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  #1  
Old 09-07-2003, 02:25 PM
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603 Overboost relay...

Hey guys....

Quick question.... do you know if the overboost relay (I think that is what it is called) on the firewall should be plugged? I tried to improve the power on my car and haven't had any luck yet. I disconnected the vac line from the intake to the relay and from the relay to the ALDA. I tried to clean them out and replaced the rubber vac lines, but no luck yet. The car is still very slow and I noticed that I cannot blow through the relay at all. It is either plugged or not meant to pass air through. I believe it is plugged because I have seen other threads that mention that it should be free to blow through. Any thoughts.... and... how does a person clean this out?

Thanks.

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Old 09-07-2003, 02:31 PM
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1: I'ts plugged

2: Replace it, no percentage in trying to fix it. You can try some brake parts cleaner (NOT carb cleaner!!!) but it has rubber seats in it, and most anything that will remove the crud will remove the seats, and then it will leak....

Temporary fix/diagnostic is to connect the hoses together bypassing the switchover valve. You will have TREMENDOUS power in relation to what you get with a plugged valve! Don't leave this in place permamently -- if the wastegate line on the turbo leaks, you can blow the engine up!

Peter
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  #3  
Old 09-07-2003, 05:38 PM
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Anyone happen to know a part number from Fastlane I could use?

Thanks.
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Old 09-07-2003, 11:40 PM
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Marc, I'd just bypass it for now. I've been bypassing my relay for about 3 months, with no discernable problems. But be aware that this really should just be done for 'test purposes' only.

You really should pressure test your ALDA to ensure that its capable of holding pressure too. I've got a dual-action mityvac if you need one, since I'm just up the road from you

You may find that the ALDA needs to be shimmed as well.
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Old 09-08-2003, 12:14 AM
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Marc, I'd just bypass it for now. I've been bypassing my relay for about 3 months, with no discernable problems. But be aware that this really should just be done for 'test purposes' only.

You really should pressure test your ALDA to ensure that its capable of holding pressure too. I've got a dual-action mityvac if you need one, since I'm just up the road from you

You may find that the ALDA needs to be shimmed as well.
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  #6  
Old 09-10-2003, 05:31 PM
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Ok, I received my order from PartsShop today and I hope they sent me the proper switchover relay. This new one appears to have 3 connections on it, while the one in the car only had 2 lines connected to it and a cap on the top (I think). Does anyone have a good picture of what the relay (replacement) should look like?

Thanks.
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Old 09-10-2003, 05:58 PM
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Marc, here's a link to a picture of gsxr's overboost relay from his website.

http://www.meimann.com/images/mercedes/OM603_injection/cruise_fix_after.jpg

Mine looks just like his, though I've temporarily removed it for test purposes. Where is the extra port on your new relay? You may be able to cap it off. Do the part #'s match?
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Old 09-10-2003, 09:03 PM
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One connection on each side at the bottom and one at the top. The cap probably comes off, transfer to the new one.

Peter
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  #9  
Old 09-10-2003, 11:09 PM
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I replaced the relay and what a difference.... WOW The car feels like a different engine under the hood! I didn't realize how fast the 603 really is!!! One thing I did notice... the little white twist unit on the throttle cable appears to be broken. Does anyone know what this is for and how it can be repaired?

Thanks again for all of your help!!!
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Old 09-10-2003, 11:11 PM
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Glad to hear your "new engine" works well! The white threaded twist thingy is for, IIRC, the transmission Bowden cable. If the tranny is shifting OK it's probably fine. Otherwise it's off to the dealer (well, Rusty, anyway) for a new one...

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Old 09-10-2003, 11:14 PM
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Dave,

Do you know if the Bowden cable is something I can order from Fastlane or Partshop? Also, is it something that can be replaced easily or is it fairly involved?

Thanks.
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Old 09-10-2003, 11:22 PM
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Excellent questions to which I don't know the answer. Perhaps someone else can chime in?
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Old 09-10-2003, 11:42 PM
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Thanks Dave... I'm very curious if the cable can be repaired at the throttle end or whether the entire thing has to be replaced.
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  #14  
Old 09-11-2003, 02:06 PM
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Hi,
Could some body explain the function of this "overboost relay"...and how its failure could result in bad engine performance as indicated in the previous posts...If it is for protection from overpost, this means that it only works to block the extra boost, if it fails, does it block all the boost?...I am trying to understand!!
Also, how to check this device, taking it off and blow through it??!!
Any insights...

Thank you ALL
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  #15  
Old 09-11-2003, 02:34 PM
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toknow,

On my car, the "overboost" relay or switchover relay was completely plugged and I couldn't blow through it. Replacement brought my power back up to par because extra boost pressure was reaching the ALDA on the back of the IP. I definitely knew mine was plugged because I couldn't blow through it at all.

HTH

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