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Bob
Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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84 Chevy Blazer: V6, 4X4, water temp gauge..cluster

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On an '84 Chevy Blazer C10, V6, 4X4, water temp gauge reads 50 degrees high; have replaced gauge, sending unit, thermostat and confirmed engine temp with infrared sensor. Believe dash circuit board may have faulty ground as gauges flicker if instr. cluster is jiggled. Cannot find circuit board diagram to id ground circuit. And what is best way to get at speedo cable and other wires that must be disconnected to remove instru cluster?

If you will look at the wiring diagram below, you will note that the black wire is the common ground for the whole cluster and PC board. Many times these boards will develop cracks in the plastic covered part of the circuit where they fold over and the connector plug pushes in. Some of them also use a small resistor on the lugs of the individual gages, between the posts where the nut secures them. As to removal, it is easier if you unhook the speedo cable from the trans end and let it slide out as you pull the cluster out, until it is far enough out of the dash that you can reach and unhook the clip that retains it to the cluster. The other one to watch is the sensor the has a small black plastic connector with a 5mm screw through it, holding it to the cluster from the back side. The rest you can reach pretty easily with the cluster out far enough to get your hand behind. Thanks

Customer: replied 8 years ago.
Reply to Bob's Post:

Bob - thx, this gets me part of where I want to be. Can you tell me where the ground you are referring to appears on the diagram (?just to the upper left of the instrument panel printed circuit board?) and where it is physically on the car? I was also hoping to understand which of the three contacts the temp gauge plugs into are the lead from the sending unit, the ground, and the positive (I'm assuming those are the three).

thx - Dave

The botXXXXX XXXXX wire, last one in the row goes to the temp sender, you can trace the wires, from the gage, etc. The black is the common ground. If you put a test light or volt meter on the back terminals of the gage, you should get a 12 volt reading on the power side, the temp sender side will be either real bright or real dim, you can unhook the sending unit and watch for the change at the terminals on the gage. Open circuit at the sender should give a higher voltage reading at the gage, etc. If you have the cluster out, you can trace each segment of the printed circuit board, just by following it and then comparing it to the plug in connector. Sorry, I don't remember which way the resistor connects or if that unit even used one, if so it will be a small white resistor that connects across 2 of the terminals on the gage. Sorry, I also do not remember the resistance. I do remember that on the early S10's there was one major ground, black wire, that connects to the lower left side of the dash pad at one of the metal brackets close to the park brake release lever on the bottom of the dash pad. But, again, don't remember what year they moved it or where too. Memory is fading fast, sorry. Can only hope some of this helps you, been too long since I had one apart, but would remember quick if I had it here to look at. If I come up with anything else, I will be happy to send it to you. It may even be that they only used 2 of the terminals and used the sending unit for the only ground on it. The change in ground resistance being what changes how the gage reads. Thanks and good luck

Found the other half of the dash diagram, maybe this will help some more.

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Customer: replied 8 years ago.
Reply to Bob's Post: Bob - you've been tremendously helpful. I've accepted your answers, thanks so much. Dave
Thanks a lot Dave, really happy when we can help. If you need anything else, just drop a note. Wish you well with the project.