This applies to '86 through '89. Other years may be slightly (or a lot) different. For more detailed info, go here:

http://www.vatssucks.com

The VATS module has two outputs.

One enables the start relay. If the engine cranks then VATS is seeing the resistor in the key and found it to be correct. This isn't a 100% accurate indication because some people bypass the start relay so the engine will crank without a valid key resistor (or VATS module, for that matter).

The second output is a 30Hz (50Hz on later years) square wave to the ECM. This goes out on VATS module pin P and goes into the ECM on pin B6. If the ECM does not see this signal it will not enable the injectors. Check this signal with a DVM (Digital Volt Meter) on the 20 volts DC range. If it's 5 volts it's bad (no square wave). If it's 2.5 volts it's OK (the square wave divides the 5 volts in half).

 

Just a stab in the dark, but have you actually had your hands on the G-104 ground series of 5 wires on the block near the oil filter?

AND

have you removed the battery and its support tray and inspected ALL of the 5 or 6 HOT wires from the "jumping post"? THose hot wires DO NOT go thru the fuse panel. These hot wires are ALL fusable link, fragile wire AND they supply the ECM, fuel circuit, sensors and cabin control circuits with power.

I had an experience last week after installing a new battery tray where my eng would barely run, rich, fouled plugs in 30 seconds, backfired etc. Problem was the wire on the jumper post (all hot critical hot wires) were dirty and made poor contact. The ECM and INJ circuits were getting only part of the necessary voltage. I cleaned the contacts of all 6 wires, cleaned the post and remounted. Engine suddenly fires and runs like new again, even with fouled plugs (that burned clean seconds later)
Point is that these wire harness and connections are every bit as sensitive as your own HEART or BRAIN to electrical signals. If these signals are not clean the system does not operate. If power is interrupted or low voltage is applied the system cannot operate. If the inj seem to NOT want to pulse, they may not have power. Its a simple circuit split 8 ways.

Another "tip"...
go back and UN-DO the VATS bypass and all that DIY programming. Its only adding to the confusion. How exactly do you know that the ECM is reading the correct resistance thru your bypass? the only RIGHT way to bypass a VATS system is to take it out of the program. Its EASY to insert the WRONG resistor value in the key circuit and be close enough that the temp makes the difference. I've seen people jump that circuit with a piece of wire thinking that is all they had to do.....it takes the exact resistor (within range) to bridge that circuit and make the ECM think the right key is being turned in the ignition.

These problems are almost always more simple than people make them out to be.
UNDO the VATS thing...
Ck the jumper post terminals and clean
Ck the harness ground collections near the oil filter.

THEN,
One step at a time...key to ON, verify pump runs for 2 seconds and pressure builds.

If so, Crank eng with NOID lite and verify inj pulse. Verify Spark with spark inline tester.

Depending on what happens with these basic test, there will be a result or diagnosis.

Once so many things have been jumped or bypassed or d/c there is no way of knowing which thing did what.
One thing at a time...
you'll still get there.

 

Thanks Cliff. I bypassed the key resistor by installing a set of resistors which measured
the same value of the key (5% tolerance at 1/4 watt).
That eliminated the key lock problem which randomly triggered the vats timeout.
The engine cranks flawlessly now every time.
Once I have the ecm hanging down, I'll check pin B6 on the ecm for the correct
signal from the vats decoder with the ignition in the run position.
I know the easiest way to remove the vats from this equation is to get a prom for the ecm
that eliminates it. However, if it turns out that the ecm is at fault (which could mean
that the injector driver is dieing or has somehow died), then it would be better to get a
new ecm with a new prom without vats.
But before I get to that point, I'll first check for any faulty grounds in that circuit and
repair them as needed, then go from there.

Thanks leesvet. The only thing I've changed so far is bypassing the key resistor with
a set of resistors (5% tolerance at 1/4 watt) that measured to the same value.
I haven't been able to get underneath the Vette since I don't have a way to lift it far
enough to let me crawl underneath and get at the big ground connection near the oil filter.
But, I'll pull the battery and tray so I can get my hands on all the links and check
their condition. If anything checks doubtful, I'll repair them.

------------------
The test with the noid light revealed on one day that everything was working when
the engine fired up. Then the next day and since then, no injector signal.
The simplest test for now is to check the connections for grounding and power.
Could be one or the other, but also could be a combination of both.
The recent changes in weather could have a lot to do with ground connections.

 

Regarding the ground bundle, sometimes it can be seen from the top BUT you have to pull the wiper motor and move some harness around to get a clear view or get a tool on the bolt.

Its most definately easier from below. The ONLY advantage to working from above is that you sometimes accidentally find a bad splice in the harness because thats where they ALL come together, right where the ground wires exit the harness trunk on the firewall.

 

Thanks for the info leesvet.

