Engine stumbles when you hit brakes even with vacuum for booster disconnected...

[KRBishop]

Have a 1984 c10 2WD that stumbles only while in gear and only when hitting the brakes. Originally I thought it was a vacuum issue, but I disconnected the vacuum line from the booster and it still does it!

The video below is the truck with the vacuum from the booster removed and plugged. The front wheels are chocked so the truck will not roll.

In park and neutral I have no issues... I can hit brakes and everything is fine.... When I put the truck in gear with foot OFF brakes... everything is fine... as soon as I touch the brakes it starts to stumble. Again, WITH the vacuum line to the booster disconnected and plugged.

I have since also disconnected the cruise control vacuum lines. So with no vacuum lines connected to the brakes (booster and CC) engine still stumbles only when in gear and only when foot on the brake.

1984 C10 w/ Power windows and locks, AC and Cruise Control... thank you in advance!!

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[gmbellew]

there is a vacuum release switch that releases the vacuum in the cruise servo when you hit the brakes as a redundant feature. maybe disconnect that line from the cruise servo and plug it and see if that helps?

 

[KRBishop]

there is a vacuum release switch that releases the vacuum in the cruise servo when you hit the brakes as a redundant feature. maybe disconnect that line from the cruise servo and plug it and see if that helps?

Cruise was next on my list! Where is the cruise servo located? Thank you!!

 

[gmbellew]

Cruise was next on my list! Where is the cruise servo located? Thank you!!

on my 1990 tbi 350, it is on the back of the block next to the distributor cap on the driver side. you wlmay have to take off the air cleaner to see it...

 

[gmbellew]

another potential option is electrical. you could have a short to ground in the brake lights or several circuits connected from mice chewing. I had that problem with some window/lock harness mice damage. it also caused a stumble when you hit the window switch and pegged the volt meter.

 

[KRBishop]

another potential option is electrical. you could have a short to ground in the brake lights or several circuits connected from mice chewing. I had that problem with some window/lock harness mice damage. it also caused a stumble when you hit the window switch and pegged the volt meter.

Connected battery to multimeter...

• 12.83 with motor off
• 14.3 with motor running
• 14.3 with motor running and in gear with brakes pressed and engine sputtering

I also disconnected cruise control vacuum and the booster vacuum so the brakes are not connected to the vacuum lines at all. See pics below.


I am at a loss...

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[gmbellew]

i don't know, man. somebody smarter than me will hopefully chime in.

 

[KRBishop]

i don't know, man. somebody smarter than me will hopefully chime in.

All good brother... thank you

 

[Bextreme04]

Where were you testing voltage at? Are you testing at the distributor? That's most likely the circuit where you are having the issue.

 

[KRBishop]

Where were you testing voltage at? Are you testing at the distributor? That's most likely the circuit where you are having the issue.

I was testing directly at the battery. How would I test the voltage of the distributer using a multimeter? Thank you.

 

[talltidefan]

I read on one of these threads........

When cranking hold foot on brake, if it depresses down when cranking the brake vacuum booster is bad.

Keep us posted on results

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[Bextreme04]

I was testing directly at the battery. How would I test the voltage of the distributer using a multimeter? Thank you.

Not trying to be sarcastic but..... place multimeter lead on the Batt+ wire on the distributor and the other lead on a clean metal surface of the engine. Then try One lead on the Batt + and the other lead on the ground wire from the distributor. It should be close to the same and also greater than 12volts. If you see it bouncing close to zero, or reading low, or getting two different reading between the two measurements, you found your problem.

 

[KRBishop]

Was trying to figure out how to

Not trying to be sarcastic but..... place multimeter lead on the Batt+ wire on the distributor and the other lead on a clean metal surface of the engine. Then try One lead on the Batt + and the other lead on the ground wire from the distributor. It should be close to the same and also greater than 12volts. If you see it bouncing close to zero, or reading low, or getting two different reading between the two measurements, you found your problem.

LOL All good! I was trying to figure out how to connect the multimeter to the alternator... I tried to poke through the line the line but would not hold... other kids than this that make it easier?

https://shop.advanceautoparts.com/p/innova-auto-ranging-digital-multimeter-3300/12071148-p

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[Waylon]

If you have some extra wire and crimp terminals, build yourself a jumper wire to test it. Just disconnect the B+ wire at the distributor, plug in your new wire and run it straight to your battery positive terminal. An alligator clip at the battery would be best. You will have to disconnect it to kill the engine.

 

[bedwards]

Just a hunch, but take your connector off the brake light switch and see if it does it. You have cruise control in one of your pics and it turns it off with the brake light switch if I'm not mistaken.