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testing the temp gauge in situe

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Matt Wood

testing the temp gauge in situe

Hi guys, is there a way to test the temperature gauge whilst still in the truck? i have recently put a different cluster in and now the gauge doesn't move, issue is at the same time i cut and remade a new loom on that side so i cant confirm that the cluster is the fault thus my reasoning for testing

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: testing the temp gauge in situe

Administrator

Easy. Pull the wire off the sender and ground it. With the key on the gauge should go to the top of the range.

If that works then put a DVM on the sender. Cold it should read ~72 ohms and with the engine hotter than it is supposed to be then it should read ~10 ohms. However, I've not had good luck testing the temp sending units with a DVM. Those I've tested didn't read how they are supposed to read.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Matt Wood

Re: testing the temp gauge in situe

thanks for the tip! worked a treat!, now my question is, i have cut the sensor wire and extended it by about 2 feet, will that have caused the resistance to be wrong thus making the gauge see a higher resistance therefore show cold? i don't think it should matter seeing as the only wire that says not to cut is the resistor wire. i'm not convinced the sender is broken either which puts me at a bit of a stopping point, i will try tomorrow with the DVM and see the outcome but i suspect it will work just fine

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: testing the temp gauge in situe

Administrator

No, if you used #18 or larger wire it won't have any impact on the reading.

So, I take it grounding the wire made the gauge go full scale so your wiring is good. Good luck on testing the sender. You could put it in a pan of water and bring it up to a boil, at which point it should be down to about 10 ohms.

Gary, AKA "Gary fellow": Profile

Matt Wood

Re: testing the temp gauge in situe

not sure why every issue i have with the truck is always a massive drawn out process.... but, the issue deepens,

I have taken the sender out and tried it in boiling water, the gauge didnt move... however the resistance of the sender went from about 600 ohms down to 400 before i took it out because i was burning my hand off lol. now... this seems way outside of the 73 to 10 ohms you said about so my plan is to get some resistors of varying values (from 73 to 10 ohm) and see what the gauge does. im also going to buy a new sender as they cant be that expensive, even if i do need to order one and have it shipped over here. Is it a 3/8 thread?

Before i conduct my tests are there any other suggestions you have? I have a show on sunday and wont be able to get a sender in time so will be driving 'blind'

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: testing the temp gauge in situe

Administrator

Matt - As said, I've not had good luck measuring the resistance of senders. I had several and one or more of them was for a later truck which uses a different resistance range. Never did find out which was which, so ordered new.

Not sure the thread size, but I think it is 3/8" NPT.

But when you checked with the sender in boiling water, did you have a ground and the hot lead connected?

As for resistors, check the bottom of this page which specifies 10 ohm and 73 ohm resistors. But note that you may be pulling 500 ma. And at that kind of current you'll need pretty hefty resistors. The formula for wattage is W=I*I*R=.5*.5*10=2.5 watts.

Gary, AKA "Gary fellow": Profile

Matt Wood

Re: testing the temp gauge in situe

and the saga continues...

so i haven't had time yet to go and test the gauges with resistors however i took the truck to a show yesterday and it would seem that my oil gauge has jumped on the band waggon and is also acting up lol. I have fitted a mechanical gauge right at the sender so i know the correct pressure luckily however the gauge seems to think it has oil pressure when the truck isnt even switched on! All i do it put the key in turn it to ACC and hey presto i have perfect oil pressure according to the gauge haha. I'm going to chase some grounds next and see where it takes me.

After which i am still going to check the gauges with resistors (thanks for the heads up on the power) to make sure they are giving 'accurate' results

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: testing the temp gauge in situe

Administrator

You should see the gauges come alive in Accy. As shown below, power is fed to them in both Run and Accy. But, that doesn't explain having a reading other than below zero.

The Instrument Voltage Regulator, aka Instrument Cluster Voltage Regulator, provides a pulsing voltage to all three gauges that averages about 5.4 volts. And if it glitches, which is very common, all three gauges will react, and if that glitch is to give higher voltage then the gauges will read high.

I like to replace the IVR with a real regulator, and one way of doing that is shown on the page at Documentation/Electrical/ICVR. In my experience that makes the stock gauges work very well.

Gary, AKA "Gary fellow": Profile

Matt Wood

Re: testing the temp gauge in situe

So I tend to agree that if the regulator is acting up it would cause the gauges to act up. However, I tried the original cluster that I removed and it did the same thing. This would suggest that I dont have an issue with the IVR and that its somewhere else down the line

'Shelby' - 1985 bronco XLT, 351W C6, Limited slip 9 inch

Gary Lewis

Re: testing the temp gauge in situe

Administrator

That is correct as the IVR is screwed to the back of the cluster. There's not much else in common that would cause a big difference. I wonder if that 8 - 9 ohm resistance wire would? If, somehow, it was giving full battery voltage to the IVR then perhaps either cluster would show the problem.

What about a lack of a ground to the cluster causing the problem? That would drive the IVR barmy, but I don't know what the results would be.

Gary, AKA "Gary fellow": Profile