 

Can't believe I had to pull the breadbox to get at the vats module.
I checked Pin P during ignition "on" and found the signal voltage of ~2.5v.
At least the vats module is functioning.
Accessing the ecm is going to be even more of a PITA, since the lower knee pad
appears to have been glued into place.

Attempted to start the engine:
Engine cranks and tries to light up. Same situation as previous.
Temporary start up for only a few seconds feeding off the cold start fuel feed.

1. No pulse indicated at injector 1 using the noid light on a 5 second "long crank".

Something is apparently not being picked up to switch over to a constant "run" condition.
Is it possible that a defective ignition control module (in the distributor) could
be causing this situation?

 

It is possible the icm, pickup coil or the related wiring have issue(s). This was mentioned earlier in thread- thought you checked all that already - did you not?

Note: It may take more than 5 seconds of continual cranking to see the "pulse". Turn the "key" all the way off then go all the way back to the "starter"position each time you go to "crank" it.

The next step would be to pull the ecm.

The vats is officially "good" then?

 

I replaced the icm in the distributor when I changed out the injectors.
I remember many years back in the 80's that I had to change an icm
on a different vehicle. The new one that I installed turned out to be defective
right out of the box. That caused a situation similar to what I'm experiencing now.
So, I'm half-way expecting that to be this case,
but I'll double check it and the wiring to make sure.

The first time I cranked it up after installing the new injectors, it took more than 5
seconds to start up. But afterwards, it fired right up. As it also fired right up when I did
the first noid light test. That's what really threw me off.
I did crank it for more than 5 seconds on the third try and did it the way you suggested.
(Turning it all the way off). It stumbled on the cold start feed for a couple of seconds on
the first and fourth tries, but failed to fire up.

Seems the vats is ok, at least it's sending out the signal to the ecm.
I'll be checking the other output to the ecm, but think that'll be okay.
One other thing I'll need to check for is the distributor reference signal that's
supposed to be going to the ecm. Looks like it will be easier to check once
I pull the ecm down.

 

That passenger side lower panel has 3 phillips screws IIRC....one near the lite, one to the far right edge and one in the rear. It then drops and slides toward the rear of the car. Its more fun trying to get it back.
The ECM has two 10mm bolts that are easily reached with a ratchet and 4" extention. You should remove the 2 harness plugs first, release the catches, then rock the plug out of the ECM socket. THEN remove the 2 bolts and it drops out. Odds are against the ECM being bad just as they were for VATS being at fault IMO. When one person writes up his encounter with a bad ECM or VATS module everyone see's a potential scapegoat for the misfire or starting issues. There are a thousand things that have to go right for a Vette to lite up....Vats and the ECM are just 2 of them and they are among the most reliable. Both are inside, not exposed to weather and heat, so they are less likely to fail as other componants are.

That lower panel will send you to the chiropractor if you;re not careful...so go slow. Some folks that are too big or too old (like me) have to remove the seat to get enough room to get in there comfortably and do the under-dash work. Besides, pulling a seat is like a lottery ticket...never know how much money you;re going to find pulling one...

 

Thanks leesvet,
I could have sworn that I got all three of those screws loose... but maybe my own screw is loose if I missed one (namely that back one).
I tend to agree that since the ecm and vats are inside the cabin, they they would tend to be more reliable.
Jon (FIC) just brought something to my attention that makes a lot of sense which I'm gonna check on tomorrow. If the engine fires up after this check .... it's gonna leave me with a few other questions that's gonna beg for answers.

 

You can get to the ECM by removing the kick panel with the footwell light in it. It's held on by 3 7mm screws.

 

This intermittent problem of yours screams WIRE (to me anyway) and knowing that these splices are simple crimps with black tape and how that lots of the wire has NO insulation under the black plastic conduit...it simply rots and cross connects. Fuel inj sending signals to each other in effect shorting each other is a common thing,.
You also could have inherited the "long crank" times that come with Bosch-III injectors. Nobody knows how or why, but its very common.
Be sure to do the Key ON 2 seconds, key off. Key on crank for 2 seconds, key off. KEY ON CRANK it lites off....
The alternative is grinding on the starter for 5 + seconds...and it'll fire up. but damn...you'll be ordering a new starter doing that too much.
Mine is a bear (cold bear) to start when cold. I was worried that it would be impossible with 24 degree temps yesterday...well, it was hard but exactly the same as when its 90 degrees ambient. I use the same process and it fires and starts everytime.

Jons very good,. and I'm sure he can send you down the right road.

On that lower panel,, that thing is jammed in the rear and held TIGHT sandwiched between ducts and console parts. You have to pull out then down once the screws are located. Gotta wrestle it a little bit.

 

Got that one out.
Still a lot of rug attached/glued to the main dash assembly.

 

I'm not understanding that....

The lower panel should be all there is between the floor and the underside of the dash, exposing the ECM , harness, ducts, everything.
Can you post a pic of whats glued there? did someone glue some sort of sound dampening junk or insulation up there?

