Factory Tach Issues.... Looking for some Help

[Six862m6]

Hey guys, looking for some opinions on what the deal is with what I believe is a factory tach connected to an aftermarket (Accel) HEI ignition on my truck. Pretty sure I've got the real deal; the gauge itself looks correct, just replaced the original printed circuit last night (due to half the cluster lights usually not working, but sometimes) and it had the correct cut out area for the tach as well.

My issue with the tach is that it usually doesn't work, but sometimes will perfectly. Then it will work, then not work... like a loose connection. The connections aren't loose though! Do I need to install an electrical filter that I've seen on some of the parts vendors' websites? I've read about them being really sensitive to not having a good ground; I have it ground to the parking brake handle mount/steering column to dash mount (my truck is a '68), so I would think this would be a pretty good ground location? Guess I could always take a jumper cable and hook it straight to the battery ground to test, haha! Any suggestions? Do you think the gauge is just slowly going out?

Also, there are 2 small light bulbs behind the tach gauge (same bulbs, sockets as the rest of the guage lights). What the hell are these for???? They don't seem to do anything and I couldn't see them being "dash lights" as the tach is never actually connected to the printed circuit, meaning they would have to take their power from the regular 3 wire tach connection and would be on at all times. There are no cut out holes in the front of the tach face as a "warning indicator" (think temp/brake on the fuel gauge).... so what in the world????

Lastly; this is not tach related but a question nontheless; what is everyone's opinion on defrost vent hosing. (Non AC Truck) The PO replaced the ones in my truck already, I'm almost certain. It looks to be a plastic material and way too new looking to be the originals. Plus I think the originals were a cloth material, right? My problem with them is that they don't clamp to the defrost vents or the heater box toward the floor worth a CRAP!!! Is there any out there that does or should I just resort to using a few staples or very small screws to keep these things connected properly? What are your guys' experience with this?

Let me know any words of wisdom, thanks!!

David

[ray_mcavoy]

Hi David,

The fact that your tach will sometimes work perfectly and sometimes not sure does sound like a loose conection. Since you've checked all the external connections, maybe it's time to take a look at the tach's internal circuit board to see if there's a bad solder joint or any loose connections there.

As far as I know, those filters are only to prevent radio interference from the tach wiring. I've never had any problems using a tach (factory or aftermarket) without a filter.

It's been a while since I swapped the factory tach cluster (from an old farm truck) into my 72, but if I remember correctly, the tach shares the same ground as all the other gauges ... that being the metal housing on the back of the cluster. So I think a bad ground would cause problems with more than just the tach.

I don't recall mine having any small lights directly behind the tach. Just the turn signal indicator bulbs that are roughly at the 10 & 2 O'clock positions relative to the tach. And a couple of cluster backlights at the 9 & 3 O'clock positions off to each side of the tach. All of which are connected to the printed circuit.

You're right, the original defroster ducts were a cloth type material over a spiral metal wire reinforcement. A previous owner installed some of that plastic replacement ducting in my 72 and used zip ties to hold it in place ... seems to be staying put okay. Some radiator hose clamps would probably work even better.

[Coley]

You are probably going to have to remove the tach and 'bench' test it out in the open...on or in the vehicle.
The needle swing mechanism is very sensitive. So you need to first visually investigate it up close to make sure it is not hanging up on anything. Honestly, sometimes it seems like a misplaced thread in the wrong place can keep it from swinging to position.
With it out of the dash and available, carefully 'blow' out the dust, etc out of the circuit/needle swing area.
Remove power from it and carefully tighten and reseat all connections on it, noting that it gets its ground from the metal casing. Did you repaint the casing? this might prevent a good ground.
Then (of course) try it first without re-installing in the dash.
Keep us posted.
Coley

[Six862m6]

Thanks for the reply guys. I'm going to take some pics tonight and try to get them posted (might be tomorrow). Mainly of these light bulbs that I'm talking about. They're not the signal bulbs and they're not the cluster bulbs.... very interesting that no one seems to recognize these. BTW, I need to pick up a couple of the cluster bulb sockets (the correct ones), do any of the local parts places still carry the right ones or do I need to purchase these from the specialty vendors?

Definitely did not repaint the housing. Is there anyway to truly bench test a tach? I would assume it would involve a potentiometer to replicate the signal from the coil? (BTW, what is the voltage range that would come from the coil to make the needle swing?)

I also have two cut wires that I'm going to take a picture of and post to see if anyone has any clue to what they were for. I've studied the wire schematic pretty well at this point, but I still can't identify these two particular wires. One is about a 14 ga, red that is in the same part of the harness as the windshield washer pump and motor. The other is a smaller, 16 or 18 ga, brown wire... forget at the moment what part of the harness it comes from but will find out tonight and post some pics.

Thanks again for the help guys.

David

[Six862m6]

Oh, and as a side note, my tach has an actual grounding lug on the back of it that I will take a pic of as well. It does not come into contact with the printed circuit at all. Also, the gauge cluster doesn't ground off of the housing. At least mine doesn't. I plugged it in last night without putting the metal housing back on just to test all of the bulbs out before I put everything back together. So the tach definitely has to get ground off of that lug.

[Six862m6]

Ok guys.... got the tach problem figured out last night. I still don't know what the hell the bulbs are for, they still don't seem to do anything even now that they're in correctly (the pics I post below will help explain everything)! Also I have a couple of pics of some wiring I need help with. Take a look, any info will be very helpful:

Front of my tach still in the gauge cluster:



Back of my tach still in the gauge cluster:



Back of my tach without the printed circuit in the way. What is the item I have circled in the pic? Looks like a potentiometer to fine tune the tach to me. Anyone?:



Mystery Bulb #1 at the 9 o'clock position:



Mystery Bulb #2 at the 3 o'clock position:



Tach removed from cluster, both mystery bulbs:



After studying the tach for just a moment, I realized that the bulbs/sockets were in backwards! There was nothing for the contacts to touch when installed in the direction that they were, whereas there is on the FACE side of the tach board:



This turned out the be the cause of my tach woes; the solder had become very loose and this part was easy to wiggle around. Did a quick resolder job, tested the tach and it worked flawlessly, no more "loose connection" problems (I guess it really was a loose connection)!



