LS1LT1 Forum banner

issue with 94, VATS??, runs for 2 seconds

6592 Views 17 Replies 5 Participants Last post by  nophix
Hi all. I am new to the LT1. I just picked it up. But I have friends and my cousin that is familiar with them.

I am having an issue that I believe is related to the VATS system. First the story:

I bought this with out it running. The guy replaced the starter and still couldn't get it to run. I figured I and my friends can get it running. So checked the starter, it works fine. Read on here about grounding the Yellow/Black wire from TDR to ground. Now I can get the starter to engage.

Next issue, now she will run for 2 seconds and shut off. I figure it's now the VATS shutting down the injectors. So I read the resistance on the key, and it is key 2 (523ohm). I go pick up some resistors to wire in to the ignition wires. I do this the first time. No luck. I check the resistors and fitting, no go. I up the resistors to 550 ohms, still will crank and run for 2 seconds and shut off. Next we put the key in the cylinder and then read the resistance of the 2 cut white wires, it says 518.

So if thats true, that I can read 518 from the cylinder with the key in, then it makes me think that it is not a cylinder or key issue. So what can it be? Can the actual VATS system go bad? If so how do I trouble shoot it and or replace it?

Here are 2 video's. 1) First is just normal crank and run for 2 seconds. 2) I have seen occasionally the electrical system flicker. It's not every time, and not every 5 times. Some times it clears it's self, other times I have to pull the negative from the battery.

1) 94 chev z28 lt1 -1 - YouTube

2) 94 chevy camaro lt1 z28 - YouTube

I'd really like to get this Old girl running and fixed up. Any and all advice would be great.
1 - 18 of 18 Posts
Ok, so here is some more info. I am the third owner of this car. PO, Had new trans with shift kit and stall (installed), and new Rear end gears installed. About a week later he had an issue driving home, that the lights and gauges all started to flicker and go out. He thought it was the alternator. He took it out and to a shop, alternator tested fine. So he put that back in. He then took the starter (now he couldn't get it to crank) in and got a new one. Put it on, still no start... He did this 5 times thinking the starters he kept getting were bad (I know it was VATS)

So it sat for 2 years. I come across it, friend of a friend. Thought well the price seems good and I can tinker enough to get myself into trouble. So I purchased it, trailered it, and now she is here. I can hear the fuel pump when I turn the key on.

I had to replace the AC pump as the pulley was locked up. And other than an oil pan gasket leak, she looks to be in good shape. And has all sorts of mods done to it from the first owner. Who ironically is a 5 generation cousin (didn't know at the time until I pull the glove box and found original papers).

Here is the list of things that I know have been done:
K&N cai
Granatelli plenum gms
Granatelli Maf
Big mout throttle body
Long tube header
Edelbrock tower tie
8 over factory injector
Edelbrock front sway bar
Track and swaybar edlebrock
Front to rear chassis edlebrock
Edlebrock springs
Rebuild tranny with shift kit and stall
Rebuilt rear end. New center moser. Rear gear richmond gears 3.73 moser axles
Drilled rotors
Pace setter header
upgraded fuel pump (unsure of how much or from whom)

See less See more
There is one thing in common with the Theft Deterrent Module, the instrument cluster, and the PCM. It's a fusible link at the power terminal on the passenger side wheelhouse next to the battery. That link connects power to all of those modules. I would check that by moving them around one at a time while someone watches the instrument panel for flickering. I can't really tell you how to identify which of the several there are in that location is the one of interest. Just use some trial and error. Or disconnect them one at a time until you lose the instruments.
See less See more

Yes I've seen that. We thought that or a loose ground. What we checked was that all of those wires on the fuseable link were tight and that I had 12v going from the one side to the other. I will check today to see how it react's to removing one at a time.
I only suggested removing them in order to see which one was the supply to the instrument panel (and VATs and PCM). Once you know which one that is, reattach it and try moving it around to see if it is intermittent.
By the way, the "fusible link" isn't the mounting post. They are the "fuses" internal to the wires mounted there. Each wire marked as a fusible link has a short length of smaller gauge wire inside, that serves as a fuse. Sometimes they get corroded inside the insulation or get burnt out, but continue to make a loose contact.

We have removed the distribution block, and attached the 4 eyelets and positive directly together. We also can remove and find the 2 eyelets that provide VATS, Instrument and ignition. We also have a probe that we put in the wire, below the fuseable link and we have continuity from the eyelet past the fuseable link. All same symptoms. Will run for 2 seconds, and die. If I remove the TDR starter ground, no starter.

OK then, so much for the fusible links. I think you are correct about the VATS stopping the injectors after 2 seconds. I have recently duplicated that symptom on my bench-top 94 PCM mock-up. The VATS 50HZ signal must be missing before the engine is started however; or at least, the PCM doesn't see it. If you lose the 50Hz signal from the VATS module while the engine is already running, it will not stop the injectors. Perhaps the instrument panel flashing is just a coincidence. I would unplug and replug all the connectors on the PCM and VATS module just to see if that signal path is intermittent. I know that that circuit is fairly heavily loaded by the PCM and a poor connection would kill it. Obviously the VATS module is allowing the starter relay (on a separate circuit) to function, unless that is bypassed somehow. I assume the Security lamp is not lit or flashing, or burnt out... correct?

