I tried to start the used ( 3 starts) Ram 750F today that I bought from a guy on here. The problem is that the Temp would never show above 20 deg. C. I think that either the temp probe is bad or there is an ECU problem. While I was setting the ECU up to my TX at home the temp probe wires were getting warm.

What should the resistance be across the temp probe wires? I am a technician but have never used thermocouples at work...

I get 7.8 Ohms across the temp probe. Is this good or bad?

Gene,

Not sure that would be the correct way to measure whether a thermocouple is good or bad but here are the results I get from a simjet thermocouple...

5.6 ohms on a good sensor
7.6 ohms on a known bad one

The Ram and the Simjet are the same temp range I believe.

Dont know if this helps but....

Todd

Gene,
If you are reading 20 degrees on the start box,
then the thermocouple should be good. When
you push the start button do you get ignition
on? Then when you push the air and gas switch,
do you get a pop? If there is a pop does the
temp stay the same (20 degrees) or increase?
If you got a pop and the temp stayed the same,
unplug the glow connector and the temp should
increase and after 150 degree there should be
a fuel ramp and start up. I have several 750
and have had the same problem.

Johnny Woods

Huh? This is really weird. I have a 750f and have had no problems with it, but I think that to get a good temp indication by unplugging the glow plug connector indicates a short somewhere. It is certainly not a normal condition. Are you out there, Tony F? Maybe you can explain this strange malfunction.

Hi, I just had the same problem with a Ram 500. The probe is most likely bad. They should never get warm on their own. A way to ckeck if it is calibrated is to put it in a bucket of ice and it should read 0 degrees and then into a pot of boiling water and it should read 100 degrees. I will bet a dollar to a donut that the probe has a internal short and that is why it is getting warm. This is what was happing with the Ram 500 we had such a time with a few weeks ago. A lot of people gave me things to try, but when I touched the HOT and I mean HOT probe I knew it was bad;since I had just turned on the ECU and had Not tried to start the engine that morning. We changed it out and everything works great now. Hope this helps, Ron Stahl

Who did you buy this engine from, since he on on here, maybe we all would like to know.........Perry

I have already contacted the person that I bought the Ram from. He called me last night and told me that it had no problems the last time he ran it ( last September). He was more than willing to pay for the problem to be fixed! A very nice guy that I would buy from anytime! I really don't want to give his name because he is one of the good guys and I don't want this to cause him any problems.

Thanks for the replies!

I tried to go to the Ram web page Last night and today and can not get to it. Have any of you had this problem? Is it down?

Gene

At room temp. the ECU reads 32 deg C. I heated the probe with a heat gun ( used for SMT part removal ( at least 500 F) and the ECU still reads 32 deg C. If I unplug the temp probe the ECU then reads 0..... It looks like the probe is bad.

Does it appear that it has ever had a severe bend or kink in it? If so, I know that can cause it to short out. Always make gradual bends in these thermocouples, not sharp bends.

I tested the temp probe today. When they are heated they produce a small voltage ( even when not hooked to the ecu). I heated the probe with a heat gun and checked the voltage. It started low and increased as it got hotter to about 40 or 50 millivolts. It looks like it may be working! And the ECU may be bad.

I was with Gene when we attempted to start the 750.
The engine would start normally when air and propane were
applied. I was obvious it was running on the propane .
The display would jump between 12 and 24 degrees c .
Because of the low temp indication (???), the ecu would never initiate the fuel ramp sequence . We had good glow and the fuel pump functioned normally during prime.

It is not impossible that the ECU is bad, but it is much more likely that the thermocouple has gone belly up. You might take the little yellow connector apart and make sure it is terminated right. The other thing you can do to check the ECU is take the thermocouple wire, and cut it about 2" from the connector. Strip back about 3/4" and twist together as evenly as possible, then try heating it. The wire from the thermocouple is ....... you guessed it type K thermocouple wire and will function as a thermocouple as well. This way at least you can verify your ECU is functioning correctly.


David Reid

David's way of checking the thermocouple is good. Do not try to run the engine on propane to many times to check the probe. There is no oil going to the bearings and you may fry the bearings. Also make sure the probe ends are in the slots of the ECU. It is very easy to slide the ends underneath in the ECU. You may want to check with RAM as I think and not sure if the unit will start with a faulty temp probe.
Burt

Burt is describing what I once did. The prongs on the yellow plug can easily go under the PC board and the plastic case, but you will have a zero temp displayed (if I remeber correctly) and you had a temp displayed so that does not sound like the problem.

I took the Temp probe to work today and connected it to one of our Temp meters and heated the probe... It worked just fine! I then brought a new probe from work to try on the ECU... It worked fine also... Then I plugged the Ram probe into the ECU and guess what..... It now works just fine.... I had plugged the Ram probe in several time and always had the same results. Now I'm not sure what to think.....


The seller has suggested the he buy me a new ECU and probe so that we are sure that there are no more problems. He has been a pleasure to deal with and I would buy from him again anytime!

Thanks for all of the help you people have given me!

Gene

Gene, you may want to ask Andy Low of Electrodynamics about that ECU. He's the original builder and a great guy, always a pleasure to deal with. He may be able to refurb the ECU or sell you a refurbed one quickly. And there is another issue that comes to mind: a huge batch of early ECUs had funky connectors (actually an intermittent connection where the wires met the connector). Its the connector that is the same as an Apple's connector, the one that the ground data terminal plugs into, on the airframe side. I had at least two of them that got goofy. Plug the whole set up in, turn the power on and wiggle the hell out of that connection and see if you get an intermittent display, or it may even make the temp go intermittent. Any way, talk to Andy. He's the man when it comes to that ECU.

Gene,
Just curious, where is the end of the temp probe when you were having problems? Was it inserted into the tailpipe and if so how far or was it laying on the exterior of the tailpipe. It should be inserted about 1/16" into the tailpipe.

Cobrajet

It was in the tail pipe about 1/16". It has ran that way before and looked good. I even heated it with a heat gun ( a very hot one) and the reading never went up at all.... It always said 32 deg. unless I unplugged the probe and then it was zero.

Did you try wiggling the connector (both the thermocouple connector and the one I spoke of above) while heating it with the heat gun?