Dash heat 12v booster pump- no juice

[Rick]

Perhaps the relay was replaced and wired wrong or has gone bad.

[InfiLights]

Sure looks like it.....

If I read correctly the diagrams, this Groco booster pump will run all the time the engine is turned on, regardless if heat is needed or not and the Mercedes valve regulates the heat based on the slider switch position next to the driver left knee.

I am not sure I like that or see a reason why this pump should run all the time in the summer.....for one, wear on the pump ( which I found it is shot - need a new pump) and second, sending hot coolant upfront when it is not needed.....

Maybe I should consider feeding power to the pump via a relay and separate switch on the dash ( plenty of open blank positions) and only have it come on when it is really needed.....I can see this fighting the dash A/C in the summer...

[gcyeaw]

Originally Posted by InfiLights

Sure looks like it.....

If I read correctly the diagrams, this Groco booster pump will run all the time the engine is turned on, regardless if heat is needed or not and the Mercedes valve regulates the heat based on the slider switch position next to the driver left knee.

I am not sure I like that or see a reason why this pump should run all the time in the summer.....for one, wear on the pump ( which I found it is shot - need a new pump) and second, sending hot coolant upfront when it is not needed.....

Maybe I should consider feeding power to the pump via a relay and separate switch on the dash ( plenty of open blank positions) and only have it come on when it is really needed.....I can see this fighting the dash A/C in the summer...

Just a warning, if you have the electroluminescent dash, never drill a hole in it, it will destroy it. Add switches somewhere else. Sometimes there are available unused switches labeled accessory on the dash.

[InfiLights]

Of course....there are 6 open accessory switch positions now with blank plates

[wasper67]

Originally Posted by InfiLights

Of course....there are 6 open accessory switch positions now with blank plates

On my Aux pump switch, everything is in place the switch and wires all there but they are capped off and it looks like it was from the factory that way. You may have the same and can use that switch.

[InfiLights]

I don't have an Aux Pump switch on the dash, nor a Winter/Summer switch....91's seem to be different than earlier models in this respect.
Our friends have a 90 that does not have an Aux Pump switch as well and no Engine Preheat switch to activate the second pump when you would want the Primus/Webasto to preheat the engine block.
Mine has an Engine Preheat switch on the overhead panel. According to the manual, this pump has dual purpose. One to preheat the engine if desired when you turn on the Webasto and also to provide engine coolant to the interior radiators while driving for heating purposes.....not for dash heat....that is why they have the other booster pump for the lines that go only to the dash.

[wasper67]

I have that same pump in the same location as yours but I am unclear if it actually works or if it is just the engine water pump pushing the hot antifreeze up to the dash and around. I do know my dash heat did not heat at all until I bled the air out of the front heater core which required me to open a small valve located in the front access panel to let the air out.

[nbedinger]

Originally Posted by InfiLights

Sure looks like it.....

If I read correctly the diagrams, this Groco booster pump will run all the time the engine is turned on, regardless if heat is needed or not and the Mercedes valve regulates the heat based on the slider switch position next to the driver left knee.

I am not sure I like that or see a reason why this pump should run all the time in the summer.....for one, wear on the pump ( which I found it is shot - need a new pump) and second, sending hot coolant upfront when it is not needed.....

Maybe I should consider feeding power to the pump via a relay and separate switch on the dash ( plenty of open blank positions) and only have it come on when it is really needed.....I can see this fighting the dash A/C in the summer...

Blue-bird changed things at some point around 89-91. According to this post from an earlier surge on the question of SP dashboard heat, the Mercedes valve had a Groco aux pump power switching function in a microswitch. Big aha!

https://www.wanderlodgeownersgroup.c...&postcount=134

[Clueless]

On my 89SP that booster pump was totally removed eons ago. I still get all the heat I need from that coolant loop from the way powerful water pump on the CAT.

Try hooking the groco pump inlet and outlet lines together bypassing the pump and see what you got.