Finished Gauges, front and rear:





Ok guys, aside from the mystery bulbs that STILL seem to do nothing even though they are now correctly installed (seriously.... anyone have ANY clue???) Here are three more pics of items I have a question on:

What is this wire supposed to go to? This is coming out of the part of the harness that connects to the windshield wiper motor and pump:



What is this wire supposed to go to? This is coming out of the part of the harness that has the little bulb/socket for the heater control:



And lastly, can anyone show me a pic of how this wiring is supposed to PROPERLY nest behind the gauges? This is the wiring connectors for the horn, turn signal switch, hazard switch coming from the column, it looks like the one part of the connector is actually supposed to rest ON the column guessing from it's round shape:



That's it for now guys, again, thanks in advance for any help you can give in shining a light on these remaining issues!

David

[VA72C10]

Those two bulbs inside the tach assembly shouldn't be there...so no clue on why they were there???

THe last picture is an adapter for a newer turn signal switch to the curved factory style connector...goes to the turn signal/steering column wiring.

[Six862m6]

You wouldn't think they were supposed to be there, but the holes in the tach's board are cut out for them EXACTLY, no hack job and like I said; there are contact points on the front side of the board to make contact with the bulb socket's contact points. All looks factory..... So I dunno. Hoping someone can shed some light on this one.

Ah.... so my signal switch has been updated at some time? Odd.... I wonder why. I'm really curious as to how this wiring/curved plug is supposed to be physically mounted to the truck.

[VA72C10]

the hole on the back of the tach is for adjustments....I have never messed with it personally but I have heard some have...

[Six862m6]

Bump. Anyone have any info on those two wires or what the backwards bulbs in the tach are for?

[Six862m6]

Daily Bump

[Six862m6]

Hey guys.... little update. I found out what one of the cut wires in my pics was for. The P.O. of the truck strikes again with his marvelous wiring skillz.... the cut red wire was actually one of the ground wires for the wiper switch to wiper motor (that had been cut and jumped with this red wire of course, as red isn't it's original color at all...). Anyway, the OTHER wire, (the tan-ish one) thats cut is for the brake warning switch wire shown on the wiring diagram as a 10T wire... can anyone tell me what this is exactly? (I mean, what IS the brake warning switch?) and can anyone post me some pics of both the switch and where it's located and also what it connects to at what looks to be out in the engine bay from what I can gather on the wiring diagram?

Also... no update on those two odd ball light sockets on inside the tach....

Thanks guys!

[VA72C10]

Originally Posted by Six862m6

Hey guys.... little update. I found out what one of the cut wires in my pics was for. The P.O. of the truck strikes again with his marvelous wiring skillz.... the cut red wire was actually one of the ground wires for the wiper switch to wiper motor (that had been cut and jumped with this red wire of course, as red isn't it's original color at all...). Anyway, the OTHER wire, (the tan-ish one) thats cut is for the brake warning switch wire shown on the wiring diagram as a 10T wire... can anyone tell me what this is exactly? (I mean, what IS the brake warning switch?) and can anyone post me some pics of both the switch and where it's located and also what it connects to at what looks to be out in the engine bay from what I can gather on the wiring diagram?

Also... no update on those two odd ball light sockets on inside the tach....

Thanks guys!

On the tach bulb sockets....yes, the holes are in factory tachs...my point I was trying to make is with the dozens and dozens of factory and aftermarket tachs I've worked on, I've never seen a bulb in one and see no reason for one to be either...

[Six862m6]

Originally Posted by VA72C10

On the tach bulb sockets....yes, the holes are in factory tachs...my point I was trying to make is with the dozens and dozens of factory and aftermarket tachs I've worked on, I've never seen a bulb in one and see no reason for one to be either...

Ahhh.... I see what you mean now VA72C10. I don't really see a reason for them to be there either... just an oddity really.

[jaros44sr]

That brake warning lite is there to notify you that you have lost fluid in the brake system. The lt brown wire goes to the proportion vavle along side of the m/c and comes into the engine harness side of the bulkhead connector at the upper left side first row. Should be a push on connector w/rubber boot at the porportion valve

[Six862m6]

Originally Posted by jaros44sr

That brake warning lite is there to notify you that you have lost fluid in the brake system. The lt brown wire goes to the proportion vavle along side of the m/c and comes into the engine harness side of the bulkhead connector at the upper left side first row. Should be a push on connector w/rubber boot at the porportion valve

Thanks jaros. What's it physically connect to on the inside of the cab? The "BRAKE" indicator light in my dash is operated with a bulb on the printed circuit. Does this wire go to the wire harness that connects to the gauges?

David

[jaros44sr]

Originally Posted by Six862m6

Thanks jaros. What's it physically connect to on the inside of the cab? The "BRAKE" indicator light in my dash is operated with a bulb on the printed circuit. Does this wire go to the wire harness that connects to the gauges?

David

Lt. Brown comes off of the bulkhead connector, into the main wire bundle at the fuse block and up into the dash gauge connecter. If you look at that connecter, you should see the lt. Brown wire, that connector is all numbered to reflect the location on the factory wire schmatic.....

From there it plugs into the printed circuit board, and is easily traced to the bulb