Aux Fan Troubles With Dual Relay

[owenj99]

Hi guys,

I have been trying to diagnose an intermittent aux fan issue on my 1992 500E. As summer time rolled around the temps have been rising, and I knew that the aux fans didn't work at the end of the last summer when I got the car. Both the high and low low speed fans don't work. It is worth noting that the last time I had the headlight out I replaced the resistor that is known to go bad just out of caution and convenient access, probably about 9 months ago. I went to go and check the fuses on the blue and orange fan relays (suspecting it would be the same set up that I had to fix on my 1994 E500), only to find that the 2 fan relays are controlled by one large, seemingly unfused relay, occupying both relay ports. I unplugged the temp sensor and the high speed fans kicked on only once, not to come on again. Because of this I suspected the double relay was bad. Ordered and replaced the relay only to find the fans still didn't kick on. This evening I removed the connector behind the drivers side headlight and applied 12 volts, and the fans kicked on without any issues. No squeaks, spinning freely, etc. Next, I checked power coming to the connector (from the car side) and it read 12.0V. Strange... 12V and unplugged temp sensor should cause the fans to come on. Because they didn't, I unplugged the connector again and checked the voltage again and it was reading 0.13V (probably somewhere close to zero... my multimeter has been in use for many years.) I messed with the connectors, wiggled the wires, etc. and couldn't get the voltage to jump back up again. This is what stumped me... If anyone has any ideas that could cause the intermittent issues, it would be much appreciated as the car is almost unusable anytime the weather is above 80 degrees, and forget about stationary traffic...

I have attached the part number for the relay that was where the two relays are normally located. Also, the plug locations of the twin relay setup does NOT match with the single larger relay.

Double relay part number: 0015428219

Thanks in advance for any help!

 

[gsxr]

To solve the first mystery... pre-facelift cars, through 1993 USA model year, have a single large unfused relay - what you are seeing. Facelift cars, 1994-up USA model year, have the two separate cube relays with fuses. The fuse for the early/single-relay setup is a 30A strip fuse located in an external holder behind the brake booster, usually next to the vacuum valve for the cold upshift delay. Relay 001-542-82-19 is correct for your 1992 500E.

The very first thing to do is remove the 30A strip fuse and verify it is intact. If broken or cracked, you may have solved your problem. If the 30A fuse is good, then:

Test 1: At the red pressure switch behind the driver headlight, at the AC dryer, short the 2 leads/pigtails of the switch together. This should make the fan run on LOW speed. If so, that part of the circuit is working OK. If the fans don't run on low speed with the AC blowing cold, the red pressure switch may have failed, however you would need to connect gauges and verify refrigerant pressure is normal.

Test 2: If you pull the connector off the 2-pin temp sensor and the fans don't turn on high speed, short out the 2 sockets with a wire (or, insert a resistor of 100-200 ohms). This should make the fans turn on high speed.

 

[owenj99]

Thanks for the relay clarification Dave!

I haven't checked the low speed function but I have unplugged the two prong temp sensor and the fans have kicked on high, but don't come on every time I unplug the sensor. I've spent a few hours messing with this over the past week or so, and I've only had the fans come on twice with the two pin connector removed. One time the fans actually kicked on, and the other was last night when I actually saw 12v on the multimeter...

All of this making wonder if it isnt more of a wiring issue? Is the harness for the fans part of the upper harness or is it a seperate piece, and is it prone to failure like the main harnesses?

Thanks again,

Owen

 

[gsxr]

Owen, I don't think the fans always trigger on high speed when the 2-pin temp sensor at the intake manifold is unplugged. If not, short out the sockets and see if that does turn the fans on high speed. Remember, under normal conditions the high speed ONLY runs when engine temps exceed 107°C. You can simulate this with a resistor <200 ohms.

The fan harness is only prone to problems at the low-speed resistor behind the headlights, and that failure only results in the low speed not working.