W124 Idling Problem

[peejeems]

Hello everyone
My name is Peter Lim and I am from Malaysia.
I bought a W124 200E (1990 model) about a month ago.
I have this problem in the idling speed - it is set at about 850 rpm but when the aircon is turned on, it dips to almost 0 but the engine doesn't die.
A mechanic told me that this is due to a faulty Fuel Pump Relay but I have had this changed and the problem is still there.
Could someone with a similar experience help me or point me to a past thread that has already discuseed this before?
Thank you for reading and your help!

 

[Hertfordshire Merx]

Which relay did he ask you to take a look at, does it have fuses in the top of it or is it enclosed in a black case with many tubular connectors coming out of it (simple terms!!) May be an over voltage relay.

 

[television]

On a 1990 car you could have an idle relay, with this system when D or the air con is turned on it should increase the RPM by 50 to compensate