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11

Electrical / Re: TH350C orange test connector use?

« Last post by Scaryfastatx on March 03, 2026, 12:17:24 pm »

For what year model truck are you seeking an answer?

For a 1983 C10 with MV4 (350C) transmission, the orange test connector is used to monitor and verify when 12-volt accessory power is supplied to the transmission TCC circuit through the service brake [applied] switch and low engine vacuum (i.e., high engine load) switch.  There is also a minimum road speed limit switch inside the transmission.  All of the described switches are connected in series with the TCC solenoid.  See diagram.

Thanks for the reply, it's an 81, I think I have everything hooked up correctly according to your diagram

Only difference I see is my 12v supply to the brake switch is pink

My truck had cruise control originally, if that changes anything

I didn't check the brake switch for continuity, will check that next

But was wanting to put a light on the test port to see when I should be engaging the lockup

Sent from my Nokia 8.3 5G using Tapatalk

12

Electrical / Re: TH350C orange test connector use?

« Last post by bd on March 02, 2026, 01:40:25 pm »

For what year model truck are you seeking an answer?

For a 1983 C10 with MV4 (350C) transmission, the orange test connector is used to monitor and verify when 12-volt accessory power is supplied to the transmission TCC circuit through the service brake [applied] switch and low engine vacuum (i.e., high engine load) switch.  There is also a minimum road speed limit switch inside the transmission.  All of the described switches are connected in series with the TCC solenoid.  See diagram.

13

Electrical / TH350C orange test connector use?

« Last post by Scaryfastatx on March 01, 2026, 06:24:03 pm »

Hi all

I'm trying to get my lockup working on my TH350C to squeeze every bit of mpg out of the old Scottsdale

I want to know what kind of behavior I should be seeing on the orange connector:

- 12v when the clutch should be engaging?
- 12v only when the clutch is engaged?

Thanks?

14

Electrical / Re: Fusible link or grounds?

« Last post by zucchi on February 23, 2026, 04:32:21 pm »

Don't underestimate the importance of good grounding. Thoroughly clean all grounds; engine to frame, body and battery – frame to body, battery and engine – battery to body, frame and engine. The body has lots of components, so; fenders to firewall, firewall to hood, cowl to door, etc.
Think of grounding like insurance; it's better to have more than you need than not enough.
Redundant grounding is, admittedly, tedious and boring and usually doesn't "solve" an acute problem but getting it done eliminates that variable. You'd be surprised what systems and assemblies poor grounding can affect. The worst I've seen was a transmission that needed a rebuild after only ~60k regular miles. Upon inspection, the bushings were eroded. It was ultimately determined that current flowed through the transmission so as to get back to the battery. You see, current is like water; it flows the path of least resistance. If a circuit is incomplete or poorly grounded, current will flow through a related or shared circuit to find ground. In the case of the transmission issue, current flowed between the output shaft and clutches to the case. Electricity was arching between the shaft bushings and the shaft. Aluminum bushings don't hold up like the tungsten contacts used in relays and ignition points.

I also second what the other replies recommended; clean the wires and contacts at the battery and starter.
If you're still having issues, use a multimeter to check for voltage at the distributor and solenoid.
If everything tests ok but the starter solenoid won't engage, use a remote starter switch to confirm the problem is with the starter rather than the ignition switch.

Keep us ported.

15

Electrical / Re: Fusible link or grounds?

« Last post by bd on February 23, 2026, 12:07:45 am »

...So I went and jiggled battery cables and the headlights came on...

Make sure that the battery cable connections to the battery, starter, and engine are clean and tight, and that the copper strands of the cables aren't corroded where hidden under the insulation or severed by corrosion or fatigue.  If the cables check out okay, give the fusible links a firm tug to see whether they stretch or separate.  An '86 should have two fusible links at the starter and two fusible links at the firewall junction block above the back of the engine.

16

Electrical / Fusible link or grounds?

« Last post by Chanman09 on February 22, 2026, 08:20:53 pm »

I took my 86 c20 out for a great little drive today.

She’s a little cold natured, but after she was ready I drove down by the lake and enjoyed the wind in my hair.

I pulled back into the yard and put her back into her spot and killed it.

Then, I remembered I was going to cycle the ac and heat through all the settings to make sure my vacuum hoses were still ok and that they would pull the vent doors closed and switch to feet and defrost and bilevel. 

I went to crank the truck again and no start.

And, the radio quit. I pulled the headlight switch and didn’t have lights either.

So I went and jiggled battery cables and the headlights came on.

I had dash lights and cab light. But when I turned the key, nothing


I thought maybe there was some heat issues with the starter so I let it sit for a couple of hours.

I just went back out there and realize there’s no power to any ignition, and the things that controls - radio, blinkers, fan etc.

So, I’m going to go testing tomorrow.

Obviously I’ll start with the easy stuff an go from there. But with my luck it’s always the hardest and most time consuming thing that it ends up being.

What are y’all’s  bets??

A wonky ground? A fuse? A fusible link?

I’m going to pray it’s the - battery ground at the alternator bracket. But I have a feeling it’ll be a fusible link at the starter.

I’ll end up on my back in the cold dirt cussing that I don’t have a third hand to hold a starter while I lose the bolt or washer from the starter lug. 


Such is life.


Anyway, any cool things I might need to look at before I cry too much??

Thanks!

17

Performance / Re: Oil additives

« Last post by VileZambonie on February 21, 2026, 05:50:20 am »

Add this at every oil change if you are running a non roller cam. Although you can buy oils that should have what you need, most modern oils aren't made for this generation of engines.

18

Heating, Ventilation & Air Conditioning (HVAC) / Re: 74 c10 hvac control

« Last post by bd on February 20, 2026, 10:34:29 pm »

Thanks for the update.  Glad you got it sorted out.  We're always happy to help a brother in need.

19

Heating, Ventilation & Air Conditioning (HVAC) / Re: 74 c10 hvac control

« Last post by doneebaker on February 20, 2026, 04:30:23 pm »

Well I did some probing so to speak. I replaced the inline fuse as suggested and verify power to the relay,  all working .  Next i installed a new control panel to spruce it up inside with a new switch. Plugged in the switch and it would work  intermittently. I discovered one of the terminals inside the plug was broken and not making good contact. Repaired the broken terminal and it works as should.. Thanks for all the help I  really appreciate it.