Another possibility is that the ignition switch is intermittent and makes good contact in the Start position but not in the Run position. That might explain the flashing instrument panel.
See less See more
So we did that. Disconnected PCM and BCM. I even sprayed them down with Deoxit to ensure that the connections were good and clean and lubed.

We also verified via Noid light. On Key run, it was Bright. As the car was running it went dim and then went out when the car died.

We also have found fuel soaked plugs. We are in the process of replacing all plugs.

Next thoughts?
I would monitor the voltage at fuse #5 (PCM IGN) in the inside fuse panel, with the fuse installed. See if it drops when the I/P cluster dies. That may indicate a bad ignition switch. Do the same for #4 (PCM BAT). If those look ok, I would check the voltage at all the others and see if any die when the I/P cluster dims. That could indicate a short.
Well we did that check the fuses. They held steady 12v until it stopped running.

I just want to be clear we are looking at the same issue. We have not experienced the flickering of the gauges from vid2. So we are right now at the VATS shutting down the starter/injector.

Security light does not come on, until the car runs, and you see that flicker a little bit when the car stops running.

Thanks again for the steps to trouble shoot this thing. We won't let her win.
Sounds like your having fun. Have you checked the ignition switch down on the column. The contacts overheat and become loose over time.
I don't understand about the instrument panel. You say you aren't having that problem but there it is in your post at 3:36PM: "As the car was running it went dim and then went out when the car died."
Sorry about the confusion. On the Noid light, it went dim, as it lost voltage (nearly simultaneously). When we did the fuse 5 & 4 check, voltage remained until it was cutout (via BCM?)

Here is what I feel is happening and where I am currently at.

I currently have a key2 resistor bypass in place, that is at 530Ohms. 1) oddity here. I actually have 550 ohms (1 510, 4 - 10 resistors in series) when I test them on the bench I have 549 ohms. When I hook it up to the VATS down line, it reads 534 ohms (both are within spec 502-564) Regardless I have that setup and yet still BCM/VATS is shutting it down. I also have the TDR/starter shunted to ground. If I disconnect this, the starter is not enabled and will not crank.

I also have the + Distribution Lugs, bypassed. All the wires at this point are directly connected to the main battery +12v. Since I have done that yesterday, it did not do the I/P and voltage jitter. I think we have that issue solved right now.

So something via the BCM/VATS is stopping signal. I am wondering, if I am not "throwing a code" from the PCM, and it is telling the BCM to shut it down, that is not security related. Just a thought.

I will be ordering a ALDL cable, and hopefully then I can see if I have any codes. But that wont be here until probably midweek.

I appreciate everyones tips so far. I will get this old girl to run. I know she will be worth all the aggravation in the end.
See less See more
First, there is no BCM. You have a Theft Deterent module (VATS) instead.

Second, the VATS module does NOT shut down any voltages. If you are losing voltage at fuse 4 and/or fuse 5, that is the problem. Fuse 4 comes directly from the battery. It is the memory backup power to the PCM. You cannot turn it off without disconnecting the battery. The voltage from fuse 5 comes from the ignition switch and from there to the battery. The VATS module does only these two controls: 1) enable the Starter relay and 2) send a signal to the PCM that it is ok to run the injectors.

This statement is very telling: "voltage remained until it was cutout (via BCM?)". If you are ever seeing the voltage at fuse #4 die, that is what you should investigate.
Sorry, I just re-checked. Your right, I have 12v at the fuses. It doesn't cut out.
Since you sound like you know you way around electronics, do you happen to have a signal generator you could use. One way to know for sure if vats is causing the problem is to send the 50 hertz fuel enable signal directly to the pcm and see if car runs for longer than 2 seconds.

Fuel enable signal comes into pcm on connector A cavity 25 (dark blue wire).

If you don't have access to a signal generator Baker Electronix sells a 50 hertz bypass module for around $40. Get at Baker Electronix - GM VATS Bypass

or if your good with building circuits, there's diagrams floating around the web, to build your own 50 hertz signal generator. About $10 in parts.
I don't have one, but I do have access to one. I may try that. I've also ordered ALDL cable to be able to plug in and see what I am up against. Whether it's a DLC or other issue.

I know 2nd PO did have the PCM re-programed/tuned so that all the engine mods would work properly. I don't know what they did though.

So the ALDL cable, with some software, I should be able to check things out, and thought maybe I could disable the VATS that way. Will that work, or do I still need the VATS bypass module that you mention?

On other note. There are 2 "relay's" I think by the gas pedal on the firewall. What do these 2 relay's do? One I hear dinging with the Key in ignition. But it sounds tired.
Vats is a separate module, independent of the ecm.

Its essentially a relay with a lock.

Sent from App
1 - 18 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.