Alright, found something interesting.

I checked for 12v across B and G on the back of the tach, circuit paper in place. No voltage at all. Checked across B and a known good ground under the dash (I know it's good because my radio grounds through it), zero volts. Finally, I set my meter to 200 ohms, checked across G and the same known good ground, and got a very low reading (something like 0.46). So we know the ground is good. Also double-checked, and I'm still getting good tach signal.

I traced the circuit line from the B post along the circuit paper; it led me to the contacts for the right turn signal bulb at the top of the cluster, then split into two separate contacts that ended where the cluster connector fits in. The wires going to these places are red with yellow stripe AND red with yellow dots (both sharing the same pin) and purple with white stripe. I checked both of these for voltage, and I got zero. BUT, when I turned on the right turn signal, red/yellow stripe and red/yellow dot read 7 volts. Purple/white stripe read zero.

This doesn't make a bit of sense...but hey, at least I know why my tach isn't coming on.

 

Find a good source for 12 power, like the fuse box, jumper this to the B post an see if the tach will work if it gets power.

 

https://www.ford-trucks.com/forums/9...4-f-150-a.html

have you read this thread yet?
post # 11

 

No, this is the first time I've seen that topic. I've bookmarked it now though.

Ran a short lead off the fuse box to the tach....SUCCESS! lol. Tach works fine, and it's pretty much spot-on with my Sunpro tach. Apparently it's not getting power, for whatever reason.

Do I run any risk of damaging anything by keeping that wire connected to B (using the metal nut on the post), even though it's sending 12 volts through the B post's circuit? I'm not bothered by having an extra wire, I just don't want to burn anything up.

Or should I just custom-wire it all like DrCharles did and do something with the tach part of the circuit paper, like cover it?

 

I do like DrCharles approach to adding a tach, but I am not sure it is the best way for you to go. At this point all you are lacking is 12v power to the B post. I am also worried that this lack of power feeding to the inst cluster might be related to the gauge problems you had in the past.

Reference DrCharles post #11 photo. He states the 12 volt switched power in is the lower right of the connector. If the wire feeding this connection still has 12 volt switched power, then I would tap into this wire and feed this to the B post.
<O</O
Using the wire that you are running from the fuse block could cause some unwanted back feeding if the feed source was _always hot_ or hot in ACC and key ON. I would not do this without more research, as the power source would need to be the same as original, meaning power with <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com</st1:City>KEY <st1:State w:st="on">ON</st1:State> only, which is how I believe it is stock from the factory.
<O</O
The simplest and cleanest would be to use the lower right feed wire, if it has power.
<O</O
Not sure if you paper circuit film is bad or maybe the connector plugging in at the back of the Inst cluster, those would seem the most likely areas to cause a failure.
<O</O
Good luck, at least you have a good tach and it is working.
<O</O
Jim

 

Sometimes the circuits on the circuit paper get crud on them and don't make contact, make sure all contact points are clean and have good connections.

-Enjoy
fh : )_~

 

One of the traces on a printed circuit I had had broken, I'm assuming from bending. The result
is the same as a cut wire.

 

All the connections appear clean. No corrosion anywhere on the paper. There was a bit of crud on the white/red stripe wire on the harness, but I cleaned that up. It wasn't even enough to cause a problem anyway, I don't think.

Would a break be visible to the naked eye? I followed the trace from the B post, past the left turn signal indicator socket, to two contacts where the main harness connects, and didn't see any breaks. I have to remove the cluster again either tomorrow or Monday to replace a burnt-out indicator bulb, so I'll double-check all the traces then. I just don't get why the trace for B is the same one for the left turn indicator. I don't get how that'd work...

I had the tach wired to an add-on fusebox I put in myself that's on a relay that's hot in ACC and ON. I've since pulled the fuse while I learn more about how I need to wire it up. What kind of problems would I have if it was backfeeding?

I'll check that wire and if it has power, I'll tap into it.


Not to change the subject, but I'm starting to wonder if my gas gauge problem is due to the sending unit, and not the gauge. I hooked up my original cluster just the other day and got the same gas level reading as the new cluster's gauge.

 

It doesn’t make any sense that the B stud would be on the same circuit as the left turn signal, give that another look when you take it out next time.
<O</O
I don’t think you would be able to see a broken or cracked trace with the naked eye. If you use your ohm meter you will find it. Hook one lead of the ohm meter to the B stud and see where it comes out at the plug in connector on the back of the Inst cluster. It should be the lower right, based on DrCharles post #11.
<O</O
The power would not back feed if the trace is broken (open circuit) and worse case is it would blow the fuse if it did back feed to numerous other circuits in the ACC position. It would try to feed all the RUN circuits while the key was is ACC through your new wire.
Logically the tach would not need to be powered in the ACC key position as the engine is not running, I would guess that the tach is only power in the RUN key position.

Jim

 

I would imagine it depends; I have/bhad one in the garage where a trace had
been broken up by the IVR, I think it was even peeling off the plastic.

 

I'll see if I can find any broken traces this evening. I'm gonna be able to do this with my eyes closed before long, lol.

I had the tach wired up through my add-on fusebox for about two days on a standard 4.5 amp fuse. If it were backfeeding, wouldn't it have blown that fuse right away?

Something occurred to me just now though: My gauges come on in both the ACC and RUN positions, which means that the IVR (and its trace on the circuit paper) is receiving power in both key positions. With this being the case, if I tap into that wire DrCharles used, wouldn't that be the same as using my wire from the fusebox, in that the tach would be turned on in ACC as well as RUN?

I suppose if I had to, I could always just do some research and locate a Run-only circuit, and tap into that wire with an inline, low-amperage fuse for the tach. Or I could swap out the circuit paper and use my non-tach one from my original cluster, and just custom-wire the tach like DrCharles did.

 

I see your point, you could just leave it as is with your extra wire and see if it blows the fuse, if the gauges are power in both RUN and ACC it should be OK as is.

Jim

 

Cool, I'll see what it does and report back over the next few days. I'll still check everything and see if I can find any broken traces. If possible, I'd like to eventually have everything wired as factory, at least as far as the gauges go.

Thank you very much for your help, everyone! I appreciate all the advice and knowledge you've shared. And thanks, Jim, for all the pics, links, and advice.

 

The gauges shouldn't be on in Accessory (ACC) position should they???

-Enjoy
fh : )_~

 

I just checked my truck, I moved the key to ACC and the gas gauge slowly went from E to 1/2 tank.