A "what is this part" question

[Bruce Appel]

This morning did maintenance stuff on the 85 sd. Changed the tranny oil and filter, adjusted the valves, cleaned the banjo bolt at the back of the manifold, and replaced the broken air cleaner bracket. Anyway, while doing this I noticed that towards the back of the manifold there is a part that looks like a temperature, or pressure sending unit that is weeping what looks to be a mix of fuel and oil. Is this a pressure sensing switch of some sort for the tubo, and what exactly does it do? It looks like it needs replacing because it is leaking quite a bit.

[rg2098]

A picture would be most helpful.

[1985 300SD Sady]

I know what he speaks of, because I own an 85SD ... but I dont know what it does... probably somekind of pressure sensor for the computer...

I do beleive that there is a sealing type ring on this that could be replaced...

Call Phil. (Fastlane)

[Breckman99]

Most likely it is the over boost pressure switch. It tells the switchover valve to stop delivering boost to the alda therefore cutting the supplemental fuel flow. It is a system safety, so if not working and you have an over boost condition there will likely be damage. If it is leaking I would pull it out, clean up the threads, and put a good automotive grade thread sealant on the threads to see if it stops. That "oil/fuel" looking stuff is what has built up inside your intake manifold due to the EGR and crankcase ventilation system.

Originally Posted by Breckman99

Most likely it is the over boost pressure switch. It tells the switchover valve to stop delivering boost to the alda therefore cutting the supplemental fuel flow. It is a system safety, so if not working and you have an over boost condition there will likely be damage. If it is leaking I would pull it out, clean up the threads, and put a good automotive grade thread sealant on the threads to see if it stops. That "oil/fuel" looking stuff is what has built up inside your intake manifold due to the EGR and crankcase ventilation system.

that is exactly what it is

[Bruce Appel]

I thought the over boost pressure switch was mounted to the firewall over towards the alda, and has a vacume line going from the banjo bolt at the back of the manifold on one side, the alda on the other.
The part I am wondering about is simply a sensor screwed into the intake manifold. It has an electrical connector, no vacume line. Unfortunately i have no way of photographing it, but when I look at Fast Lane they have a part called a boost sensor that looks like it. I am reaaly curious as to what exactly it does. I did not trace where the wire runs to. Does it send the signal to the overboost protection switch?

[stayalert]

http://www.ultrahot.com/vacuum/turbo_system.jpg

My opinion(s)

Part in question Boost pressure switch.

Thing on firewall is switchover valve. which the boost pressure switch is wired to....

Helpful? Hope so.

Best,

[Bruce Appel]

That's it. Thanks.

[Fisherman]

You might just try to tighten it a bit.

This switch is on the ground side of the circuit and when it senses overboost it completes the circuit to the switchover valve so that it turns on and your alda then reduces the fuel amount to cut power.

I have had two of them go bad (complete the circuit) by getting gunked up inside the manifold. You can test it by using a ohm meter and checking the top pin (where the wire attaches) and ground (manifold) to see if the circuit is open.

[Bruce Appel]

Thanks for the explaination. Unfortunately it appears to be leaking not from the threads, but actually around the plastic part. I cleaned it up good, will drive it a few days and see if I can see it better. The part is over $50, so if I can just clean it up and tighten things to seal it up it would be great.