[Multi-Point Thermometer] [LASER]


This seems to be a very useful and popular way to keep track of  bearings, hubs, tires etc. There have been at least two incidents, where alert GMCers using this tool, have caught problems before they became trip ending events.

  Steve F. found the different temperatures shown below and upon further inspection found bad bearings in the offending Hub.

Mr.C found a 100 degree difference in one of the rear drums, and when he opened the wheel, he found a spring (replaced about 1999) had broken and allowed the brake shoes to drag.  This wore out the brake shoes and probably was cooking the bearings.

 The best source I have heard about is
 provided by Al Kruebbe.

 I thought it would be good to start a list of what readings we should expect on various points.


FRONT HUB 140 205







Newly Discovered Diagnostic Tool

I was getting our GMC ready yesterday for our yearly fishing trip up in the northern CA when I discovered the engine was missing.  I was dreading having to remove the front wheel wells to get to the sparks plugs to find which cylinder was missing.  When I was walking out the entrance door I just happened to notice the small hand held infrared temperature indicating device that I keep next to the door for checking  tire and wheel temperatures when I stop during traveling. 

The old brain woke up and said why not use it to determine which cylinder was missing.  With the engine cover removed, it was very easy to determine the temperature of each header pipe next to the engine.  In about one minute I was able to determine that the # 8 cylinder exhaust pipe was much cooler then the others.  I was about ready to remove the right front wheel well when I happened to notice a rubber cap on a vacuum port on the intake manifold was severely cracked  The vacuum port was on the rear side where I had disconnect the vacuum line for the OEM cruise control when I put on the electronic cruise control.  The vacuum leak was causing a lean mixture going to # 8 cylinder to the point it was misfiring.  It only took about five minutes to find and fix the problem so the infrared device paid for it's self today in saving me a lot of  time looking for the problem.  This is probably "old hat" to some of you, but I had never heard it talked about before.  I hope it helps someone else. Chuck Aulgur La Mesa CA

Multi-Point Thermometer

There have been tons of questions about temperatures on the GMC motor home.  It seems that we are always asking what is the heat rise of the Refer, the engine compartment, the batteries, the transmission, etc.   I have often wondered while driving down the road if my wheel bearings were getting hot, were the brakes hot?  did I hear a strange noise back by the generator, was something getting hot there , was I over charging a battery on a hot day.  I wanted a multi-point thermometer that I could switch from the drivers seat that was inexpensive.  Accuracy of plus or minus a few degrees is ok with me since I am looking for large changes of temperature.

Click for Detail
I have modified a  dual readout thermometer from Radio shack that costs  nine dollars to Twenty five dollars. The thermometers store the max and min temperatures and these reading can be viewed by pushing a readout button.  They even have one that will give voice readings every hour and will give a vocal warning if the temperature goes over a set point.  I have modified the instrument so that the two  channels are fed by  two external 6 position switches which are connected to 12 thermistor sensors that I also manufactured.  The Sensors cost $2 each.  The thermometers have the advantage of operating from internal batteries so there are no ground loop or noisy power supply problems. The unit shown above is the one I will use in my GMC.

Click for Detail
Building Sensors
Radio Shack has all of the parts you will need for this project.  The Sensors are made from a RS -271-110A ($2) Thermistor. (Note not all instruments use this 10,000 ohm sensor.   Some are using a 50,000 ohm sensor for which I do not have a source.) A thermistor changes resistance with temperature and this is the sensor used by these inexpensive instruments.  The wire to the sensor is the smallest two conductor speaker wire you can get.  Radio Shack has several types.  Strip the wire back 1/4 " and solder a wire to each leg of the Thermistor which is placed over the top of the zip cord as shown.  It is a good idea to protect the thermistor with pliers while soldering the connections as shown.

Click for Detail Use shrink fit tubing over the connections , leaving the thermistor exposed.  We will attach the thermistor to a mounting appliance later.   The picture compares the fabricated sensor to the Radio Shack sensor that comes with the instrument.

Click for Detail The two pole, 6 position  (RS-275-1386a )switch will allow for 6 sensors , 5 fabricated plus the one that came with the instrument.  The wire from the instrument that went to the external sensor is cut and attached to the swinger contacts of the switch.  The sensors are then attached to the rotary connections of the switch so that a pair from each sensor is selected at each position.  There is no polarity on the sensors so either wire can be used on either connection.  There is no practical limit on the length of the wire going to the sensor.  Now when the Digital Thermometer is in the   OUTSIDE switch position, the rotary switch selects the sensor to be used.   The instrument shown cost $9 from Radio Shack on a closeout sale.

Click for Detail The fabricated Sensors can now be attached to terminal lugs as shown for different applications.  The top terminal shows the Sensor will slide into a terminal which can then be bolted to device to be monitored like a battery.  The Thermistor should be epoxyed  to the terminal with J-B weld and then shrink fit placed over the wire and terminal for strain relief.  The lower terminal shows that a smaller terminal can be used and then a hose clamp used to attach the terminal to a pipe to be monitored.   The Sensor could be J-B welded to a position like on a Hub, but this would make it difficult to remove for repair or relocation.  J-B weld was selected as the epoxy because it seems to have good thermal conducting properties but it does not conduct electricity.

This system does have some limitations.  The accuracy is about plus or minus 2 degrees and the reproducibility is about the same.  The upper limit of the temperature is about 130 degrees F which limits some of the engine measurements.  However, if you are looking for failure conditions, it might be possible to measure a location that does not routinely reach that temperature, but would on a failure condition.

I hope this project will encourage GMC owners to make measurements  and post the results on GMCnet.  For example, I am sure you would  like to know it really helped to place the rear panel 1/2 inch behind the coils in the Refer.  Now you can take temperature measurements before and after at the top, bottom, freezer , and cabinet of the Refer.  I know many of you want to know if your batteries get hot during charging /driving / discharging.  I know I want to know , and I will soon let you know.   All of this for less than the cost of dinner.