RoadrashOU812

I have a friend who is having major difficultiy with the VAT system on his 90 vette in which he is trying to sell . The car will not turn over unless you jump the solenoid on the starter nor will the fuel pump come on nor the injectors pulse . The Ignition key cylinder switch and key has been replaced and the ECM computer has been replaced as well but still the same problem . My friend purchased two different electrical diagnosis books so that we could observe the wirring and see of a way to bypass the VAT system . There is supposed to be a VAT relay behind the stereo that interupts the Central Control Module and the Voltage down to the starter through the Nuetral safety switch . Can anyone tell us exactly where this relay is located or a way to bypass this VAT system so he may sell his car ?
The Unbelievable thing is , is that the local Chevrolet dealer could not even get the car running or even to crank . They are the ones who told my friend to replace the key cylinder and key .

Any info would be highly appreciated .

1Patriot

Does he have the original ignition key for the car and can he measure the resistance of the implant in the key? Use a multimeter set to ohms and measure the rsistance of the button in the key, then go to Radio Shack and get a 1/2 watt resistor close to the value of the resistor in the key. At the base of the steering column inside the car are 2 wires, don't recall the color for a 90 model, but you can unplug these wires and insert a fixed resistor across the wires going under the dash, not the wires from the steering column. After attaching the resistor across these wires, leave the two wires from the SC disconnected. Now when the switch key is turned, the VATS will see the correct resistance in the circuit and should start and run. All this is predicated on all the other components in the starting/run circuit being servicable.

Had a temporary problem in the 88 coupe that I owned with the drivers door lock and this was a temporary fix untill I had the door lock replaced.

RoadrashOU812

Understood and appreciated but as I stated .....The key and key cylinder is new and replaced . We areTrying to find the relay that is supposed to be around the radio in the dash that controls the VAT system starting circuit .

skybolt31

On the 86 cars, the vats disables the starter and the fuel injectors and bypassing it is not as easy as it sounds. bypassing the starter enable realay would be fairly straight forward, but getting the injectors to fire would be a bit more challanging. The starter enable relay on an 86 is behind the information center above the radio. I would first measure the key resistance as has been suggested and get the resistors to make sure the vats module is getting the right signal.

newbecorvetteguy

Why not just buy the $60 or whatever it is VATS bypass kit from mid-america or something and forget it?

1Patriot

How about $2.00 versus $60.00; no brainer to me.

RoadrashOU812

Hyperteck company here in Memphis has already done this , this is why it has a new key and cylinder . Why would we need to measure the resistance if the new key(with resistor in it) and cylinder has been replaced with suposably the right one ?

RoadrashOU812

Hyperteck company here in Memphis has already done this , this is why it has a new key and cylinder .
Why would we need to measure the resistance if the new key(with resistor in it) and cylinder has been replaced with supposably the right one ?

Hooked on Vettes

On the 90 Vette, the Pass key system works like this.

When you insert the ignition key in the ignition cylinder, the value/resistance of the key pellet is measured/read by the CCM.

If the value matches what the CCM was programmed for, when you turn the key to run, the Security Light on the left side of the dash cluster should go out. If the light stays on, either the key pellet value is wrong, the wiring from the steering column is bad or the CCM is bad.

There is a CCM diagnostic that can be run to see what key pellet value the CCM is reading. You said they installed a new key cylinder. Did you use the same key pellet resistance for the new key as the original?

If the security light goes out, the CCM will ground the Starter Enable Relay. The Starter Enable Relay will energize when the key is in Start.

The Starter Enable Relay is located on the Left side of the Steering column. If you drop the carpeted panel above the driver side knee area and look under the steering column, you'll see 2 relays next to each other. The one closest to the driverside door is the A/C clutch relay and the other one is the Starter Enable Relay.

At the same time, the CCM will also ground pin 6 Dark Blue wire Red Connector of the ECM which will allow the injectors to pulse.

So you need to see what the Security Light does before you can further isolate the problem.

RoadrashOU812

Thanks Hooked and the others ,
I will try all this first thing in the morning . You have explained things a little clearer to me and I appreciate it !

I am walking into this blindsided of the original problem of the car . My friend bought it from Chuck Hutton's used car lot without it being able to crank because they had lost the keys to it . Hyperteck supposedly checked the CCM to see what coded key it took , so my friend got one and had it cut . The Security light is not going off so I am thinking that he has the wrong key code . No telling at this point .

Thanks again !

RoadrashOU812

How could I personally check to see what resistance (key) I would need if the original key is not available ?
Could I take my ohm meter and check the wires leading to the CCM from where the key cylinder plugs in to it ?

Hooked on Vettes

It sounds like the key pellet resistance value is the wrong one.

You can use your ohm meter and measure the key pellet resistance of your friends key and see which key value it is in the chart below. As far as I know, there is no way of telling what value of resistance the CCM is programmed for except by trial and error.

There are 15 possible values.
Code key 1 is 392 ohms
Code key 2 is 523 ohms
Code key 3 is 681 ohms
Code key 4 is 887 ohms
Code key 5 is 1.13K ohms
Code key 6 is 1.47K ohms
Code key 7 is 1.87K ohms
Code key 8 is 2.37k ohms
Code key 9 is 3.01 K ohms
Code key 10 is 3.74K ohms
Code key 11 is 4.75K ohms
Code key 12 is 6.04K ohms
Code key 13 is 7.50K ohms
Code key 14 is 9.53K ohms
Code key 15 is 11.80k ohms

Here's a URL which describes how a basic Pass Key system works.

Note: On the 90 Vette, if you try to start the car and the wrong resistor value is detected, there is a 2 to 4 minute delay where you will not be able to start the car.

You have to wait for the time out. Even disconnecting
the battery will not disable the time out.

http://home.t-online.de/home/O_Scholz/vats.html

The resistance values don't have to be exact but probably close to +- 10%.

You could purchase a linear variable potentiometer from Radio Shack and set it for the different values.
Disconnect the two pin harness at the base of the steering column and clip the resistor across the harness going to the CCM.

You'll know when you have the proper value because the Security light will go out when you turn the ignition to run.

RoadrashOU812

Thanks again Hooked !

RoadrashOU812

Tried every single code key made and the security light still does not go out . Now what ?

RoadrashOU812

Yes .... it ended up being the CCM was bad .
Thanks for all your help !