rotorman Posted June 11, 2020 Report Posted June 11, 2020 The light came on solid on takeoff. Went to a flicker during part of the climb. Then when out and stayed out for 1.5 hr cruise. Then came one solid at the start of taxi after landing. Then a flickering at the end of taxi until shutdown. Vac gauge showed in the green the entire time. HSI worked as it should. I think this is electrical. Where is the vac hi/low switch located on the 1980 20J? Quote
PTK Posted June 11, 2020 Report Posted June 11, 2020 Talking about vac hi/low annunciator mine started to flash on off recently on the ground. When I increase rpm it goes away. Also when I pull the standby vacuum on, it goes away. It seems to come start to flash at rpm lower than about 1200 or so and goes away when I increase rpm above that. Vacuum pump fail light on standby vac system never comes on. Only the hi/low. This is indicative of low vacuum at those rpm’s but not sure what it means. Never came on in flight. Any ideas? Quote
rotorman Posted June 11, 2020 Author Report Posted June 11, 2020 1 hour ago, rogerl said: According to 1998 M20J SMM: Thanks for that info. It makes sense. I recently changed both filters associated with the vac system and as anyone who has changed the large canister one knows they are not easily accessible. Good chance I disturbed some wiring in the process. Will inspect the area soon and follow up. Quote
carusoam Posted June 12, 2020 Report Posted June 12, 2020 Consider for a moment.... The pump The regulator The gauge The sensor The instruments The filter If there is a leak... enough to tip the sensor... Is it possible to not affect the gauge? It helps to have a vac system layout to better consider the options... the POH is a pretty good place to find that... Our sensors and gauges are usually trying to warn of a pump breaking.... a vacuum leak can be a bit more subtle... Best regards, -a- Quote
Guest Posted June 12, 2020 Report Posted June 12, 2020 14 hours ago, PTK said: Talking about vac hi/low annunciator mine started to flash on off recently on the ground. When I increase rpm it goes away. Also when I pull the standby vacuum on, it goes away. It seems to come start to flash at rpm lower than about 1200 or so and goes away when I increase rpm above that. Vacuum pump fail light on standby vac system never comes on. Only the hi/low. This is indicative of low vacuum at those rpm’s but not sure what it means. Never came on in flight. Any ideas? It’s an indication of the pump wearing. As it wears it takes ever increasing RPM to create sufficient vacuum. As the vanes continue to wear and shorten the pump will fail. Newer pumps have a port and a gauge which allow you measure the vane length and hopefully replace the pump before it fails. Clarence Quote
PTK Posted June 12, 2020 Report Posted June 12, 2020 9 hours ago, M20Doc said: It’s an indication of the pump wearing. As it wears it takes ever increasing RPM to create sufficient vacuum. As the vanes continue to wear and shorten the pump will fail. Newer pumps have a port and a gauge which allow you measure the vane length and hopefully replace the pump before it fails. Clarence Thank you for that Clarence. I was trying to differentially diagnose it among the regulator, filters or pump. On today’s flights it didn’t flash. Go figure! Quote
rotorman Posted June 12, 2020 Author Report Posted June 12, 2020 Sometimes you just get lucky. I got on my back with my head at the pedals and found the Vac switch. It's a 3 wire. One was tight, one was loose, and one was disconnected with no screw. But the luck was with me. Whenever I find a screw on the floor, I put it in the fuel selector tray hoping I will figure out where it came from. This screw fell out a while ago and I was puzzled because it wasn't the usual small interior type screw that normally find. Not only did I have the screw but I had the tiny washer as well. I had to disconnect the 2 HSI connectors to get the job done and in 15 min and I was back in business. Thanks to all who contributed. 1 Quote
carusoam Posted June 12, 2020 Report Posted June 12, 2020 Great insight, Clarence! For a second opinion... or back-up logic... check the hours on the pump... At 500hrs, you are in the window where some people just replace it while it is convenient... Sounds like, at 500hrs, follow Clarence's advice, and look in the window! Its the added knowledge, that allows a CB aviator to fly safely at a lowered cost.... Go MS! Best regards, -a- Quote
