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SP (Single Axle Pusher Unique Issues) If you have a unique issue with your SP model coach and it can't be answered in one of the other forums here, then this is where you can list it....List your SP parts here too.

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Old 1 Week Ago
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Default Dash heat 12v booster pump- no juice

Hello all,

Hope somebody can shed some light on this issue.

There is no hot coolant coming to the dash when the sliding switch is set to warm and the dash fan heater switch for driver and/or passenger is turned to either low or high speed - according to the manual.

As I dug into it and read some posts, I did find a 12v Groco booster pump on the hot coolant hose located above the muffler, hidden up on the frame and the coolant is hot on the hose before it and cold after....the pump is not moving it.
Traced the wiring and it goes from the pump to the rear of the mudflap behind the drive tire next to the landing light. There is an orange wire that enters the body there and should energize the pump when heat is called upon but has no 12v to it. The ground is tied to the frame.

So I tried the normal Warm switch combo with the heater fan switch like before, no 12v on the orange wire......then turned on the Engine Preheat switch that activated the other Groco pump that is connected into the Webasto circuit and preheats the engine....still no juice to the booster pump above the muffler....then I though maybe it is wired into the Webasto circuit and turned On the Webaso front and rear hoping to get 12v to that pump...still no juice.

So...does anybody know if this is maybe a fused circuit ? Maybe I have a blown fuse ? Where to look ?

What switch, button that I may not be aware of, would deliver 12V into this orange wire that goes to the pump ? The wire enters the body just behind the mudflap in the rear and I have no idea where it goes from there.....

Any ideas ?

Logically, in my mind, power to this circuit ( orange wire) should come from the Colder/Warmer sliding switch that is on the left panel by the driver and activates that pulsating Mercedes valve....but, no orange wire that I can find connected into that circuit board with the switch.

Whatever the pulsing valve is doing should be after hot coolant gets to the dash.....and for that, this pump has to pull and push the hot coolant first.
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'91 36SP
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Chassis Heat switch on the Dash? Perhaps a Winter/Summer switch on the Dash?
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Quote:
Originally Posted by Rick View Post
Chassis Heat switch on the Dash? Perhaps a Winter/Summer switch on the Dash?
front heat switch on dash , my Winter/Summer switch is on the overhead dash panel. There are several old threads on the Mercedes Switch
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There is no Winter/Summer switch on the 1991 36SP.

The 12v diagram shows a #35 wire going from upfront to the Chassis Heat pump.....now I have to find wire #35 upfront....
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There are 2 coolant valves on coolant lines by the engine. Make sure they are not turned off.. They are there to shut off coolant flow in the hot months.
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They are on, hot coolant from the engine block travels on the hose to this booster pump above the muffler and then does not go further....hot hose before the pump, cold after the pump.....need to find out what energizes this #35 wire that would turn the pump on....as a first step. If I have 12v on the wire and the pump does not run or blows a fuse, then I need to change the pump.
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Gotcha! Do you have access to a toner? It is very handy to have with our complex birds. You attach the base unit to the wire 35 at the pump and use the probe up front to find the wire. There is a tone sent thru the wire to its termination.
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I don't but I am about to buy one.
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FWIW, that coolant pump near CAT muffler (on the frame rail) might run only with ignition on. I’m not sure, just a possibility.

The SP section of WOG downloads has a sticky post with ‘91 SP36 12v build sheets showing wire color, routing thru body, distances, ... I still intend to index them so they’re easier to search, but for now, a good pair of reading glasses is a big help 8-)
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All the testing is done with ignition on, since the dash heater is only supposed to work with engine on....
I have all the diagrams in the bus as well. Bought a tone feeder and found wire 35 coming under the dash into plug #28.
From there, this #35 wire becomes a thinner orange wire in a group of thin all orange wires that go up into the fuse panel above the driver.

I was able to trace the signal up into the fuse panel to fuse position SK05, where a wire tagged #28, Red, is called "Heat Enable" and is fused in the 5th fuse down from the top.

I had no fuse in that position. I put one in and I had power on wire 28, then 35, all the way to the pump, BUT......with the fuse in that slot put in, the circuits remains energized when the key is turned off, I get the Air dump, leveling red lights on the dash and constant voltage to the pump.

The only way I can cut the power off to these lights and pump is to cycle the A/T switch. Brief turn off, then on, it resets everything and all is good until the next time the key is turned on then the circuits remains energized again when the key is off.....

So.... something is not right, a relay in that #28 Heat Enable circuit stays latched when the key is turned off. Got to figure out which one, and why....

If I pull out the fuse on that SK05 circuit, all is back to as before, all lights turn on and off on the dash with the key as they should but no voltage the pump....
I found a receipt in the folder that the PO had the ignition switch replaced 2 years ago...wonder if they wired it wrong.

Doe anybody have any ideas how to proceed ?
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