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Unhappy The little yellow box

So I am driving down the expwy at about 75 MPH and I notice that the small boost gauge on the tack is past the 1.0 bar level yet my 3" VDO boost gauge shows nothing abnormal. Up to that point it had been working fine. I had been driving about 2 hours. I decided to push the throttle and the 3" boost gauge starts to move then the ignition shuts off. I let off of the gas, back to cruise, and everything is fine again. Factory boost gauge still is pegged. At my off ramp I let off of the gas and the engine ignition shuts off.

Now it wont start. I change out all of the red and black relays nothing. Engine turns over but it wont start. I tap the little yellow box (left side engine compartment), clean off the terminals etc. nothing.

I finally get the car home (yes i had it towed). leave it for a few hours and presto it starts right up. Now the little boost gauge is at .5 boost at idle but only when I start the engine. The big 3" boost shows nothing under 2500 rpm and begins to climb at about 2800. This reading is normal for my car.

Any suggestion on the cause of this problem? I have another yellow relay on order. But I am thinking I might need to look at something else?

thanks,

steve M.
Old 03-26-2007, 02:21 PM
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I had an issue with my 930 where if I really ran it hard it would cut out on my. Turned out the plugs were pretty fouled up and I essentially was flooding the engine. If I let it sit it would fire right up.

Can't offer much on the boost gauges though...
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'88 Blk/Blk 930 ('Lucy') - Not Stock & Not Running
Old 03-26-2007, 04:01 PM
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I had new plugs installed less than 300 miles ago. Thanks for the suggestion.
Old 03-26-2007, 04:46 PM
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It's late, but are the gauges both referencing from the same location? Could the problem exist between the two or is the OEM gauge just $h!+s?
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Old 03-26-2007, 07:27 PM
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I am not sure if they both are registering input from the same location. I will check.

thanks,

Steve
Old 03-27-2007, 05:03 AM
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The stock boost gauge get's it's signal from the big signal sensor on the BOV assembly (red circle..see next post below), so it is electronic. Most, if not all, aftermarket boost gauges run on a vacuum/boost line, non electrical.

I would recommend you also check the overboost switch connection on the BOV assembly (yellow circle..see next post below).
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Last edited by mark '87 930; 03-27-2007 at 06:10 AM..
Old 03-27-2007, 06:08 AM
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Old 03-27-2007, 06:10 AM
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Thanks for the suggestion on the BOV. I will check.

As a side note I just started the car. At idle 900 rpm. the factory boost gauge registered .7. While sitting at idle I abserved the boost gauge peg the meter and the engine shut off. Now it won't start. It definitely does appear to be electronic.

thanks
Old 03-27-2007, 07:42 AM
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Disconnect the wire going to that red boost signal sendoer (red circle). You will no longer ger a signal at your tach, but I'm unsure about the engine cutting feature tied to that. Try it and see.

Have you taken a really close look at your yellow relay? Many folks, including myself, have resoldered the legs as there are fine crackes in the solder joints.
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Old 03-27-2007, 10:15 AM
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Thanks for the suggestions.

I will resolder the the legs on the "little yellow" box this weekend. I will also try disconnecting the red wire. One of these wires must connect to the fuel pump shutoff . I will see if I can find a wiring diagram.

thanks,

Steve M.
Old 03-27-2007, 04:50 PM
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Don't disconnet the whiute wire to the overboost sensor. If you do, you're car will not start. That is the one connected to your fuel pump cut off. You could consider grounding it though, only if you know your car is not overboosting. By the sounds of it, you are ok, especially with your manual boost gauge ther, you can monitor that.
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Old 03-27-2007, 04:56 PM
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Boost Sender

Ordered a new over boost sending unit from our host. Installed this afternoon. Engine starts right up. Drove around with no failures. Stock boost gauge does register .2 at idel and does move up the scale faster than the 3" VDO but even when it is pegged it no longer cuts the fuel pumps off. This appears to have fixed the problem.

Thanks for the input from everyone.

Steve M.
Old 03-31-2007, 05:35 PM
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enigne shuts down "still"

Ok today after driving for about 1/2 hr. at a stop sign the engine died. I also noticed that the factory overboost was pegged past the 1.0 bar, engine not running. Prior to that it had been running within .1-.2 of the 3" vacuum boost gauge. I also noticed that just turning on the ignition switch caused the factory boost gauge to peg.

