What is the location of the headlight actuator and isolation relays on 1988 corvette

Brett has the answer to this. Bad headlight relay.

Brett Duxbury Mar 16, 2022
Hi Anonymous, check the headlight relay under the bonnet in the relay compartment . Get a voltmeter or 12v probe tester and see if coil on relay gets 12v from the head light switch
If no voltage at relay coil then check switch and the fuse.

https://www.google.com/search?q=1988+ford+festiva+headlight+relay+location

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There are two motors, and three relays in the system. Here's how it works:

The headlight door motors reverse direction by reversing polarity. However, the automatic shut off (high torque switch, or HTS) makes that a rather more complex task than at first it might seem.

That's because when a motor reaches its limit, one of the 'wires' is disconnected from the motor by the HTS. Reversing the polarity at this point wouldn't do anything since the circuit is broken by the HTS.

Consequently, each motor has three wires for control. The combination of the blue wire, gray wire, and green wire provide polarity reversal to the motor (to make it able to run in both directions) and to run each motor from its resting state, after it has shut off.

There are of course two motors. They operate and are controlled identically.

Here's the matrix:

RIGHT HAND MOTOR

Blu Grn Gry
up: (+) (-)
dn: (+) (-)

That is, in order to run the motor up, the system connects the green wire to the hot side and the gray wire to ground. In order to run the motor down, the system connects the blue wire to the hot side, and the green wire to ground.

The left hand motor operates the same, except for its blue wire is actually blue with white stripes.

There is an actuator relay for each light mounted on the body of the light housing. There is also an isolator relay, mounted on the fender wall on the driver's side. When operating properly, the two actuator relays act identically.

The actuator relays get their instructions from the isolation relay. When the motors need to run down, the isolation relay is not energized, the blue (and blue/white) wires make with the hot side through the closed contacts of the isolation relay (which is not energized), and the ground is furnished to the motor through the closed contacts of the actuator relay until the circuit is broken by the HTS. When operating properly, neither the motors nor the relays draw any current in the full down position.

When it's time to go up, the actuator relays are energized when the pink wires coming from the isolation relay are connected to the hot side through the newly closed contacts of the isolation relay, the isolation relay is energized because the yellow wire coming from the head light switch is now hot.

When the actuator relays are energized, they furnish 12v from the always hot red wires, through their newly closed contacts, to the green wire. The motor is grounded through the newly closed contacts of the actuator relays. The motors will run up until the circuit is broken by the HTS. When operating properly, with the light doors in the full up position, all relays are energized and draw current, but the motors do not.

There are very counterintuitive things about this circuitry: When the motors are running UP, both the hot side and the ground are furnished by and through the actuator relay. However, when the motors are running DOWN, only the ground is furnished by the actuator relay because the blue and blue/white wires do not run to or through the actuator relay.

Similarly counterintuitive, when the motors are running down, the hot side is supplied by and through the isolation relay, but when they are running up, the hot side is supplied not by and through the isolation relay, but through the always hot fusible link.

That's how the headlamp door relays and motors work. The relays are not cheap but not terribly expensive, either: $17 ea for the actuator relays and $25 for the isolation relay. Proper trouble shooting can save a buck or two.

It is fairly easy to swap the actuator relays from side to side. If the symptoms change with them, then its probably the relays.

Although the actuator relays look similar to the isolation relay, they are very different, they cannot be swapped. Please note that the system is very interdependent, and it's easy to see why Chevrolet changed to electronic (as opposed to electro-mechanical) controls.
Hope this helps.

blown fuse or relay

The fuse block is located under the right side of the dash. Check the fuse called "INST LPS" fuse # 15, it's the one located in the 5th row across and 3rd one down. If the fuse is good, with the ignition in the on position check for 12 volts at the dimmer switch assembly this is a dark green wire. If you have 12 volts replace the dimmer switch assembly, if not repair the dark green wire. Hope this is helpful to you.

There is a silicone gel in the headlight system that acts as a drive between 2 gears. The fix is very inexpensive but the labor is pretty intense. almost all corvette supply places sell pellets that replace the gel which has more then likely dried out over the years. The only reason I can think of is the gel is used in case the headlight is frozen up or down the gears won't strip. On the headlight assembly you'll see a round area about 2 inches across held on by 3 screws. When you remove the cover you'll seee a gear inside. When you pull the gear out you'll see the area where the gel used to be. There will probably be dried up powder inside. You'll see 3 indentions in the back side of the gear where you place the 3 pellets. Keep one headlight assembled so you can see how to put it back together. I actually bought the pellets from a guy on ebay for less the $10.00. I had to sand the pellets down a little to get them to fit into the gear but it actually worked great

Those are vacuum actuated headlights and the vacuum leaks down when the engine is off. Probably have a leak in the headlight actuators, will need to replace or rebuild one or both of them, not a big problem for them to pop up some people just do not like for them to pop up while parked.

check wire going to motor for proper voltage or open circuit

Maybe a relay but maybe and more probably a broken wire Get a diagram Chilton or Haynes manual

Most of the time you will find that it's rotting rubber hoses. If these have never been replaced, they're 28 years old! Engine heat, exposure to the elements that splash up under the car, and age can rot them.

