Rooftop RV problem: Shutting down when compressors kicks on.

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

Texabama

Member
Joined
Jun 2, 2021
Posts
7
Location
Alabama

I'm having trouble finding the problem with my Dometic Penguin II AC. The fan starts and runs about 12 seconds then the compressor kicks in, runs about 3 sec and then both shut off. Repeats every 1-2 minutes.

I switched boards with the another AC unit and still not working. The other unit continued to work with no problems.

Called out a mobile RV Tech and he got dometic on the phone and ran through the recommended tests and found nothing. They had him bypass the board by connecting 3 wires together to make it run continuous. It ran and cooled. Dometic said it must be the board or thermostat. Doesn't sound right to me since I've already switched the boards and the other 2 ACs are working fine with same thermostat. Any suggestions.
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
73,944
Location
At our Silver Springs FL home
Sounds more like a failed start/run capacitor to me. Without the capacitor assist, it takes 2-3 seconds for the amp overload to cause a shutdown. As I recall, on my Coleman units the capacitors were separate from the board - is the Dometic that way too?
 

Henry J Fate

Well-known member
Joined
Jun 14, 2018
Posts
1,412
If the board was bypassed and the unit ran without shutting down, it wouldn't seem to be a capacitor problem as the capacitor still should have caused the same condition if the capacitor is weak or bad. The capacitors are cheap and it wouldn't hurt to replace them.

You replaced the board with a known to be working board from another unit and the problem continued. It could be the thermostat and I would figure out a plan to prove it as good or bad but I agree it to be an unlikely cause. After that it's a crap shoot. Maybe a bad connector to the control board either the low voltage side (12 volts) or the high voltage side (120 volts)
I would look closely at those connectors as it sounds like those connectors were bypassed when the ac unit was wired direct and worked. From there I would suspect any dedicated wiring to the unit both 120 volt and 12 volt
 
Last edited:

Texabama

Member
Joined
Jun 2, 2021
Posts
7
Location
Alabama
Thanks for the responses. There are 2 seperate capacitors, start and run. I'll be able to check them in a couple of days. I'll post an update then.
 

OldMarine

Well-known member
Joined
Apr 30, 2020
Posts
77
could be a bad compressor fan. I had VERY similar problem last season. My fan would come on...run for about 45 seconds...then shut down. If I switched from fan to cool, the fan would come on, then I would hear the compressor kick in, then it all shut down after about the same 45 seconds to 1 minute. I put new capacitors in, no difference. Put a new FREE! used fan motor that a local RV shop had leftover from a job and gave me....and problem solved. Guess the old fan motor's thermo - switch was shutting it off when it started to overheat.
 

OldMarine

Well-known member
Joined
Apr 30, 2020
Posts
77
Thanks OldMarine, I look into that.
oh, forgot to say what kind of unit I have, it's an older Coleman Mach III 13,500btu I believe. This type has NO thermostat, just knobs on the unit itself for on-off-fan-cool, and other knob is temp setting.
 

Texabama

Member
Joined
Jun 2, 2021
Posts
7
Location
Alabama
Both capacitors test good. Fan runs on all speeds with no cutoff. Stops only when the compressor comes on in cooling mode. The compressor runs in Diag mode (fan off) without problem. I checked all the connections I could find and reseated them. I'm at a loss for what to do next. If I pay another tech to not fix it, I'll be getting close to spending what it would have cost to just buy a new one.
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
1,541
Out of left field here but are the electrical connections good and solid? Are the connectors hot/warm immediately after shut down.

Just thinking a high resistance connection causing a low voltage shut down. The connections are most stressed when the compressor kicks in (obviously).
 

Texabama

Member
Joined
Jun 2, 2021
Posts
7
Location
Alabama
I've monitored the voltage and it never drops below 120. I removed and reseated all connections to board and capacitors.
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
73,944
Location
At our Silver Springs FL home
If I pay another tech to not fix it, I'll be getting close to spending what it would have cost to just buy a new one.
Now you understand why RV dealer shops rarely try to fix a flaky rooftop a/c unit. At their labor rates, diagnosis and testing labor cost quickly exceeds the price of a new unit. Good thing you don't have to pay yourself $125/hour!
 

Henry J Fate

Well-known member
Joined
Jun 14, 2018
Posts
1,412
So back to the beggining.

The unit provided cooling and worked as it should (fan and compressor) when the board, thermostat and switches were bypassed. Assuming that test will produce the same result if repeated (and I would repeat that test and use a longer run time), this test provides the best clue. It eliminates lots of possibilities and begins some focusing on certain things.

Did you take voltage readings at the compressor during start up?

Look at the 120 volt connections. I think those colemans used line voltage thermostats up on the ceiling. There were 12 volt control kits offered if the builder wanted a wall mount thermostat. Check to see if it is a line voltage thermostat.

Bypass the thermostat and see if the unit runs.
Bypassing the thermostat should be easily done. Shut off power to the unit while you set up a bypass.

There are also some switches up there as well. If you could get a model # or the year of the coach, a wiring diagram would be very helpful.

In general, open up the sealing control area. Follow the 120 volts. Check all the connections. Everything. All those controls and wiring were bypassed during the direct wiring test. Chances are the problem is in there somewhere.

A wiring diagram would be of great help
 
Last edited:

Texabama

Member
Joined
Jun 2, 2021
Posts
7
Location
Alabama
It's not a Coleman, it's a dometic penguin II. I don't think it's a line voltage thermostat. Thermostat is connected by phone type cable.

Voltage only changed 1-2 volts while testing (120-122). Max amps increase on compressor start was 10.

RV is 2016 Riverstone 38TS
AC model #936000744
IMG_20210408_152221947_HDR.jpg
 

Henry J Fate

Well-known member
Joined
Jun 14, 2018
Posts
1,412
Sorry. Got things mixed up with OldMarine's post.

However, it still comes down to what was bypassed in that direct wire test.
 

Texabama

Member
Joined
Jun 2, 2021
Posts
7
Location
Alabama
Well I hate to do it but I guess I will replace the whole thing. I'm going to replace a seal on one of the other ACs tomorrow and if I don't have any problems removing and reinstalling it, I'll order the replacement and install it myself. Any other ideas would be appreciated before then.

Thanks for all the help. I've learned a lot about ACs so all this troubleshooting wasn't a waste of time.
 
Top Bottom