Sick Tach

[BigBrother-84]

Hi Gentlemen!

As some of you know, I am installing a Speed Control kit in Big Brother.

I told myself, «well, while I'm at it, why not change the cluster for the Tripometer/Tachometer one that is collecting dust on a shelf?»

So, I put the Speed Control installation on pause and opened a Cluster parenthesis.

Made a test, the speedo/tripo works very well, although it indicates 3%-4% faster than reality, for speeds over 40-45mph. Never mind, I consider this as a Cop protection...

For slower speeds, it appears very accurate.

The problem is about the tachometer (6000rpm model): Not functional at all, dead.

Looking closer, I saw that one electronic part looks burned:

But I also noticed that two wires are cut, behind the connector (black wire and blue/red wire):

• Are these wires related to the Tach operation?

• Can I fix the Tach itself?

Thanks for advices!

[Gary Lewis]

Your tach is certainly dead. That resistor is toast. Literally.

If we can figure out which resistor it is, or the value thereof, you might be able to replace it. I can look at my tach on the shelf and see if I can read the value on that resistor. And I included the schematic below. And in the picture the same resistor looks to have a color code of Brown, Black, Brown which is 100 ohms.

But, that may not be the only problem as something else is probably causing too much current through the resistor, therefore burning it out.

And there's the cut wires. From your very good picture it looks like it is the BK/LG wire on Pin 1 and the LB/R wire on Pin 5. The BK/LG is either Ckt 30 or 48, and it can't be 30 as that is power to the gauges and your gauges work. So it must be Ckt 48 and I haven't yet figured out what that is, but it doesn't go to the tach. Maybe we won't worry about that.

But the LB/R wire is Ckt 19 and that is the power to your instrument illumination. Which means your gauges aren't lit. Right?

 

[BigBrother-84]

But, that may not be the only problem as something else is probably causing too much current through the resistor, therefore burning it out.

I don't remember when, but I bought this whole Tripo/Tach cluster at my local salvage yards. Probably years ago. The donor truck might had an electrical tach issue.

I may be wrong, but I guess Big Brother electrical health is ok.

So, if there's a chance to fix the tach, I would try to, hoping that the Tach itself didn't cause its resistor to burn. The rest of its circuit looks good, no overheating signs.

But the LB/R wire is Ckt 19 and that is the power to your instrument illumination. Which means your gauges aren't lit. Right?

Nope, the cluster is illuminated, and its bulbs dim correctly from the light switch. They might take their power from another wire...

[BigBrother-84]

But, that may not be the only problem as something else is probably causing too much current through the resistor, therefore burning it out.

I don't remember when, but I bought this whole Tripo/Tach cluster at my local salvage yards. Probably years ago. The donor truck might had an electrical tach issue.

I may be wrong, but I guess Big Brother electrical health is ok.

So, if there's a chance to fix the tach, I would try to, hoping that the Tach itself didn't cause its resistor to burn. The rest of its circuit looks good, no overheating signs.

But the LB/R wire is Ckt 19 and that is the power to your instrument illumination. Which means your gauges aren't lit. Right?

Nope, the cluster is illuminated, and its bulbs dim correctly from the light switch. They might take their power from another wire...

EDIT:

About the blue/red wire...

I am wondering if this could explain the fact that my cluster is never very bright, even when dim at 100%.

On dark nights, I prefer to set my cluster lighting very low, wanna see the road without being blinded by a light show.

But with Big Brother, I have to keep cluster lights at maximum all the time, incandescent bulbs are too "soft".

I sent you some questions about your May 2020 HIPO LED experience.

I am planning to LEDify my cluster in order to correct this too low lighting. But maybe my cluster isn't wired correctly, I should start by verifying that?

[Gary Lewis]

But, that may not be the only problem as something else is probably causing too much current through the resistor, therefore burning it out.

I don't remember when, but I bought this whole Tripo/Tach cluster at my local salvage yards. Probably years ago. The donor truck might had an electrical tach issue.

I may be wrong, but I guess Big Brother electrical health is ok.

So, if there's a chance to fix the tach, I would try to, hoping that the Tach itself didn't cause its resistor to burn. The rest of its circuit looks good, no overheating signs.

But the LB/R wire is Ckt 19 and that is the power to your instrument illumination. Which means your gauges aren't lit. Right?

Nope, the cluster is illuminated, and its bulbs dim correctly from the light switch. They might take their power from another wire...

If your gauge lights work then lets not worry about the wires that are cut.

And I'll see what I have in tachs that I can look at to determine the value of that resistor when I get to the shop after while.

