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450SEL kick panel relays

Started by Jenno1981, 17 December 2022, 08:26 PM


Brains Trust,

1978 450SEL on straight LPG.

4 x relays behind the driver kick panel  connected to wiring numbered 6, 12, 8 and 5.

1. What do the numbers correspond to? Out of interest I matched them to the Fuse box and it kind of made sense...

2. The relays all appear to be the same. Where is the best/cheapest place to get replacements?

1978 450SEL - Green
1999 ML430 - Also green 🤷�♂️


Here is what I have for these numbers:

5: "Change over valve" - I think this is ignition related.

6: "Auxiliary Fan"

8: "Change over valve with additional fan"

12: "Air conditioning system"

They all seem related to the AC somehow.  Hopefully some more learned member will chime in.
1977 450 SEL 6.9 - Astralsilber


Thankyou Daantjie, this aligns with my issues that led me to these relays.

A long time ago my AC and Aux Fan stopped working and I never really had a decent go at troubleshooting.

I searched this forum re Aux fan issues and followed a couple of tips (Fuse good, fans works when isolated, AC heat sensor good), so I thought down to the relays?

I pulled the relays and grabbed my Multimeter... but it's busted 🙄

That's as far as I've got. Open to suggestions.

1978 450SEL - Green
1999 ML430 - Also green 🤷�♂️


Hello Jenno! I would suspect that if the AC and aux fan stopped working at the same time, it´s likely that your AC system ran out of gas, or at least had a leak. For what I´ve seen, the aux fan only comes on in only 2 situations:

1- When engine temp rises near to 100°C

2- When the AC is on. But the fan uses a pressure switch located in the AC dryer to start, so: No gas = no pressure = no fan coming on.

Hope this might help! You can check the circuit by locating the switch in the dryer and jumping the contacts, to see if the fan starts.

Good luck!


I should elaborate...

When I say the A/C doesn't work, I meant the cabin fan does't work either...

So the cabin fan doesn't work, therefor the A/C doesn't work, therefor the AUX fan doesn't work.

Its been a long time so dei.85 you're probably right, the gas would be empty but I'm assuming the internal cabin fan would work regardless?

So far I've found:
- Fuses are correct/intact
- Relays behind the kick panel are all working
- The AUX fan works with direct power
- Internal cabin fan switch clicks around but no air coming out

Any clues on where to go to from here would be appreciated. I'm thinking I need to diagnose this internal cabin fan first, then get the A/C gas filled up.
1978 450SEL - Green
1999 ML430 - Also green 🤷�♂️