When a Mercedes 2007 came into Brendan’s workshop with the pressures on the gauges indicating that the compressor wasn’t pumping properly - high side was low, low side was high – it was obvious something wasn’t right.

After checking the control valve operation, the valve was drawing 800 mA, so Brendan suspected that it was likely to be a control valve issue.

Brendan changed the compressor, condenser, receiver drier and flushed the system and performed a standard A/C service. The system flushed clean and there was no restriction in any of the hoses – job done, right?

Wrong! Further testing found what appeared to be the same problem, although with slightly better pressures, the A/C was still under-performing.

Despite being a genuine Denso unit, Brendan concluded that the only explanation could be that the new compressor was faulty – we sent him through another new compressor, and he went on to replace the TX valve and flush the system for a second time. Frustrated, Brendan discovered that the gauges still showed the same issues. 

Martin, Highgate’s Sales Director, had a lightbulb moment and suggested it may be an issue with the hoses internally delaminating. Delamination is when the inside wall of a hose comes away from the outer, and as the compressor is pumping, it sucks the inside wall in and restricts the flow.

Brendan wasn’t convinced that this would be possible, especially when there was nothing that had shown this problem on his gauge readings. However, running out of options and ideas he thought the only thing to do was to double check that the hoses weren’t delaminated.

So he reluctantly cut the suction hose open… problem found!

The inside lining of the hose had completely detached from the hose wall and that was the cause of everything. Restricted flow, compressor didn’t have enough refrigerant return = underperforming AC. Easily mistaken for a control valve issue.


Although we know cutting open $400 hoses is not ideal, what we do know, and we often suggest is to invest in, is the Lokring and Lokclip hose and pipe repair tool.

This tool allows you to remake and repair hoses with the existing fitting; be it pad or o-ring fittings.

We also recommend having an electronic control valve tester, which allows you to professionally test the operation of a control valve and determine what signal is coming from the HVAC module.