carusoam Posted June 12, 2020 Report Posted June 12, 2020 Great follow-up Rotorman! Just remember to use caution when switching tanks with random hardware living around the valve handle... Best regards, -a- Quote
rotorman Posted June 13, 2020 Author Report Posted June 13, 2020 I had 500 hours on my pump a few months ago and said to my A&P that I was thinking about changing it out. He sort of shrugged his shoulders and wrinkled his brow so I did nothing since I do have the backup vac system if needed. About 10 hours later it failed as I came into the traffic pattern after a 1.5 hr flight. The gear, which is made of something other than metal, failed. With respect to the loose wires on my vac hi/lo switch, I am pretty sure this was the result a sloppy installation the last time the ADI was out for repair. Those small pan head screws have a lock washer behind them. And since one of the the three was on tight as can be and the others loose, it is hard to put together another story. Quote
rotorman Posted June 23, 2020 Author Report Posted June 23, 2020 Since I fixed the loose wires on the hi/lo vac switch the light comes on at taxi with the throttle nearly full back. I don't really mind that but if I could easily make an adjustment I would. I understand there is one on the regulator. Can some verify that? And also which direction for increase and decrease' Quote
carusoam Posted June 23, 2020 Report Posted June 23, 2020 Things to consider... 1) Vac pump increases vac with rpm.... 2) Vac in the system depends on having excess.... then using a regulator to cut off the excess in a controlled manner.... 3) Without enough rpm, there won’t be a controlled amount of vac.... 4) I have no idea what the minimum rpm of the vac pump needs to be to generate the proper amount of vacuum... the internet can find this detail easily... 5) Many people get tired of this game and add a vac gauge just to know better about what is actually going on... 6) Idiot lights are not very helpful in the real world... 7) there is a sensor for the idiot lights... that probably has a diaphragm switch... it may be cleanable, adjustable or replaceable.... it’s specs are probably available on the internet as well... 8) When you have a gauge, the idiot lights become less important... 9) The ammeter works in a similar way... at low idle, the alternator provides very little current, and an idiot light comes on... PP thoughts only, not a mechanic... Best regards, -a- Quote
rgpilot Posted June 24, 2020 Report Posted June 24, 2020 I believe that vac light flashing indicates low vac and vac light on but not flashing is too high vac. Quote
rotorman Posted June 24, 2020 Author Report Posted June 24, 2020 15 hours ago, carusoam said: Things to consider... 1) Vac pump increases vac with rpm.... 2) Vac in the system depends on having excess.... then using a regulator to cut off the excess in a controlled manner.... 3) Without enough rpm, there won’t be a controlled amount of vac.... 4) I have no idea what the minimum rpm of the vac pump needs to be to generate the proper amount of vacuum... the internet can find this detail easily... 5) Many people get tired of this game and add a vac gauge just to know better about what is actually going on... 6) Idiot lights are not very helpful in the real world... 7) there is a sensor for the idiot lights... that probably has a diaphragm switch... it may be cleanable, adjustable or replaceable.... it’s specs are probably available on the internet as well... 8) When you have a gauge, the idiot lights become less important... 9) The ammeter works in a similar way... at low idle, the alternator provides very little current, and an idiot light comes on... PP thoughts only, not a mechanic... Best regards, -a- Agreed on all of the above. I have a gauge. I just find the light annoying during taxi. I thought there was some way to set the lower limit on the switch. The other option would be to increase min RPM. And of course the last option is to live with it. Quote
Hank Posted June 24, 2020 Report Posted June 24, 2020 ignore the light while taxiing, check it during runup. That's where it is on the Before Takeoff checklist in my Owner's Manual. 1 Quote
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