I unhooked the wire from the "brand new" overboost switch turned on the ignition and the boost gauge did not register. I tried to start the engine and it would not start (maybe not the overboost switch). I re-attached the wire to the overboost switch and low an behold the engine started? I have been fighting this problem for over a month. I have changed out all of the relays red, black and the infamous little yelllow box cleaned off the contact ends of the related fuse ends.

This problem is intermittent. The first time it happened I ended up getting the car towed home. Tried to start it after 3 , 6, 10 hrs sittting finally 24 hrs later it started. I then replaced the overboost switch. 20 miles later the engine hesitated a couple of times cruising at 60 mph (factory boost gauge indicated past 1.0), but luckily the engine never stopped.

I am getting the impression that the problem is not the the overboost switch, but everytime the engine has stopped there seems to be a relationship to the boost gauge pegging past 1.0 bar).

I am visiting my mechanic next week to look at a wiring diagram to see if I can trace down the wiring.

Any thoughts on anything else I might check?

thanks,
Old 04-21-2007, 12:58 PM
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does any1 know the wire color for the stock boost guage? or does it vary from year to year?
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Old 04-21-2007, 03:28 PM
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So I continued to try to figure this intermitent problem. I took the car out again this afternoon driving around with occasional blasts up to 1.1 bar, no problems. Then I remembered when I had the engine shutdown incident this morning I had just turned on the A/C. So I thought maybe this is the common thread, so I turn on the A/C and the engine immediately shutsdown and the boost gauge pegs.

Finally I think have found the source of the problem although I can't imagine how the A/C is interferring with the engine. I will isolate tomorrow to the blower or compressor. Then isolate it to the ignition or fuel pumps.

I have only owned this car for a months and it appears that perhaps the former owner had miss wired something.

At any rate I will try to sort out tomorrow.

Anyone else every run a cross this type of problem with the A/C?

thanks,
Old 04-21-2007, 05:46 PM
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There is a manifold pressure switch on the back of the manifold behind the recirculation valve. When .8 bar is reached this switch opens the ground circuit cutting the fuel pumps so you don't exceed .8 bar. Failure of this switch will result in a pegged boost gauge needle. It sounds like the AC circuit is interfering with this operation.
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Old 04-21-2007, 07:07 PM
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The factory manuals say that the manifold pressure limiting switch (overboost protection) should read infinity (open) at 1.5 bar, and 0 (short) at between 1.1-1.4 bar. You can check it on the bench with a footpump and a multimeter- a Schrader tire valve will screw straight on. Remove the valve core and be careful not to overpressurise...

The boost pressure sensor is a separate unit; The boost gauge in the tach will peg at maximum as soon as you turn the key if it is disconnected from this sensor, and should return immediately to zero when grounded. When checked against the connecting lug and housing, this sensor should read between 5 and 13 ohms at 20C (68F) at atmospheric pressure (engine not running).

They both bolt onto the pop-off valve housing after 1978.

I'd guess that you have an electrical issue that is isolating both the overboost switch and the boost sensor for some reason, causing both the tach gauge to peg and the overboost protection to kick in, despite the mechanical boost gauge showing normal boost levels.
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Old 04-22-2007, 01:19 AM
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Thanks for thes suggestions Spuggy
Old 04-22-2007, 02:39 AM
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Here is the test for the Boost sending unit.. hard to read but the point is when you remove the lead it should deflect the gauge to max with the ignition on.

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Old 04-22-2007, 04:12 AM
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OK - Hopefully this will end this thread.

I went back over all of the above suggestions particularly the ones regarding how to check if the over boost sensor/switch is bad. What I found was I had misread the instructions. I replaced what I thought was the over boost sensor when in fact I replaced the boost pressure sensor for the guage. I grounded the white wire from the Over boost sensor and the engine started right up. Not only that but the air conditioning when turned on did not shut the engine off. Still not sure what the relationship is between the A/C and the fuel pumps/over boost sensor but everything seems OK now.

Thanks for everyones input. This forum was invaluable.

Steve
Old 04-22-2007, 02:03 PM
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