Temperature Gauge Woes

[pabusnut]

I spent so much $$$$ fixing the steering on my 4905 that I let some lesser costly things go undone recently.  One of those was my temperature gauge problem.

History-- Immediately after I bought the bus(years ago) I installed a newly rebuilt 8V71 in place of the tired smoking one that was in it.  But "new" engine had been in a 12VDC bus and the 4905 is 24VDC, so I knew I needed to change the sending unit, but never got around to it since the bus was unfinished, and my trips were extremely short.  About 2 yrs ago, I changed the sending unit, but then the engine seemed to run hot(per the gauge, not the IR gun).

The Problem--Started the bus to take on a short trip to the local state park for a "shakedown " of the house systems(went all the way 9 miles!), but before I even built air pressure the temperature gauge was pegged at 260!-- so I shut it down!  Walked around to the back and checked the engine temp(I could lay my hand on the thermostat housing) and it wasn't hot.  When I climbed back in the seat, I realized that with all power off the gauge still read 160.  I ran it to the campground figuring that since the engine wasn't hot, I could risk a 9 mile trip with one short hill.

When I got home I called Luke, but he needed the number off the gauge.---well I finally got to tearing the gauge out of the dash Monday --3 weeks later!  Even out of the dash, it only went back to 160, so I called Luke and ordered the new one.  He assured me that the new one goes back to 100.

Being now smarter than I was last time I ordered parts from Luke, I immediately informed the "better half" before they showed up on the doorstep!  I replaced a faulty downstairs  bathroom fauced without being nagged as an added measure of "CBHI--Continued Busnut Hobby Insurance."

Steve Toomey
PAbusnut

[sledhead]

Hi Steve  I use a hand held laser thermometer  to check tires,Eng. temp.As soon as I get out of the bus after driving .helps to see if tires are low or if something is wrong . As for the gauges on the dash they all lie ,I drive with two gps units as one will eventually be wrong .But they do look good at night when the lights are on (dash gauges )    dave

[Geoff]

I had an electric water temp gauge with matching 12v sender in my RTS, but I didn't like the 1/3rd dial because beween 180 and 240 it was a small jump.  I happened to have a 24v temp gauge with a better sweep and more detailed readings so having both 12v and 24v available in my RTS I installed the new 24v gauge with matching sender. 

I went on a test run and used a laser temp gun to measure the temp at the sending unit and the reading on the gauge (forty feet of wire up to  the front), and found the new gauge wasn't reading correctly with the laser temp gun.  So I took the 24v sender out and put the old 12v sender in, and guess what?  The sender for the 12v gauge was dead on correct with the laser temp reading at the sender for the 24v gauge.

I know you had a bad gauge if it was stuck at 160 cold, but using the laser temp gun is the way to dial in your temperature gauge on the dash.  Senders have a resistance built in but it is for trucks with a short wire from the sender to the gauge.  When you have a forty foot bus it may not read correctly.

--Geoff

[gus]

There is a string on one of these boards about a new digital water temp gage.

I, too, am tired of my old mysterious analog gage and have ordered two them from this guy
http://www.ebay.com/itm/-/180983683629?item=180983683629&ViewItem=&vxp=mtr.

I notice he has raised his price $2 but it is still worth it, I hope, since mine haven't arrived yet!

[Len Silva]

The voltage should not matter to the sender, only to the gauge.  The temperature sender must be matched to the gauge by resistance.

[luvrbus]

Len, sure glad you posted about the voltage I have been telling it people for years voltage does not matter on a sender but they still shop for a 24v sender when they do find one the sender cost twice as much

[belfert]

All of the 24 volt gauges in my bus are just 12 volt gauges with resistors to drop the voltage down.  They do have 24 volt bulbs to illuminate them.

[pabusnut]

Continuation of the Saga--Part II

I installed the new gauge, but still have the same problem, the gauge pegs shortly after starting the engine.

This time I checked that the sending unit was getting 24VDC.  With the wire off the voltage measured 24 VDC (engine not running, but power on), and 15.6 VDC difference between the post and block.  I did not have the IR Temp gauge this time, but the engine had not run for more than 5 minutes, and was mildly warm on the thermostat housing.

Luke is sending another sending unit, so we will see.

Steve Toomey
PAbusnut

[Len Silva]

Lets start with the brand and part number of your gauge and sender.  Automotive temperature gauges are basically ohmmeters which measure the varying resistance of the sender.  The problem is that different gauges use different resistances.

For instance, VDO gauges use the resistances from the attached chart but other manufacturers may be very different.

With temperature gauges, generally the lower resistance is the high temperature so a gauge that is pegging probably has a short in the sender lead.  (it's just the opposite for pressure gauges)

If you know the resistance scale for your gauge, you can substitute a resistor for the sender and check it that way.  For instance, for the first gauge shown in the chart, a 62.2 ohm resistance substituted for the sender should read 100 degrees.

Checking the voltage at the sender will not give you any useful information.  If you ground the sender lead, it should peg the gauge.  Having the sender lead open will not tell you anything.  Different gauges come to rest in different places with no connection at all.

[eagle19952]

Lots of good advice and tips here.
or..google
Capillary Type Temperature Gauge
Electric Contact (CAPILLARY) Temperature Gauge With Thermowell
These are available with up to .S.316 Armor Capillary (1 meter - up to 50 meters available) and are capable of incorporating an over temp shut down switch.

[loosenut]

Continuation of the Saga--Part II

I installed the new gauge, but still have the same problem, the gauge pegs shortly after starting the engine.

Steve Toomey
PAbusnut

I have the same problem intermittently, usually after dark when I'm using headlights.  Don't have any idea as to what is going on.   

I bring this up because I don't believe my problem is either gauge or sending unit and while I have links verwendbar wiring you too might have a problem with wiring not equipment.

I agree with everybody else about the temperature gun.  It is a great tool for peace of mind. 

Mike

[Len Silva]

You can probably cure most of you gauge problems by running a ground wire directly from the engine block to the gauges.