Okay, so i finally am getting around to putting a zener back in the 90 minivan after I broke it. . Well i installed a new one and hit boost cutout. What is the boost cutout voltage on the van? I bench tested the pressure rise vs. voltage and the 4.3v zener is clamping the signal to 3.5v to the computer. Even tried a few different ones all were within a .1v . Didn't have a problem before with the old zener.
Why the Zener? You can get a map sensor and a chip for pretty cheap these days. I'm sure the diode is much cheaper, but I like the extra insurance of the cal.
i haven't worked since 10/09 is the biggest issue. I only run 14 psi. Problem is I'll get a 1-2 psi spike at times on a long pull, like when I am racing or towing, hitting the boost cut.
try a 4.7, but honestly i wouldnt risk it
I was running '89 Caravan 2.5L TI electronics in my Omni GLH along with an adjustable overboost eliminator from M&M Racing. I found the overboost threshold to be at about 4.20v, although I believe this voltage can differ depending on your elevation because of the baro-read solenoid.
3.5v MAP output at WOT sounds pretty low.....too low if you ask me, you won't hit overboost, but you could very possibly run lean AND too much timing at WOT and break something...
Well altitude may very well be the issue then, I am sitting at 4800 ft. Yes 3.5 volts seemed low hence why i changed it around to make sure that i didn't just have a bad zener. Highest one was 3.72 volts lowest was 3.44. Van will run just fine sans zener with good a/f ratio up until it will hit the boost cut. With the zener it will hit the hard cut and then stutter with the a/f ratio dropping off.
I think I may still have some 4.7v zener diodes left over some where from the old days when the Shelby still had a MP LM in it. Maybe I will try them tomorrow.
Ha well I am at about 4550 ft here in Wyoming so I'm betting your threshold won't be very far off from where I found it to be. I would highly suggest getting the adjustable overboost eliminator from M&M Racing. You will really be able to maximize the fueling out of the stock computer that way, giving you a little more overhead. It's pretty easy to dial in with a voltmeter if you have an assistant riding shotgun recording the voltage at the moment of shutdown. You gotta have the grainger valve (MBC) adjusted just right so it just creeps up on overboost slowly, and not spike right into it so fast you can't zero in on the voltage. I would set my eliminator at about .05v below the cutout point.
I can't remember if mine were 4.3v or 4.7 volt, but now I run an adjustable cut out eliminator that's probably very similar to the one iTurbo has mentioned. There's nothing wrong with running a zener as long as you add the additional fuel. It seems you're only after avoiding the cut out during heavy load runs and probably not above 15psi.
The one I got was from a guy on Neons.org and it was sold as a "map clamp". There has been talk of making one yourself, but I have no idea where the link went. Try M&M racing as iTurbo suggested and it'll work fine for you.
Edit-- The voltage where the computer turns off the fuel is the same regardless of altitude, right? It should always be "around" 4.5V
Yeah I had planned on going down the horsepower road after i move back to Ohio and get back on my feet. No need for a map clamp with a programmable sbec and 3 bar map. The irritating thing was the old zener diode i had on there worked great, broke the wire off where it went into the cathode.Nothing different aside from the new zener replacing the old, which was an old 4.3v I took off my Starion. Puzzling indeed.
And the answer to the altitude question is kinda. The ecm uses the barometric reading to zero the sensor, however the barometric pressure is not merely affected by weather (i.e. high and low pressure fronts), but also by basic atmospheric pressure which varies by region, season, and altitude.Long story short the ruler changes in size slightly, and so will the target values the ecm will use.
I think there were a lot of variance in the zener diodes or maybe the quality control just wasn't very good so you never quite know where it will limit the MAP. That's another reason I like the adj unit because you can account for that as well.
Once I dialed the adjustable unit in though, it does seem very consistent and unaffected by temperature. I would only have to retune it if there were a large change in elevation. For example if I dialed it in here in Gillette WY (4550 ft), and then went to Laramie WY (7200 ft), I would be hitting the overboost cutout again. Seems like the further you go up in elevation, the lower the voltage threshold to hit overboost will be. In Laramie it seemed to ping the overboost cutout at just around 4.05v. At sea level I imagine it would be closer to 4.5v.
FWIW, with the key 'on' and engine off, my MAP output voltage for ambient pressure is 2.02v usually according to the OTC scanner I use. Here is a link to MM Racing: www.mm-racing.com
I just saw a motor almost melt down this friday night because someone thought it a cheap way to make HP. the turbo made +25psi when a waste gate hose blew off and the motor started to lean out
The price of a map clamp is not worth the price of a motor +towing bill !!! and you have not address the problem of timing causing KNock!
GEt smart and get a chip burned!!!!
Well that same scenario could have happened with a cal. And that someone you mentioned needs to zip tie their vacuum lines ASAP! I was running an adjustable MAP clamp for the longest time in my SL and then I got a cal from ShelGame with the overboost disabled so I removed the MAP clamp. Really what is the difference since they both pretty much defeat the safety feature. Even then it's possible to blow a motor if it spikes too hard/fast for the OB to catch it so there are no guarantees.
For what the OP wants, doing it the 'right' way...i.e. cal, bigger injectors, 3-bar MAP...is overkill. Sounds to me like an adjustable MAP clamp is probably perfect for him considering they are only $15 and it sounds like he knows what he is doing...
a proper cal will have the over boost set to the desired level and proper timing talbes!!! Ashtraying a motor is not worth it!!!
regardless of what i think of map clamps, does anyone have that old dennis doza map clamp pic... the one showing the diodes and the potentiometer?
i cant find it anymore.
brian
Originally Posted by turbovanman
The potential for heavy damage through a custom cal is just as high. I'll stick with my "wrong" way because it hasn't broke anything once I figured it out.
Dollar for dollar, hour of tuning for hour of tuning, you won't beat the capability of a custom cal. The first baby was born in a cave only because it had to be.
a cal is only as good as:
1. the person that creates it
2. the person that installs it and monitors the conditions for proper operation.
ill never go back to the old school hacks. they are silly when doing your own calibration is so documented and cheap.
brian
When you make it dollar for dollar I'd still go the cheated 2 bar route. Considering the money for a chip burner, eraser, computer vs trips to the junk yard and some pressure switches. Both require a wideband to do properly and both require knock sensor input to do properly, so the time to tune should be about the same. Having said that, I do have a burner and eraser and computer upstairs waiting for me to get into the whole custom chip burning. I tried a vendor and wasn't happy with the results and resulting damage.
Edit - This thread is WAY off course so I'll bow out of any continuing discussion.
the problem with 2 bar trickery is that it has no control over RPM variations. pressure yes, but RPM no.
there is also the problem that the manifold doesnt flow as even as you might think.
i know, i know, i know, gus said it works fine and for the most part it does.... but its not as even as when you are controlling port fuel injectors though.
get out the burner and start doing some reading.
oh and you dont need a chip eraser if you use 27sf512 chips. i know they are obselete but you can still find them here and there. they still make the larger 27sf010 and 27sf020 chips though they require a bit of rewiring to run them.
Brian
Burn a 2 bar map with no boost cutout. Problem solved.
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