red86man

It's been about a year and a half since I last posted about an ongoing "cranks but won't start issue" with my '86 coupe. Some of you may remember my extensive troubleshooting posts starting back about this time 2 years ago.

Yes, the car has not run in 2 years. I have put literally hundreds of hours into fixing my coupe's no start / no run issue. This problem has baffled myself and a number of savvy mechanics. There were actually numerous problems with the car which have been fixed, only to reveal yet another problem. It's been like peeling and onion.

Here are some of the failed components that have been fixed / replaced so far during this process:
- Fuel pump
- Fuel pump relay
- Fuel pressure regulator
- Clogged EGR
- Vapor cannister check valve
- Ignition module
- Idle Air Control

Other work that has been done in the interim:
- Reconditioned fuel sending unit along with replacing fuel pump.
- Cleaned, ported and polished manifold, tube runners and TB.
- Ultrasonically cleaned FIs
- Cleaned fuel rails, reconditioned & re-ringed, installed new FPR.
- Installed new knock sensor.
- New cap, rotor, wires, coil
- Rebuilt distributor with new components.

Basically, there isn't much that hasn't been done or many systems that I haven't been through several times already. I recently finished the detail painting and put the fuel / air system back together again - praying that it would finally start. Exhaust system is disconnected to eliminate the possibility of clogged cats / exhaust. Still no dice.

Back to the drawing board. So, what do I know for sure? I've got good spark, I've got good fuel rail pressure, I've got good compression on all cylinders, timing is right on, exhaust is unblocked, the car doesn't throw any codes - so what the hell could be wrong?

I was going through some old C4 tech posts on the forum and came across a thread that GIJoe had posted on and talked about the VATS. Given the fact that everything is new and all systems appear to be in order, what else could possibly keep my car from starting up? The freaking VATS, of course! There's not much else left.

So, I went out and did some searching around and came across GIJoes very helpful site at www.vatssucks.com. After some studying and reflection, I am convinced now that I have a problem in the Cranking Fuel Enable part of the VATS. If that part of the system is not working properly, then the car would crank but not start because no enable signal (30Hz square wave) would be sent to the ECM for the ECM to pulse the injectors. Go figure...

I spent my day today digging into the DIC and "breadbox'', yanking the 2 VATS modules, cleaning the pins and the PCBs and soldering test point wires off the 7 pins on the gray module and 6 pins on the blue modules. The outputs of each module are hard wired into a Radio Shack 8 phone jack module. The idea is that I now have 2 diagnostic patch panels that I can jack into anytime for troubleshooting purposes - without having to mess with yanking the dash panels. I'm going to mount both phonoplug jack panels to the front of the otherwise useless breadbox bezel and then jack into them tomorrow for some troubleshooting.

I also found and ordered a very cool dual freq (30 / 50 Hz) VATS Bypass module from Baxter Electronics' store on Ebay. Base price is $25for a single freq VATS Bypass module. I opted for the dual freq upgrade ($10) and the quick disconnect option ($10), also. Can't beat that for less than $50 delivered!

Obviously, I'm betting that the Cranking Fuel Enable circuit is bad. I wasn't able to check it conclusively today, but I will be tomorrow. Any bets that this is, in fact, the final answer to my crank-but-no-start problem? I really hope it is.

Thanks, GIJoe, for your excellent website. I'm starting to believe that VATS Sucks, too. I'll let you all know tomorrow.

CentralCoaster

I'd first look at the FSM and figure out how to completely disable the VATS and try again.

You said you have fuel and spark. So which savvy mechanic decided to replace these parts that can have absolutely nothing to do with the problem?

- Fuel pump
- Fuel pump relay
- Fuel pressure regulator
- Vapor cannister check valve
- Ignition module
- Idle Air Control

red86man

That was my thinking, too, before I checked out the www.vatssucks.com site. If you refer to the schematic diagram, if there is an issue in the circuit from the output of the VATS decoder module (pin P, purple wire) to the 30Hz generator module (not referenced) to the ECM, then you would have crank, but no start. Interesting, huh?! Finally, there is a ray of hope for my car.

zr1fred

aaaa, if the car is cranking, you are getting a VATS enable signal! Really, seriously!

red86man

As I mentioned, there were multiple problems with the car to begin with. Each one of the above mentioned parts were, in fact, bad and needed to be replaced. Just like peeling an onion.