[Gary Lewis]

EDIT:

About the blue/red wire...

I am wondering if this could explain the fact that my cluster is never very bright, even when dim at 100%.

On dark nights, I prefer to set my cluster lighting very low, wanna see the road without being blinded by a light show.

But with Big Brother, I have to keep cluster lights at maximum all the time, incandescent bulbs are too "soft".

I sent you some questions about your May 2020 HIPO LED experience.

I am planning to LEDify my cluster in order to correct this too low lighting. But maybe my cluster isn't wired correctly, I should start by verifying that?

I answered those questions in the other thread. But I'm at a loss as to how your lights even work with that wire cut. Here are what the wires are for.

[BigBrother-84]

I answered those questions in the other thread. But I'm at a loss as to how your lights even work with that wire cut. Here are what the wires are for.

Gary, I know you really don't like Haynes Manual.

For Big Brother, the electrical diagram is accurate. Well, almost, since I have to use the 1980 one (the 1984 doesn't match most of the time).

But using the 1980 diagram, it matches Big Brother's wiring.

So, I took a look in this diagram. The #5 cut blue/red wire corresponds to the Seat Belt Alarm. And yes, Big Brother doesn't have a working one. That matches.

Following this diagram, Cluster illuminating takes its power form #18 (green/white). Will test it tonight, but pretty sure it will be accurate.

[Gary Lewis]

Gary, I know you really don't like Haynes Manual.

For Big Brother, the electrical diagram is accurate. Well, almost, since I have to use the 1980 one (the 1984 doesn't match most of the time).

But using the 1980 diagram, it matches Big Brother's wiring.

So, I took a look in this diagram. The #5 cut blue/red wire corresponds to the Seat Belt Alarm. And yes, Big Brother doesn't have a working one. That matches.

Following this diagram, Cluster illuminating takes its power form #18 (green/white). Will test it tonight, but pretty sure it will be accurate.

Wait! You have an '84 truck. So why use the 1980 circuit diagram? Has someone put a 1980 instrument cluster in it?

Yes, the 1980 instrument cluster wiring is different. And the 1980 EVTM doesn't bother to give me the pinout on that connector so I can't easily check what the Haynes manual says.

But you shouldn't have to use the 1980 diagram unless someone has put a 1980 cluster in your truck. And then it would have just taken de-pinning and re-pinning the connector, not cutting wires.

[Gary Lewis]

Jeff - I've checked all four of my un-installed tachs and they are of the "closed" style, meaning they have a case over the electronics. But yours is an "open" style w/o case, and I'm convinced that those two styles have different circuitry as the components are not in the same places by any stretch.

In the pic on the left below, courtesy of Jonathan when we were back on FTE, you can see the power resistor that is clearly brown/black/brown = 100%. And in your pic on the right you'll see that yours is probably the same as it starts out brown and has black for the second band. So I'm pretty sure it is 100 ohm as well.

But in the schematic at the bottom that resistor, which is the one in the upper left, is 180 ohms. So that tells me the schematic is not for your tach since there is at least one change, and there are probably others.

So if you want to try to repair your tach you'll need a 100 ohm resistor that is probably rated at 1 watt. But you should also look the other components over closely. For instance the diode I've outlined in yellow looks questionable. Something caused that resistor to overheat.

[BigBrother-84]

Wait! You have an '84 truck. So why use the 1980 circuit diagram? Has someone put a 1980 instrument cluster in it?

Ok, I’m a whole, complete, entire, M-O-R-O-N.

Went to theatre with my wife (movie was good) but couldn’t go to bed without testing the cluster connector. Before this morning, I never used the 1980 diagram for that specific part. But I swear, for the rest, it was really helpful to help me understanding Big Brother’s wiring. The Haynes 1984 differs too much.

SO, tester on hands, I jumped in the truck… and after I tested ALL of the cluster harness wires, NO ONE was illuminating the cluster.

?@#*%?!?

«Ok» I thought, «Gary is right when not understanding how the cluster is illuminated».

«But it WAS, before I pulled it out from… wait a minute, is it possible that… no, can’t be true…»

YES, it WAS true: Behind the dash, well hidden, was lying the rest of the blue/red wire.

Can’t say how stupid I feel.

This tester session confirmed that the cluster connector wiring corresponds entirely to the one you posted, Gary.

Can we zap this part of the thread? And why not zap me at all…

Ok, now let’s focus on the tach issue and forget this connector “moron chapter"!

I’ll study your diagram and see what I can do. I have a question about the resistor identification: You don’t mention the first "gold" stripe, just the 3 other ones. Any reason?