I'm wondering what the overboost solenoid does when the boost passes its pre-set limit?

I ask because twice now while doing some serious highway cruising, my wastegate hose had popped off and didn't realize until I got home (the first time at least, the second time I noticed the turbo whine.) I'm sure I was getting full boost for minutes at a time going up mountain passes and such. So what would the solenoid do? Just regulate at 15 PSI or what?

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B
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1983 300SD

At 15.9psi, the pressure switch on the intake manifold will complete the electrical circuit (The ground side of the circuit). Then it will vent the air pressure going to the ALDA. That will cut your fuel delivery to no-boost settings until the boost drops below 15.9psi again. At that point, it will let boost build again and it will run as normal until boost gets past 15.9psi again.

If it's working properly, you will feel the vehicle bucking (Accelerating/decelerating) as the boost is vented and rebuilt.

If it was not working properly, you could have seen the water temp get into the 100*c range and/or a loss of power after a minute or two.

Holy crap! No kidding? Since it happened before with no ill effects, I didn't think that my other issue while driving would be related. The second time this happened, the car was acting as if it didn't have much power, like a fuel delivery problem. I changed the prefilter and the same issue persisted. I put in a bunch of fresh diesel and some Power Service additive thinking there was some crap in there and I'd check it out later if only I could get home. I think I still will, see if there is some gunk on tank strainer or something.

Quite the trip really. Three hours one way and the cuirse and car worked great. On the way back, the damn starter wouldn't work a few times, well every time, (smack! with tire iron while the girlfriend kept on the ignition) and cruise was acting up. Happy on the way out, "man, just get me home" on the way back.

You know, it probably was a fuel issue though. The lack of fuel seemed more constant, and not really off and on, which is how I'd imagine the overboos solenoid.

If the solenoid doesn't work, would the turbo just boost to its it heart's content, or is it failsafe, meaning it wouldn't boost at all if it was busted?

Can't go by the temps, my car has always run kinda hot. 100 while cruising is not uncommon.

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B
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1983 300SD

If the solenoid vent is clogged by EGR soot from the intake and/or the electrical part is not working, it will continue to boost as high as it can on the fuel (accelerator) you give it.

I've run in the mountains with 20+psi boost *once* before. That engine is still in use but it really hurt my idle quality. I'd bet it scored my pistons/cylinder walls and/or burned the rings. It runs great under load and when it's warm, but when the coolant is cold I get misfires, smoke, and it won't idle well below 800rpm. It's been well over a year since that happened, but I'm looking to replace it with a used engine soon.

I have a boost gauge in my car and when I was adjusting boost pressure it would buzz a little and the needle would rapidly swing from 14 to 15 psi until the pressure went down.

I never saw it drop to zero.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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I wonder if the whine I was hearing wasn't my turbo spooling, but the pressure venting off.

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B
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1983 300SD

in the 603 engine, there is an air-recirculating valve, what is the function of it? is it to relieve the overboost situation? Also, the vent side of the over boost switch is capped hardly, which implies that it cannot open to vent? is there a way to test the switch?
thanks for clarification.

If I remember correctly, I've read that the '85's don't have the overboost solenoid. Yet my boost line is hooked into something on the firewall...can anyone tell me what it is? Or is it an overboost solenoid after all and I'm just confused?

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Ralph

1985 300D Turbo, CA model
248,650 miles and counting...

My '85 still does have and overboost protection solenoid attached to the driver side firewall. I think what you are talking about is the blue thing on the driver side inner fender wall, where there is a line attached to the ALDA where the other end of the overboost solenoid line is hooked up.

(Banjo bolt from the intake manifold---> Overboost protection solenoid ---> ALDA---> Vacuum Amplifier.)

I think what this blue thing called is a "Vacuum Amplifier". What it does has something to do with the automatic transmission shifts. As the manifold pressure increases, The vacuum amplifier has a venturi which also increases the vacuum Hg, thus affects the shifts at high RPM.

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'85 300D Turbo - CA Version

On the 123 turbos, the gadget mounted to the firewall, just left and up from the brake booster is the turbo switch over valve. New style is square with 3 ports on it. I got mine from Diesel Giant. I got the hook up instuctions from this forum, since Diesel Giant never replied to me. Oh well.

On the new style, goes, banjo bolt from intake to bottom port. Middle port goes to the ALDA. Top port is the vent, so no covering it. I kept the 90 degree rubber lines for the connection.

To hook up the old style (PO bypassed mine since it was clogged), looking at it head on, passenger side to your left, the ALDA connects to the port on your left. The intake banjo bolt line goes on the port to your right, or closest to the brake booster.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

I guess I must have the solenoid, too. I was talking about the little black thing near the fuse box, not the vacuum amplifier, aka transducer. Thanks for the info., I do appreciate it.

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Ralph

1985 300D Turbo, CA model
248,650 miles and counting...

It's purely for emissions. It lowers the intake pressure to allow more EGR gas flow into the intake.

I see! I think that little black thing is also another vacuum valve solenoid, which controls the vacuum supply for the amplifier, aka transducer.

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'85 300D Turbo - CA Version

G'mornin',

I have had some vacuum problems in this area.....see my "Vacuum Devices" Thread and postings.....

There are some photos that I took inder my hood...and some explanation as to what's happening......

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG

On an older benz such as your '83, the turbo has a hard time producing much over 16 PSI anyway. The waste gate springs rust, and also weaken, which probably reduces the boost created. Unless of course you went in there and adjust your wastegate.

With this in mind, it should be safe to assume that the overboost on the firewall is not really needed. Now a brand spanking new benz with 100% working parts, well then duh sure is needed.

I've disconnected my overboost (sold it at benzbonz.biz) I plugged my wastegate hose, I noticed no temperature increase, no added power (the car is pretty quick to begin with,,, chirps in the 1-2 on WVO), no loss of power, no vehicle bucking (before or after removal of overboost). Please realize the car has 289K miles on it and burns WVO. I've made no adjustments to the wastegate,,, don't care to. EGR bypassed. and overboost by-passed.

But that's just me.