Over-boost & shutdown circuit remedy
 

Just curious how many have abandoned the factory circuits for the over-boost, and engine shut down? I've gone through two over boost senders in less than 50 miles, and have already suffered the effects of a bad shut- down relay. I'm thinking of losing the shut down relay, and the over-boost sender and replacing both with a toggle that will let me ground or disconnect the over-boost circuit manually. Anyone out there think of why it's not a practical idea? I'm tired of having to stop and diagnose ignition/fuel pump circuit related issues. I'm running a fairly stock Euro 930 motor. Garrett GT35R and Tial WG w/.9 bar spring. (Pretty sure on the spring). I was tempted just to run the over-boost wire to ground, and call it good, but then I ran into the dreaded: "my car won't shut-off problem", and had to replace the relay in the back. If I run the over boost wire to a toggle, get rid of the shut down relay, jump that connection, I can essentially manually create the same effect of shutting off the fuel pumps, before killing the ignition. Granted, I lose the over boost feature, but why not, considering my set-up?

Ground the wire that goes to the sensor. Done.

Because you then would have no safe-guard in place if the wastegate fails to open - seems like a big risk to me considering all it takes is for the boost signal hose to come off/hole blown in it/melted, etc. I once accidentally left the signal hose disconnected after doing some work (like a true moron) and the overboost circuit triggered the first time I throttled it in earnest on the test drive (within 5 minutes of heading out, in other words).

Hell I thought EVERYONE defeated their overboost.

I don't know of anyone that has if they're still running K-jet. A very unwise mod right thar!

Now I need to check and see if mine is enabled or not - cannot remember!

Jon, are you sure the switches are bad? I ask, because if the switch is not grounded properly, it will not work right. I had to add a ground wire to my secondary intake manifold after I had it powder coated because the coating prevented proper grounding.

Sure, grounding the signal wire is the easy way out, and I'm all for easy, but the added benefit of a switched ground on the over-boost sender is also not having to buy another shut-down relay if and when it fails again. You kill two birds with one switch. Ronnie, your concern is what worries me. I sure don't want to grenade the new motor. Even though I'm running a new waste-gate, new hoses, etc., Murphy always seems to hover near-by the ol' 930. I can't help but think I may be overlooking another wiring issue. It just seems odd that two senders have failed in less than 50 miles, along with the shut-down relay...

Yup, double checked. I had the same problem with powder coating when my a/c compressor clutch wouldn't work. Wasn't grounding through the new finish. That was a recent deal, so I thought about it right way. I've had two senders fail, and one shut-down relay. I'm going to open one of the OB senders. I've been thinking they are mechanical, so an electrical anomaly wouldn't be an issue with them, but I want to be sure.

I've gone through two switches in a short time as well. It's gotta be a grounding issue or faulty switches. Have you tried Alan's suggestion from your other post ? I plan on testing mine but haven't got off my lazy arse yet.

What sort of boost you running? If you cut the old faulty one open you will find they are very crude - amazing they work as well as they do. Just a single spring holding a set of contacts together.
What you could do is play with the new one - ohm meter to terminal pin and body of switch. Should be a circuit. Now wiggle the terminal and see if circuit opens. Or hook a bit of pressure in there - mighty vac- to say 0.5 or 0.8 bar and see if circuit breaks. That will tell you what is going on. Maybe a batch of cheap Chinese units or something.
Alan

Since the switch seemed to be behaving as it should, I went for a drive. Fortunately, I packed a test wire with a couple of alligator clips. It wasn't long before I had to pull over. The sender failed, and I had to jump the sender wire to ground. I guess I'm glad to hear the sender is simply mechanical inside. It seems to rule-out other wiring issues. Hard to believe they are really that poorly made. As Ronnie mentioned earlier, I'm not excited about losing the over-boost feature, but I've had enough of pulling over to install a jumper.

Jon, just for curiosity, run a temporary ground from the body of the switch to another point on the engine other than the secondary manifold it's screwed into (like one of the primary manifold studs/nuts), using alligator clips, and see if there is any change.

Paul "big" Bush, you should do the same and report back, bro (once you've gotten off with yer lazy azz, that is :eek:). :)

Do not disrespect the 70s bush

Anyone who knows Paul knows he does not disrespect the seventies.

Man-scaping is for poofters anyway!!!

Did that one already Ronnie. No joy. I've got some powdercoat under the hood, and it's already caused me a issue with my a/c clutch. I'm going to try one more sender, but I'm adding a switchable ground. At least I'll know the next time, before I get out of the car. :cool:

If the factory ones are no good anymore, you could try an adjustable Hobbs boost pressure switch, one that has normally closed contacts. They have 1/8" pipe thread, but that is close to the metric thread. If originality is an issue, then forget this.

Also, years ago when adjustable manual boost kits were sold there was a 1.4 bar pressure switch that was available. Don't know if those are available anymore.

I am trying to diagnose a problem which after doing some searching and reading I think it may be my overboots switch. First off my question. When the overboost switch kick in it is momentary or?

My situation. I have 3.3L 78 turbo. I recently got the short block rebuilt and I am breaking in the motor. When I roll into boost it bucks and is hesitant. I think it is cutting my fuel. I am running twin till 38mm wig's with 993 HE's and a GT35. I have tested the wig's and they seem to work fine.

My guess is the overboots switch?

Does this sound about right?

Over-boost switch provides ground to the fuel pump circuit. When an over-boost condition occurs, the switch causes a loss of ground, shutting down the pump. It's normally momentary. If you believe your switch is defective, ground the wire. By doing this, you will have restored a path to ground.

Sounds like you are running a little rich. Do you have an air fuel gauge? If so what is the reading when the bucking begins?

Rahl