard way! I'm sure all the years I've been turning wrenches that the instructions plainly said NOT TOO use any sealant when installing them. (but who read instructions? "we don't need no stinking instructions, this is simple!" )
But yes I admit I had been using sealant to install sending units for yrs with no trouble (at least that I knew of) until a couple years ago when I installed an auxiliary gauge in the rear of a bus we owned and it just wouldn't read right no matter what.
So I took it out and took it back for a replacement and replaced it.
Again it didn't work, so I fooled with this & that, and this and finally decided I had another "bad gauge." So again I took it back, and again they gave me another (both times with out even looking to see what I had brought back)
And again I re-installed a new one. Humpf  *%^(*#@ still not working! Weds Thurs Friday why is this stupid thing not working?
Then I take it out and return it again, and again the parts man is ready to give me yet another one. But I stopped him and told him "hold on a minute this is the 3rd one of those I have brought back, and to be honest I'm tired of replacing them just because they are JUNK!"
He looked at me kinda funny and said "really? I never have had any of these brought back that I know of. That is odd, let me check and see what kinda return rate we have on these."
So he gets on his computer and says "it shows here that in the last 24 months we've only warrantied 3 of these."
SO I said "well ain't that funny, I personally know I've brought back 3 in the past 2 weeks!" To which he replied "and yes by crossing this # over I see that 2 of those were you, and one that the gauge was busted in shipping  !"
Oh I was just about to go off on him when the manager walked up and asked "What's the problem Bryce?" SO Mr. know it all parts guy and I each tell him our side and the manager says "Well I know Bryce is a pretty good mechanic, and I know *whatever his name was* knows parts and I see a problem here that needs looked into further!"
So he says "lets take a look and see if we see an obvious defect, go grab a new one off the shelf."
While *whatever his name is* goes to grab one Greg & I open the box, and as soon as Greg pulls it out of the box he says "well look here it has sealant on it. It could never get a proper ground that way" and winks at me. And shoves it in the box. About that time *whatever his name is* walks back up and hands Greg the new one and Greg opens it pulls it out just far enough for me to see the end of the sending unit and shoves it back in the box.
He hands it to me and says "Here Bryce this one looks OK! Have a great day!" and winks at me again. As I was walking away I heard him tell the new counter guy "Here take this bad one back and throw it in the return bin, it was bad."
  BK 

[opus]

[looks around] Of course I used sealant!

[JohnEd]

I have put in 4 of these in the recent past and I recall thinking that I wasn't using sealant and mulling that "fine pitch pipe thread on brass should seal on its own and I don't recall ever using it before.  So they all went in without sealant.  Had sealant been handy, I would most assuredly used it out of habit.  Thank you Bryce and please convey my thanks to that parts manager.  That's another toe that won't get shot off.

I did dig a problem out of a neighbor's Suzuki many years ago that was related to this.  He put Locktite on the washer (?) of the block bolt under which he installed the ground cable from the battery.  It made the same sort of contact as a really corroded bat terminal as it ran the inst panel lights until he hit the starter and then everything went dark till he cycled the ignition sw again.  That guy lived two doors down and I always got the impression that he didn't really care for me nor and my dirt bike buddies that roared down the street the half block to the road's end to get to the Ca "outback".  I struck an arc from the bolt but the lug sitting there under the washer was open.  Locktite will swell between two pieces of metal and insulate them.....go figure.  Only cause he had his hat firmly in his hand and he came in at the appropriate "altitude" , knee caps  a'knocking on the pavement, did I help the AH.  Has anybody ever heard that about Locktite?

John with 2 cents again

[robertglines1]

Daaaaaa good lesson for me to learn because I would have used teflon tape.  Makes sense in the big picture. sometimes I don't see big picture.   Bob

[gus]

It beats me when using sealant how the threads don't dig into the block and make a good ground, that is a complete mystery to me but I believe it when I read about Cliff's experience!