This is a question for anyone whose Mercedes turbodiesel has a boost gauge: do you see a change in boost when you turn the a/c compressor on or off?

Here's the tale: I was driving home from Pick and Pull a couple of days ago in the '87 300D Turbo (W124, OM603). Flat section of freeway, no appreciable wind, speed a steady 70 mph with the cruise control on. The engine was fully warmed up with the coolant just above 80ºC as it usually is. Outside temp was in the mid-80s (Fahrenheit) so I had the a/c on. The boost gauge (VDO, max 15 psi), which connects to a fitting in the intake manifold next to the overboost switch, was reading about 6 psi.

Just for the heck of it, I reached over and put the climate control in "EC," that is, I turned off the a/c compressor. The boost gauge went down to 5 psi. Turned the a/c back on, boost went back up to 6 psi. Repeated several times, same result every time. No change in system voltage or anything else.

Although I know that the compressor puts a load on the engine, I was surprised that it was enough to require 20% more boost. I ask that others with a boost gauge please repeat my experiment and see what they get.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I see it change at idle in my 84 300D, haven't payed attention while cruising though. I'll give it a try tomorrow.

With the A/C off car in park after a freeway drive the car will run .2psi ish at idle.

With the A/C on it lowers to zero psi.

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light

It's not requiring, it's generating. You use more fuel to power the AC, and that also results in more exhaust to spin the turbo faster.

A/C can consume as much as 6HP off the engine. Which will increase fuel usage and boost also.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Ahhh, that makes sense. I was thinking about it backwards. Thanks for the insight!

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I replicated Jeremy's run but I don't get 5psi at 70mph on level ground with or without AC. Either my car isn't as loaded as Jeremy's or my turbo needs more exhaust to build 5psi. 5psi is the minimum my gauge will display.

Sixto
87 300D