Also the actuator has a rod that comes out. This rod has a seal around it. The seal rots too. The actuator is the large vacuum cannister, that looks like two pie pans put together. It is down near the headlight. When you have the hood open, look down towards one of the headlights.
Sometimes the vacuum tank develops a hole. This can be patched. It's up on the side of the fender.

Here, let me show you the headlight mechanism,
http://www.mamotorworks.com/corvette-c3-headlight-actuator-linkage-parts-1-249-6088.html
I use this website for my parts, but this is not a link to them for advertisement. It just shows an exploded drawing, so I can explain better.

B is the Headlight Actuator. See the long rod coming out of the middle? Has threads on the end of it? Where the rod goes up into the cannister, there is a rubber seal. This dries out and cracks, and also wears through time. Corvette parts places (At least online I know they do) sell this rubber seal.

There is also a small check valve that can go bad.
You can buy the vacuum hoses in a kit. They are color coded, and have replacement instructions.

To review:
1.Check all vacuum hoses for drying and cracking. Actually squeeze them, don't just look at them. If they have a crack from your squeezing, they're shot! (You might find that some just crumble in your fingers! Oh, by the way. Dawn dishwashing soap, or GoJo takes that black rubber residue off of your fingers)

2.Start the car, let it build up vacuum. Shut the car off, and use the override switch, (Knob under the headlight switch you pull), to keep the headlights in an up position. Put your finger on that actuator rubber seal I described above, and see if you feel a vacuum leak.
If so, replace the seal.

3.It's a good idea to replace the check valve,
http://www.volvette.com/HE76.html

4.It's also a good idea to replace the vacuuum filter,
(Vacuum reserve check valve air filter) http://www.volvette.com/HE75.html

I have a 1970 Corvette, and a 1976 Corvette. I restore and customize Corvettes.

So you're getting ready to do the Fun(?) stuff, huh? Yes sir, nothing will make your day more than replacing your vacuum hoses for the headlamps.

I'm going to detail the hoses out, and the parts they connect to. Then I'm going to give you a link, that I feel will show the headlight vacuum diagram in great detail, plus a few tricks from me.

Let's start with the hose that comes from the intake manifold. The main supply of vacuum. It comes from the rear of the intake, and goes over to a Vacuum Filter. The vacuum filter is an item to replace, as it's a possible source for a vacuum leak, and prevents contaminating particles from entering your engine.
From the vacuum filter there is a hose to a Check Valve. (Another item to replace) The check valve then splits into two fittings. Two hoses come off of it. One hose leads into the firewall, and comes up under the dash. It connects to the light switch. The light switch has two fittings on it. One towards the pull knob, one towards the end of the light switch. This hose connects to the fitting at the end of the light switch.

The fitting on the light switch towards the pull knob, has a hose that comes off, and goes to the Pull Down Switch. (Manual Override Pull Down Switch) The little plastic knob you pull to keep the headlights up, or push in, so they the headlights will close when the headlights are turned off.

The other hose that comes off of the Pull Down switch, goes along the left fender well, and ends at the front of the car. It fits into a plastic 'Tee'. The Tee has two hoses coming off of it. One hose goes to the Left headlight Relay Valve. The other hose goes to the Right headlight Relay Valve.

From here the hoses are color coded. You may not be able to see this color code anymore. The color code of these hoses, is Red, Yellow, and Green.

On the Relay Valve it has a vacuum diaphragm chamber. This chamber looks like two pie pans put together. There are three fittings on the Relay Valve body. There is a fitting on the chamber itself. The hose coming from the Tee I described above, goes to the chamber fitting. The other hose coming from the Tee, goes to the other Relay Valve chamber. Doesn't matter which hose from the Tee, goes to what Relay Valve chamber. But when you look at it, you'll know how to lay those hoses out. You don't want then twisted over each other.

The fitting on the Relay Valve that is on the body of the valve, and nearest the chamber, is Red in color. It has a hose that goes to the back of the Actuator. The 90 degree bent metal tube. The actuator looks like two pie pans put together, too. The Actuator has a rod that comes out of the middle of it, and is what actually moves the headlight.

At the end of the Relay Valve body is a fitting. The one furthest away from the chamber. It has a Green hose coming from it. This hose goes to the Actuator on the other side. It connects to a straight fitting, in the middle of the actuator. This is the same for the other side.

The fitting in the middle of the Relay Valve body, has a Yellow hose coming from it. It goes to that long cylindrical tank in the front of the car. At each end of this tank are the bumper brackets.
This hose connects to a fitting at the end of the cylindrical tank. The cylindrical tank is the Vacuum Reservoir. Same thing for the other side.

The last hose is where we left off at the Check Valve. One hose went to the headlight switch. The other hose runs along the fender well, and connects to the Vacuum Reservoir. The long cylindrical tank up front.

Tricks:
1.With vacuum built up, press the rubber seal that's on the shaft (Rod) coming out of the Actuator. If you hear a vacuum leak, the seal is bad. This seal can be replaced without having to replace the Actuator. It isn't easy, but it isn't that tough.

2.With the check valve removed, blow air into the single inlet. If you can blow air through it, this check valve is shot. It's about $14 and should be replaced.

3.If the Vacuum Filter is soiled looking by holding it up into a bright light, and looking at it, it should be replaced.

4.The best method if your budget will allow, is to buy those vacuum hoses as a kit. They are color coded, and come with instructions.