Poor AC Cooling at Low Engine Speed...Why? - PeachParts Mercedes-Benz Forum

JamesDean · #1 07-14-2012, 08:11 PM

Hello everyone,

I was thinking about this recently. A few of my cars have pretty poor AC cooling performance when the engine is at idle. It does not appear to be due to lack of air flow as I tripped my aux fan(s) and the temp was only marginally lowered. However when I increase the engine speed to 1500-2000 rpms the system cools much much nicer.

I was wondering if anyone knew why this was occuring?

I've always charged my refrigerants/ac at high idle, and attempted to match the numbers found on ACKits' website.

I was curious if this is something I'm doing wrong or just part of the design.

I've seen other people post about not being able to achieve low vent temps at idle too.

If low compressor rpm is the problem..why not just make the pulley smaller?

Anyways I'd like your thoughts on this!

97 SL320 · #2 07-14-2012, 09:26 PM

Could be that the compressor is wearing out and can't make pressure. My 97 SL 320 uses a Nippondenso 10PA17VC compressor. These have a bolt on manifold that uses a molded o ring. It is possible for the o ring to leak between the high and lo side causing poor performance at low speed. ( I have not verified this is a common problem. )

Auto AC is a tricky thing, The compressor is good for only so many RPM so the pulley need to be sized for engine red line RPM.

Use too large a compressor to cover idle cooling needs and it will drag the engine down at idle, cause a slight surge at road speed and it will rapid cycle ( clutch wear ).

Do any of your cars have a AC idle kicker? Some cars ( non make specific ) use a solenoid to boost idle to prevent stalling and provide some enhanced cooling. The added throttle covered AC drag + 100 RPM. ( not enough to make the car surge against the brakes when stopped )

Air in the system could cause this problem too.

Gilly · #3 07-15-2012, 08:48 AM

Is the vehicle in question an R12 car converted to R134a, or a factory R134a car, or the now-rare factory R12 car still running R12?

To me the R134a cars aren't quite as good in low speeds as R12 cars were. And R12 cars converted to R134a are even less so.

JamesDean · #4 07-15-2012, 02:52 PM

Well here's the cars:

1993 190E M103, 10P15C, charged with ES-12A Hydrocarbon

I charged it up when it was about 85F outside. At high idle with fans on.
I achieved about 25 psi low side, 165 psi high side.

At low idle, cooling is rubbish, 50-60s vent temp.
At >1500, cooling is excellent. Mid 30's vent temps.

The system probably could use some work done to it. I know it has a leak somewhere as my original R12 charge from 2006 leaked out in under a year. The ES-12A stuff lasts quite a bit longer. I topped off the car after 2 years since previous charge.

Would there be any advantage to replacing the 10P15C with the 10P17C? I know the 17 is higher displacement unit...

1982 300SD 617, R4, charged with Freeze12 then ES-12A Hydrocarbon

We got this car in 2007, the R4 failed, we had the shop replace it, and the dryer. I dont think they replaced the TXV as it was still in working order. They charged it with Freeze12 as R12 was considerably more expensive. The F12 stuff worked OK but a while then progressively got crappier.

So it was vented one day, accidentally, and I figured I'd try charging it up with the ES-12A stuff I had left over from the 190E.

I dont remember the pressures but it was able to achieve low 50/high 40 at speed, but like my 190E was crappy at idle.

1989 420SEL 116, 10P17C, charged with R12

Like the SD, the compressor on this car died and I had the shop replace it and charge it with R12. The performance was lackluster, and degraded over time. I had some R12 stock built up for when my other SD needed it, so I decided to try to recharge the 420...

It took a few tries but I think I got it right finally. It has a 5-7 degree difference between idle and at speed vent temps but I think its still pretty good. I was able to keep the car cool when it his 100F not too many days ago.

I know from the three cars I mentioned here that it looks like the refrigerant is the common component but I know other people use the ES-12A stuff with good results.

I tend to think that the 190E's system could use an overhaul, especially seals and valves. Maybe even the compressor, at 85F the best I could get out of the compressor (without making the low side too high) was about 165 Psi.

I think the 300SD needs to be evacuated and recharged with the ES-12 Industrial stuff (as the ES-12A is not meant to be charged into a vacuum) I think there is some remnant F12 in there causing problems. Probably should replace the dryer too. But I lack the tools for such endeavors. I just have a manifold set, no vacuuming abilities.

Air&Road · #5 07-16-2012, 07:56 AM

There ARE things beyond the refrigeration section of the climate control system that can cause less than maximum a/c performance. Pods can go bad allowing outside air into the system, preventing maximum performance. Also heater valves can sometimes allow hot coolant through the heater core and it doesn't take much to compromise a/c effectiveness.

JamesDean · #6 07-16-2012, 01:36 PM

Quote:

Originally Posted by Air&Road

There ARE things beyond the refrigeration section of the climate control system that can cause less than maximum a/c performance. Pods can go bad allowing outside air into the system, preventing maximum performance. Also heater valves can sometimes allow hot coolant through the heater core and it doesn't take much to compromise a/c effectiveness.

I know in my 420SEL I had to change the recirc door pods. That helped abit. Everything else in that car is good. I did add some R12 to the cars' charge and that really helped with performance around 100F ambient. It also seemed to stabilze the idle temp too. It has a new monovalve insert as well.

In the 190E I've not replaced any pods, I know my recirc still works. I do have less air coming out of my side vents than the center vents.. Not sure if thats just normal or..

The green 300SD needs a new defroster pod. Half of it died, taking out the center pod. I just bypassed that and shut the defroster pod for the summer. That probably should be replaced before winter hits.

compu_85 · #7 07-16-2012, 03:15 PM

I see similar performance in my SDL, and my doors are all working properly. I think my compressor is just a bit worn, it just doesn't make enough pressure at idle. You can hear the sound of the TX valve change a lot when you bring the RPMs up above idle.

-J

JamesDean · #8 07-16-2012, 04:29 PM

Quote:

Originally Posted by compu_85

I see similar performance in my SDL, and my doors are all working properly. I think my compressor is just a bit worn, it just doesn't make enough pressure at idle. You can hear the sound of the TX valve change a lot when you bring the RPMs up above idle.

-J

I do notice that my 10P15C thats on my 190E seems to have some (what i would consider) low-ish numbers on the high side. It was 85F outside and the best I could get the compressor to do was 165/170 psi, chaged with ES-12A. I mean thats within normal operational range but I still feel as if its kind of low.

I hear an occasional wisping sound coming from the dash on my cars (all of them i think) is that the noise you're talking about? Is that the TXV changing?

compu_85 · #9 07-16-2012, 05:42 PM

That's the sound of the liquid (which is turning into a gas) going through the TX valve.

-J