To be clear, to access some portions of the dash pieces you remove the lower panel, then the dash PAD..the top pad that reaches across the top. It has screws along the edge and clips that snap into recievers near the windshield. get screws out, jerk it straight back. 2 Long screws in defrost vent also.
From there you can access everything you need to get behind breadbox, fuse panel, harness ect. The center display may need to come out (no biggie there) to be able to reach around to some things like the starter enable relay. Dash pad and lower panel allow most of the access. Breadbox is about last and actually does not give you much more access. Once the dash top pad is off, you can see.

 

I removed the dash pad (across the top) which is made from styrofoam and found
hard plastic ductwork that hid the ecm underneath.
I couldn't access the vats module until I had pulled the breadbox.
The two top screws were easy, but the other two .... jeeez oh pete!
The one closest to the door was easier to take out than the one on the side near the starter relay.

I ended up mangling a screw retainer to unscrew that one from the tip of the screw.
(do not attempt to say this three times fast). Then the breadbox came off.

Something seems different about the way this Vette was put together.
The lower panel with the rug and light assembly came out easily.
Still even with that removed, it's impossible to see the ecm unless you crawl underneath
the dash enough to look straight up at it. I can find the connectors by "feel" ... .
But, it's what is obscuring that direct view which is attached to the main dash assembly
and that part seems made of styrofoam and has carpet glued to it.

 

But sadly that is what you actually have to do to get the ecm out of there - remove the passenger seat and assume the inverted positition Take flashlight, a ratchet, 10mm socket on a 8" extension and go at it.

 

Thats what I've been TRYING to tell you !

access to the ECM is DESIGNED to be from below....taking the dash pad off is only to see other things...thats got nothing to do with the ECM. Four 1/4" screws and the box is off...it takes a 1/4" drive, u-joint, extension and ratchet. careful, you DO bang the glass.

Like I said, 10mm socket on a 4" extension & ratchet. Pull the 2 plugs off first, gently and THEN the 2 bolts.
This CAN be done by kneeling next to the car and looking UP into the dash. The worst part is the plugs. They have tight latches that take a little effort to release and pull without pulling wire out.

IF you elect to take the seat out.....be wary of the studs in the floor....they tend to hurt as they puncture the flesh on your back.
Ask me how I know.

 

They probably had a special tool for that ... and it probably cost a million dollars.
Guess they called it the assembly line.
I'll get back when I find out more.

 

After disconnecting the battery:
I pulled the passenger seat out and with a LED strapped on my head took a closer look
at the ecm.
Only had to remove two 10mm screws for it to drop down
from the plastic box that was holding it.
But I had to unplug one connector going to some other relay(?) whose wiring
prevented the ecm from coming on down.

The metal ecm box itself has a "DELCO" scan tape with an SVC # of 16198259.
In fine print across the bottom of the tape strip, it says:
"may be eligible for core return"

Upon removing the prom cover I found a chip assembly with locking tabs on
both ends. The chip assmebly has HYPERTECH high performance technology
emblazoned on the cream colored chip cover.

From what I can read on the chip through one of the "windows" ...
it has two part numbers in this order ...
P/N: 128881
hypertech (c)
P/N: 126533

Okay, I'm done for the day and gonna recoup from contorting myself to get myself
into the cab inverted. Then I'm gonna look up these part numbers.

 

I've researched this ecm and found out that this it was one of the
upgraded versions, but the HYPERTECH chip seems to be an anomaly since that appears
to be a non-OEM "street rod" aftermarket version. (could I be wrong?)
Also, I can't believe how big this thing is!
The metal box itself is larger than a hard drive on my computers.
In the box is a single board with all solid state components comprising mostly of
surface mount technology, with the exception of the "drivers" which are the large
devices mounted vertically against a metallic heat sink.
The board and all components are coated with a laquer to prevent weathering.

During the disassembly phase for injector installation, I discovered a EGR solenoid which
was ironically mounted near the front of the engine. This solenoid had apparently been
tampered with since one of it's wires were deliberately cut.
I spliced the wires back together and later on after reassembly,
prepared to start the engine.
As soon as I reconnected the battery terminal, I began hearing "chatter" from a relay
just above the EGR cannister. This chatter lasted about 4-5 seconds and then stopped.
Still, the engine fired up after a long crank (which I expected). Idled up to working
temperature, then shut down. Then restarted the engine which fired up immediately.
Jon had mentioned this particular solenoid to me recently which could lead to
the no injector pulse condition that I have been encountering recently.
On that note, I fear that I may have fried the injector driver in the ecm.
It didn't happen immediately, but it may have occured later on just after the first noid light
test when the engine fired up and ran.


Does anyone know a good place where I can send this ecm off to be tested?
I've been searching the internet and can't seem to find any facility that does.