posted last week about 1989 240 dl that engine runs hot when AC comes on. Changed belts and cleaned front of radiator and changed thermostat and filled with 50/50 anti-freeze. AC works great but when you slow down to neighborhood speeds the temp light goes way up but does not come down. What are some other options. Water pump? Blockage? Temperature compensator board? Even when temp goes up AC keeps on cooling.

Which one shold I try first?

db

 

If the water pump checked out OK when the belts were off, then try the easy stuff first. Pull the dash cluster and check the temp compensation board. It may be dirty contacts, or you could just remove it and bypass by soldering a jumper wire from pin 1 to pin three, position normal and facing you numbered left (#1) to right. The second to check would be the viscous radiator cooling fan clutch. Check it when cold, and hot, by spinning to see if it has resistance or if the fluid has leaked out. If all that is good, then it is probably new radiator time.