Chevrolet Repair: 83 blazer, brake master cylinder, heater element

Question
hello i bought a 83 blazer that is in very good shape im having trouble with the  glow plugs this is a  military  truck there are printed circuits under the dash one seems to control the glow plug system im not getting any power to the plugs the light on the dash wait comes on  but not to the plugs are these circuit boards in place of the relays etc thanks any help will be great and are these replaceable or do have  to rewire it  

Answer
Hello J.
One of my favorite vehicles....

The circuit under the dash...kinda hanging there in the way usually, is the controller for the glow plug relay.
The glow plug relay is under the hood, on the firewall, just inboard of the brake master cylinder.
Two large terminals, and two small terminals.
One of the large terminals is connected to battery voltage, and should be hot all the time.
The other large terminal, usually two large orange wires and a small wire, is the glow plug side, and should have power when the wait light is on.

Here is where it gets tricky.
The glow plugs are 12 volt, but if you are checking power to that relay with a meter, it could very well show 24 volt while not operating. That is because some of them use a big resistor....looks kinda like a heater element gizmo, mounted on the firewall above and behind the engine.
When the glow plugs connect and load that resistor, it will drop the voltage to 12 volts.

OK....assuming that resistor is connected and good, or however your relay is connected to power, when the key is turned to run, that relay should click, and both large terminals should show the same voltage.(something like 11 to 13 Volts)
If not, then the contacts inside that relay are not working.

If that relay on your Blazer happens to be a little black plastic looking thing, I can almost promise it has overheated, and melted a little plastic rod inside it, and is not pressing the contact strip correctly, even if you can hear it click.
Go to a parts house, and get a relay.

This is what is needed. I just don't happen to have the part number here with me.
The relay will look like a Ford starter relay,(Fender mount SOLENOID), but it MUST have two small studs, and they MUST be isolated from ground. In other words, when measuring between the two small studs with an ohm meter, they will show continuity, but when measuring from either of the small ones to the mounting bracket of the relay, there will NOT be continuity.
It is like a winch relay for Warn winches, but much cheaper.

Also....it needs to be 12 volt, not 24.
Almost everything on that blazer is stock 12 volt GM. Only the starter and solenoid are 24 volt.
(That includes the alternators, by the way...they are 12 volt, not 24).

I found the relays at "O"Reillys, and they are only 6 or 8 dollars. Don't let them sell you a $45.00 relay.

Now, if your existing relay happens to be working, and you get the same voltage on the orange wire stud as the other stud, then your glow plugs may be burned up.
Before just replacing them, (about $11. each), make sure someone hasn't bypassed that firewall resistor and connected that relay stud directly to 24 Volts.
If you want to be sure, connect the power stud to the diamond shaped 12Volt junction block on the firewall right there beside the relay.

You can check the glow plugs with a 12 volt test light.
Connect the test light clip to 12 volts, like one of the battery posts where the two batteries connect to each other, and then touch the test probe to the electrical connector on each glow plug. If done one at a time, the light should light, but may not be as bright as if you touched the block.

Let me know what you find, and if I can help any more.
When I am at work, I have one of those relays, so can give you the number if you like. I am off this Monday, and kinda scheduled for Tuesday morning, but if you want to call me there, the number is 580-429-3007, or 429-3221.
Ask for Richard VanNatta.
OR
richard_vannatta@sbcglobal.net

Van