CentralCoaster

He was responding to my deleted post. I said the same thing as you, then I read the rest of his post and changed my mind.

jfb

VATS doesn't suck, it prevents opportunist thieves from stealing your Corvette. 99% of thefts are done by bashing the column and jumping the ignition. You bypass VATS and you might find out the hard way how much your insurance company thinks your vette is worth. I vote for keeping your car in good repair, not bypassing circuits. Its like gluing a flashlight onto your hood when one headlight stops working. Anyway, I doubt VATS is your problem, but you can prove its the fuel cutoff by connecting a noid light (available from parts stores), or use a low power 12v lamp like a side marker lamp across the socket of an injector while you crank the engine. The lamp should pulse during cranking. If it doesn't, I would next check both injector fuses and then check for reference pulses during cranking on the purple/wh wire out of the distributor with an oscilloscope.

red86man

Yes, I would agree that the starter enable part of the system is working fine, because it's cranking. My contention, though, is that the section of the VATS system that sends the cranking fuel enable pulse (30Hz square wave) to the ECM for the ECM to pulse the injectors to squirt fuel and allow the engine to start is not working.

CentralCoaster

Sounds like time for either an oscilloscope, or a depowering of the VATS module.

zr1fred

I might buy a bad computer not supplying the injector drivers, I could be wrong, it's been pointed out before, but the vats signal goes to the ecm which in turn enables the fuel pump and the starter. Not the injectors.

red86man

Thanks for the info. Certainly, the VATS serves a useful purpose. It doesn't allow a thief to steal it easily. What might suck about VATS, though, is when the system might not let the owner drive it either.

CentralCoaster

Sort of reminds me of Robocop 2.

red86man

Here's some a quote from the site: "When VATS has not detected anything wrong and is working properly, it will send a 5v 30Hz signal to the ECM to tell it to enable the fuel injection system during cranking."

jfb

When the starter motor doesn't work the owner can't drive their C4, so what do you do, put a hand crank on the vibration damper? My 87 stranded me at church about 6 years ago due to VATS and I bypassed it so I could get it to my mechanic who replaced the worn out lock cylinder and it has started every day since then (its my DD). If the lock cylinder contacts wear out again, once more I will fix it!! If my headlight quits working, I find out why and I fix it, I don't epoxy a flashlight onto the hood. If you take care of your car, your car will take care of you! Keep your car in good repair. The person I bought my 87 from ordered a 90 vette and one day they came out of work and found their driver window and steering column busted out but the vette hadn't budged an inch due to.............VATS!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!

CentralCoaster

Come on man, lets focus on the problem at hand...

jfb

I already did!

red86man

I'm not arguing the usefulness of and need for the VATS system. I am merely exploring the possibility that part of the VATS is not funtioning properly. I've posted the information about this very perplexing problem on this forum to get constructive feedback and good advise from the members of the forum, who know alot more than I do, regarding the problem with my car.

zr1fred

I believe that is in the 90 up computers, they enable the injectors rather than the fuel pump. At any rate, wouldn't it be easier to just get a new module?

zr1fred

Well, I was wrong, again, damn this memory! From the 86 FSM ....."If the proper signal does not reach the ecm on CKT963, the ECM will not pulse the injectors "ON" and thus not allow the vehicle to be started. Code 46 will set,if the proper signal is not being recieved at ECM terminal "B6" (CKT963) when the ignition is turned on."

red86man

None of the Chevy dealers or salvage yards in my area have a VATS module in stock. A quick search on the internet didn't reveal any sources that had them in stock either. What I did find was this particular aftermarket module that will help to troubleshoot the problem, if nothing else.

The blue VATS decoder module that senses the resistor in the ignition key, grounds the starter enable relay to crank the engine and also supplies circuit ground for the cranking fuel enable circuit is probably not the problem, unless it is not initiating ground on the purple wire. The second module, the gray one, generates the 30HZ square wave signal to the ECM - after the blue VATS Decoder module initiates ground on the purple wire.

If it does turn out to be part of the VATS system, I will certainly replace the culprit with a new part - as I have done many times over already with this car. I don't like to rig flashlights up on my car, either...