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Discussion Starter · #1 ·
F-350 2WD Dually Auto / completely stock.

It all started when my vacuum pump went out.
Replaced it - all was fine.
Then the brake light would come on when "idling" when at a stoplight when somewhat "pumping" the brake to move up slowly with traffic. When the RPM's go up - and more vacuum is made - the brake light would go off until the next time the brakes were "pumped" - and a repeat of above.

This morning when I started up the truck - the ABS light came on - and stays on - truck shifts hard - if at all - and OD light is flashing.

I'm thinking it might be the ABS valve?

I don't see why a lack of vacuum would cause the ABS light to stay on and the OD to flash.

Any ideas?
 

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The center brake light is tied to the computer with respect to some aspect of shifting. Perhaps the brake light being on and off is messing with your computer. I'd start with that and getting the codes read to find out exactly what the computer thinks is wrong.
 

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Discussion Starter · #3 ·
I'll check the center light on the cab.
 

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It's not a bad idea to check the brake lights, but what you really need to do is check the codes. Then you'll know what to look for. Without the codes all I can tell you is something is wrong.
 

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My thought was if the vacuum is affecting the position of the brake pedal, then the rear brake lights could be on while driving. Wouldn't that affect TQ lockup / OD?
 

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Are you saying the brakes lights on rear of truck or are you referring to a brake warning light on the dash when the "brake light comes on when brakes are pumped"?

Brake lights staying on can be caused by bad booster ans vacuum level can affect the release of pushrod out of back of booster in this situation.
BRAKE warning lamp can come on due to low brake fluid and brake pumping results in vehicle oscillating which can slosh brake fluid and low brake fluid warning lamp comes on.

My recollection on this series truck is that some rear diff speed sensor issues cause problems with some users of the signal but speedo will be OK.

Low vacuum will not make the ABS light come on.

A little more info so we can better connect the dots here.
With OD light flashing a scanner read out of codes is really needed. The trans will tell you what it is missing and the other things may be wholly unrelated. An auto parts store read out is unlikely to be of any help.
 

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This diagram & the NEXT several explain the RABS-II system, and how to read codes:

 

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Discussion Starter · #8 · (Edited)
It is the "dash" (warning /indicator / idiot) / brake light - that comes on when at "idle" - until rpm's go up.

The ABS light coming on and staying on - and OD light flashing - just started.

Center cab brake light works.

All fuses = dash fuse panel - and under hood fuse panel - are ALL good.

When first starting the engine - The ABS light goes OFF - then after a few seconds - it comes on and stays on.

OD light is NOT flashing - until I drive it.

It stays in 1st ear - unless I get to almost 2200 RPM - then it shifts hard into 2nd - and then the OD light starts flashing.

I can shut the engine OFF - and restart - and the above will repeat.
 

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Discussion Starter · #10 · (Edited)
Ah Ha!
Took her for a drive = no speedo- (will change the axle speed sensor.

However- now the brake warning light does NOT come on when "pumping" the brakes a couple of times at idle - but now the pedal gets extremely hard - after just one pump - and I almost can't hold the truck stopped at idle in gear.
When I depress the brake pedal - the "whoosh" sound (from the floor near the pedal) booster seems louder than it should be.

By the way - when I start it up - the ABS light goes off - then comes back on / steady- but it acts like it is "stuck in first gear" the OD light is NOT flashing.
If I rev up to 2200 RPM in first gear - it will shift hard into 2nd and then the OD light beings flashing - I'm sure this problem is the axle speed sensor.

I'm thinking the brake pedal might be a bad booster...?
 

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On your brake issue you may of gotten a bad pump right out of the box. Was it a new or rebuilt pump?
 

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Discussion Starter · #13 ·
The odometer reads out when turn the key on (I didn't check to see if it was racking up miles when driving in first gear because I thought that if there was no speedo movement - it wouldn't register in the odometer anyway)

The vacuum pump was brand NEW from NAPA.
 

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The odometer reads out when turn the key on (I didn't check to see if it was racking up miles when driving in first gear because I thought that if there was no speedo movement - it wouldn't register in the odometer anyway)
Correct, the number wouldn't change. But since you see a number, that suggests the PSOM (the other possible suspect in the shifting issue) is good, and points the finger at the VSS and/or wiring/connector to it.
 

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This caption explains how a vacuum booster works:



This & the NEXT several explain hydroboost:

 

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Discussion Starter · #16 ·
Thanks bugman / madpogue and Steve83 for your help.

I don't have a vacuum gauge - so-

Since I have 2 - practically identical "stock" trucks - I started them both up and checked the vacuum to each brake booster by placing my finger over the end of each hose.

The truck with problems - had quite a bit "less" "suction" power - than did my other truck that is working fine.

So - I replaced the vacuum pump and the brake problem is now fixed.

I replaced the VSS axle speed sensor - and the ABS light went off - and the truck runs and shifts fine.

I disconnected both batteries for several minutes and reset the computer codes.

Both problems are fixed.

Thanks to all three of you.
 

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I started them both up and checked the vacuum to each brake booster by placing my finger over the end of each hose.

The truck with problems - had quite a bit "less" "suction" power - than did my other truck that is working fine.
That's probably as good a vacuum gauge as you need in most cases.

And +1; I'd be lying if I said I never got confused about how I took something apart on one truck, and referred to the other one as a model.
 

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Disconnecting the battery is generally a bad idea because it clears the EEC's/PCM's adaptions for the wear & mods to the engine. It's better to clear DTCs properly, and keep the adaptions intact. It may take more than 50 miles & several drive cycles for all the adaptions to be relearned, but I'm not as familiar with diesels as gassers.
 

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Disconnecting the battery is generally a bad idea because it clears the EEC's/PCM's adaptions for the wear & mods to the engine. It's better to clear DTCs properly, and keep the adaptions intact. It may take more than 50 miles & several drive cycles for all the adaptions to be relearned, but I'm not as familiar with diesels as gassers.
The 7.3L had no adaptation on either the engine nor the transmission. The only thing that gets cleared is the codes.
 

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Good to know.
 
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