hi folks,

i have no switchover valve. OK. i think i understand that this means i do not have overboost protection, and that a boost gauge would help me to let off if things get going too forcefully.

on my '85 CA engine, i believe that the switchover valve is meant to be wired to the EGR computer. since i am DEFINITELY NOT installing that thing, is there not an earlier sort of switchover valve that i can install without any wiring?

it may be moot since i do desire a boost gauge anyway, but i would like to know what i am taking on if and when i plumb the manifold - ALDA connection without passing through this valve. thank you!

__________________
'77 240D, 504H, OM617.952, etc.

There are many switchover valves. The one that's connected to the EGR controller is for the ARV. Don't be too quick to throw away your EGR controller unless you don't want to use your tach. On an '85 the tach signal is generated through the EGR controller from a sensor on the flywheel. (I guess you could retrofit the older tach amp and take the signal off the front pulley)

Here's the basic circuit for overboost protection.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

thank you paul. i do have the older tach setup and i do not have the EGR computer, and i'd like to keep it that way.

do all the rest of the switchover valves take their signal in a manner that's not electrical, i.e., connect just to the two hoses to the IM and the ALDA?

__________________
'77 240D, 504H, OM617.952, etc.

Actually quite the contrary. The operation of a switchover valve is almost always electrical. Based on some input, an electrical signal is either applied or removed such that a vacuum signal is transferred from one line to another. Using the example of the overboost protection circuit, the normal configuration is for the boost signal (pressure) to be routed to the ALDA to be able to add additional fuel to the engine based on the boost pressure. Should an overboost condition occur, the overboost switch is closed that causes the switchover valve to receive power and in so doing supply atmospheric pressure (non-boost) to the ALDA which cuts fuel and thus boost.

The switchover valve can be thought of as a two-port vacuum solenoid. In a vacuum solenoid the application of electrical power to the solenoid applies vacuum to the associated component (as opposed to no vacuum). Using an electricity analogy, think of it as an on-off switch, whereas a switchover valve is analagous to a two-position switch - position A provides power to one component and position B provides power to a different component.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

OK, thank you for explaining. if the '85 switchover valve receives its overboost signal from the EGR computer, from where does the signal come on other 617s?

also, how agreed is it that having a boost gauge (or EGT) is a reasonable substitute for the switchover?

__________________
'77 240D, 504H, OM617.952, etc.

The overboost signal does not come from the EGR computer - it comes from the overboost switch in the intake manifold. When the overboost switch senses 15.3 psi, it closes and in doing so grounds the circuit which opens the switchover valve (boost pressure aneroid or overload relief valve) that dumps pressure to atmosphere which cuts fuel which drops boost back to a safe level.



I wouldn't trust watching the boost gauge close enough to avoid an overboost situation, but that's just me. I do have a boost gauge on my SL Diesel that I watch but it peaks at 10 psi, which is all my engine pulls (it's actually a re-purposed vacuum gauge from the original SL gas engine.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

boy have i had trouble understanding this. sorry to waste your time. i don't have the full schematic for an '85 CA donor (do you?!). also, agreed that an automatic solution is simpler.

i had wondered what that sensor on the intake manifold was for.

does it matter what year of overboost valve i get?

__________________
'77 240D, 504H, OM617.952, etc.

It doesn't matter which overboost switch you get.

Here is the overboost switch



Here is the switchover valve (your mounting will look a bit different as mine is mounted differently in my SL Diesel)



If you need a schematic, PM me with your email and I'll send you a .pdf

Attached Thumbnails

   

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

very helpful! i had never seen a picture of the part before!

thanks for the schematic and i appreciate the tips. i'm going to try to buy a whole assembly (or 2) at the yard.

__________________
'77 240D, 504H, OM617.952, etc.