c4or is right about the necessity of having a compatible sender/indicator pair. I was going to ask you about that possibility - of a mismatch - but didn't want to confuse the issue. Your truck is way newer than when the last change in sender/indicator specs were made - in 1979. So you are probably okay on that issue. I don't even think you can jam a 1978 cluster into a 1987 dash - or mount the indicators in the bucket
For Truck Years: 1967 - 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω
For Truck Years: 1974 - 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω
For Truck Years: 1979 - 1990
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω
This is good info I wish you had shared it earlier.
1. I have a 1987 cluster in my lap right now. I traced the etching for the coolant temp indicator all around the board and back to the harness socket - that pin grounds
a lot of stuff on the PC. If there were a problem with continuity to ground on that common leg, in addition to the coolant gauge problem, you'd have problems with about half the board's components.
2. Because you changed the cluster and the gauge behaved the same, it is most likely not a grounding problem internal to the cluster. So forget my suggestion about it being a bad connection at the stud/clip/PC interface. We kind of have to assume that the cluster is good.
3. Also, if the rest of the components that share that ground leg are okay, that would pretty much eliminate a continuity issue between the control coil/cluster and the grounding bus block. BTW - the final grounding point for everything inside the cab is at the ground bus block. That block is located up above the E-brake on the outside wall. It is grounded to the sheet metal and the sheet metal is grounded to the frame..... Ultimately all grounds end up back at the battery neg post.
4. Keep in mind that you are having a problem with the indicator reading high - not low. As described above (and as you now know) there are basically only two conditions that will cause that; a
short to ground (or a ground fault) in the sensing leg of the gauge - between the gauge and sender - or a
poor connection to ground in the control coil's leg.
To summarize:
You seem to have confirmed a good cluster PC and coolant gauge (by replacing with a known good).
And because everything except the coolant gauge is working (and it shares the same etched ground circuit with lots of other stuff), the cluster is properly grounded externally.
Battery voltage supply to the various components in the cluster is similar to the ground loop - it services a number of things. So, if that were a problem - again, like a bad ground it would cause you a lot more trouble than only the coolant gauge.
So it follows that, if the gauge has battery voltage to the coils, the gauge is good, and the control coil is properly grounded...the high indication at the gauge is due to a wiring issue somewhere in the lead between the sensing coil and the sender.
Have you ever traced that lead (DK GRN 35) and inspected it for insulation failure. After it leaves the loom behind the distributor, it travels through some tough neighborhoods - for about 3 feet. It is designed to be secured to the top of the LH valve cover with these nice clips:
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but, by the time you or I get the truck, they are usually long gone. The DK GRN 35 usually gets shoved onto the intake manifold and sits in hot grease/oil for years. Even worse, a lot of times it is allowed to hang down near the exhaust manifold.
I would take close look at that lead all the way from the point where it comes out of the loom to the sender button. Wipe it down down clean with a paper towel soaked in gasoline, then twist/bend it always looking for signs of insulation breakdown. It doesn't like you will find any actual conductors poking through (you might). More likely the insulation will be chafed/pinched/squished/cracked allowing a ground fault to develop at operating temperature.
Another option would be to cut the DK GRN back by the firewall and splice in a temporary lead. If it doesn't help, just splice the OEM back together. This is when meggers come in handy - but I'm sure you don't have right?
