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SR2500 09-07-2021 05:07 PM

CHT problem
VM1000C gold box. O-540 lycoming. At Oshkosh, my rpm sensor failed. So I ordered a new one, new EGT probes and new CHT probes as a couple had stopped working. Now, rpm is good. EGT is good. CHT is good with master power switch on, but when the engine is running they revert to What appears to be OAT. Any thoughts?
Probes are Dynon J probes, bayonet style.

azrv6 09-07-2021 08:33 PM

Jerry, do you know the age of the battery chip? In my experience, the VM system can start to act flakey as the battery chip approaches end of life.

Reggies article on this:

azrv6 09-07-2021 08:48 PM

Another thing you might consider are the real time processing chips:

SR2500 09-08-2021 07:18 AM

Thanks. Iíve got an email in to Reggie. I did split the box to replace the battery. Hopefully I didnít introduce a stray tron.

SR2500 09-11-2021 05:52 PM

Problem solved. I had to go to the probe manufacturers to figure it out. VM1000C used an ungrounded J probe. The Dynons I replaced them with were grounded probes. Evidently, there are a few older systems out there that amplify the signal to make it work and the grounded probes interfere with this. There’s a good discussion about this on the Alcor website. Finally, I verified this with Reggie who still helps out with the VM product line.

You can replace a grounded probe with an ungrounded one, but not vice versa.

Consequently, I have six new Dynon CHT bayonet type J probes. I paid $38 each from Stein. Will sell for $25 each plus shipping. I’d rather not separate the batch.


azrv6 09-11-2021 06:17 PM

Thanks for closing the loop, and the useful info. Glad to hear Reggie is answering questions again.

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