vacuum switch location?

**wayneg** · 7th July 2014, 01:39 PM

My P38a with sequential gas seems to be running fine although I still have not been able to get diagnostics working, KME Diago 3. Still take a long time to switch to LPG but if I drive it then stop for a while when I restart it goes to LPG within a minute even if it has not switched to lpg before the stop. I have also noticed the colder the day the longer it takes to switch to gas 15 mins plus sometimes
Doing a general service over the weekend I discovered under the air filter, within the air filter box some sort of switch. As its connected to the LPG and has vacuum hoses attached I presume this is a safety switch for the system. What I dont understand is why its hidden and insulated inside the air filter box, ambient / outside air side of the filter. Is this also a temp switch ? Why would it be located in the air box?

**bee utey** · 7th July 2014, 02:11 PM

The doodad under the air box is not a vacuum switch, it will be a MAP sensor for manifold vacuum and gas pressure. The gas ECU reads these values for calibration and changeover back to petrol on empty.

Changing to LPG is determined by the minimum temperature programmed into the gas ECU, generally read by a temp sensor in the converter body. Check that 1. the sensor is properly inserted into the converter and 2. that your engine thermostat is in good condition. A partly open thermostat will delay switch over time especially during cold weather. Replacing the thermostat in my Merc halved the warm up time. I usually set the minimum temperature to 40-45 degrees C which generally takes around 2-5 minutes to reach on the road.

**wayneg** · 7th July 2014, 06:16 PM

So from what you say there is no reason for this unit to be housed inside the air filter box.

**p38arover** · 7th July 2014, 06:21 PM

Can you post a pic of it? The original KME gas pressure sensor and MAP sensor was problematical and didn't like heat. KME issued a bulletin about it. Maybe it was mounted there to keep it cool.

This what i had:

I've just replaced mine with the latest version. I bought it from Germany and had it with 10 days as it was half the price of buying locally (when available). See LPG pressure sensor appears faulty

This is what I bought: KME MAP Sensor PS CCT4 T UND PS CCT 5 Drucksensor LPG MIT Kabelsatz | eBay

**p38arover** · 7th July 2014, 06:36 PM

See [ame]http://oldwww.kme.eu/pdf/PS-CC2_assembly_recommendations.pdf[/ame]

Oh, the new sensor has a temp sensor included (the wire hanging free in the pic). So i removed the old gas temp sensor and connected the wire to the new sensor.

**wayneg** · 7th July 2014, 06:39 PM

Yep that looks like mine. Is the new one plug and play? Now it makes sense if the installer was aware this unit did not like heat it was a good move to house it within the airbox to keep it at ambient temp

**p38arover** · 7th July 2014, 07:03 PM

If yours is running fine, then the sensor is probably OK. Mine was intermittent.

The new one isn't plug and play but conversion is pretty simple. The old sensor has a pressure hose and a vacuum hose. The vacuum hose is a straight changeover but pressure is sensed through the 12mm dia hose fittings. I cut the LPG line where it feeds the injector banks and inserted it in series with the line. I don't know if positioning is critical.

I then removed the gas pressure take-off fitting and old gas temp sensor from the injector block and put a bung in there. I picked up a left-over bung for the injector block from my local LPG/LR mechanics. If needs be, I can take pic or two for you of what I did.

The wire from the old temp sensor was joined (via a plug) to the new sensor. The small grey 4-pin plug from the old sensor was cut off and joined colour to colour to the new big plug.

Re set up, I think I have set my system to change over to LPG at 35 deg C and to step 1 second between injectors, i.e., a change over to LPG takes 8 seconds after reaching 35 deg C. If you can't get the diagnostics working you probably can't check any of that.

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