Stoplamps out - CHMSL OK - Is Shop manual wrong?

[Trio3b]

1997 C1500 Both stoplamps out . Center high mount stoplight ok.
Left 20A I/P fuse ok
Under hood 30A fuse ok
Stoplight Relay checks out
Stoplight switch replaced about 1 yr ago and checks out according to GM service manual. (not chiltons)

According to svc manual Using Testlamp:
Orange on brakeswitch to grnd always hot checks ok.
White on brakeswitch to grnd- depress switch - lamp lights ok.

Here's where things get squirrelly. There's about 8 diagnostic steps in manual and all steps follow this order:
Test lamp lights - go to next step
Test lamp does not light - repair (whatever)

Except step 4 is Testlamp from light blue on Connector C266 to grnd and depress switch:

Test lamp lights - replace multifunction switch
Test lamp does not light - go to next step.

This is out of sync with all the rest of the diagnostic steps and does NOT make sense b/c if you look at schematics( which I have) power is needed from this light blue wire to trigger the relay to rear stop/turn lamps. If I follow advice of step 4 I'm replacing multifunction switch which is labor intensive and pricey not to mention if my mechanic does it.

I have encountered incorrect information on a Buick LeSabre Factory manual. Just wanted to get thoughts on possibility that step number four in GM manual diagnosing tail light on this truck could also be incorrect. It just doesn't make any sense to me that if the testamp DOES NOT come on then we move to the next step, and if the test lamp does come on we replace the multifunction switch.
Any thoughts? Thanks in advance
P.S. leaning towards a bad body ground for the stoplights although multifunction switch is old enough for replacing. Any idea where that grnd might be? Manual is super hard to read and I'm pretty good with schematics!

[maxwedge]

Do you have voltage to the stop lite sockets, brake on of course. If ok ground the socket with a jumper does the lite come on? Do the signals work?

[Schurkey]

Yep. Test for power/ground at the stoplamp sockets, and at the harness connector to the stoplamp circuit boards. If you have power at the harness, but not at the sockets, you have a circuit board failure.

Circuit-board failures are epidemic on these trucks, including the so-called "updated" circuit boards.

You can buy new boards from GM or from the Communists. They're all low-to-middling quality. You'll buy 'em again in five or seven years.

[Trio3b]

Thanks for input. Will do this weekend and try to chase down a bad ground or PC board breaks and post back.

[Trio3b]

Wow! Brutal !. Turns out that apparently, the manufacturing tolerances of certain brand name or modern 3057 bulbs is just small enough to where the wire Tangs on the bulbs is not wide enough to make contact with the contacts in the rear tail light printed circuit board of this 97 Chevy C1500. I had to take a jeweler's screwdriver and spread the wire contacts out on the brand new bulbs for all this to work. You can imagine me tearing my hair out with fuses good, relay good, power and ground good on both sides of the REAR harness, connector, and contacts of the 3 bulb circuit board. Checked everything three or four times with a test lamp and a DMM. I checked harnesses, sockets and bulbs all connected with a wire piercing probe as well and would get good power and grnd on all wires and socket connectors. I figured well maybe something is DISCONNECTING power when I operate Cab controls. So I moved on to the front of vehicle and began to follow FACTORY MANUAL diagnosis and what prompted my original post. Any way, installing new bulbs got me nothing. I removed the bulbs and used a small pin to test hot in the socket itself and it was good, but when I put in the bulb..... nothing. I played musical bulbs with old bulbs, new bulbs, some different new bulbs, nothing until I began to wiggle the bulb in the socket. I know what oxidation, temperature related material degradation and burned plastic looks like.....But none of that here. Just the Dielectric grease looking old. Can you imagine something as low-tech as wiggling the bulb in the socket to diagnose the problem? What are the odds that all four running/stop/turn lights went bad AT THE SAME TIME! These are independent circuits so you dont have to have bulbs in both sides for the circuit to work. Wow! Sometimes we really overlook simple things, however In fairness to myself I never would have guessed that the wire contacts on the new bulbs would have to be spread a couple thousands of an inch. There goes a weekend of my life I'll never get back. Anyway as I look at the schematics, it appears the wiring colors may not be accurate according to the schematic. Also I am now fairly certain GM step #4 diagnosis for both lower stoplights not working is wrong. I'll have another look but If it turns out that the manual is incorrect I'll post back here and see if we can't get a posse up to wherever this GM manual was assembled /printed, find the 85-year old codger/engineer/tech writer who got it wrong and give him a solid wedgie.

Still dont have backup lights but I think that's a different circuit and at least I can use the D*MN truck now. Will post back.

[brcidd]

Those so-called circuit boards on GM trucks of that era are just distribution boards and are terribly susceptible to corrosion or just plain poor contact. I replaced several due to inop turn signals mostly- but soon just cut off the plastic tabs to expose the "circuit board" and then hard-wired (soldered) wires from each contact point to the bulb bases--so that I'll never have one fail again. Autozone wants $23 for each one, and guaranteed for life- but try to get a new one- they don't stock them and they get back-ordered etc.