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Discussion Starter · #1 · (Edited)
I just finished replacing several items on my '07 classic 2500HD, now I can't get it to shift out of park, brake pedal is soft, and the power steering pump whines when I press the brake pedal all the way down.

Out of this project, I replaced the power steering pump with this one: 15909827 Replaced the hydroboost with this one: ACDelco 178-1036 More Information for GM GENUINE 1781036
Replaced the brake light switch: D891A https://www.amazon.com/gp/product/B0013FVXX4/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1
Added the power steering cooler: DORMAN 918301 More Information for DORMAN 918301
And added a magnefine filter to the power steering system. https://www.amazon.com/gp/product/B0787KWZPS/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

How exactly is the brake shift interlock engaged? I know from other newer models of hydroboosts that the pushrod hole has to be enlarged to fit the '07 brake pedal pin, but the pushrod end on 178-1036 is identical to the old one that I removed, so I didn't do anything to the hole. From what I've read on here about this problem, the hole needs to be enlarged so that the brake lights will activate, but mine are activating as soon as I touch the brake pedal. If the interlock is triggered electronically by the brake light switch, maybe something is wrong with the new switch? I replaced the switch only because I was there, it's a bear to work on this area, and the new switch doesn't cost much. Old one was working fine, so I can swap them out if that's possibly the issue. Or is the shift interlock triggered by something other than the brake light switch?

Maybe I haven't fully bled all the air out of the brake lines from removing the master cylinder in order to replace the hydroboost, but I wasn't getting any air out for a while when I quit bleeding them. As it is right now, I can press the brake pedal to the floor without much effort. Power steering pump whines a bit from this, and I can hear a slight change in the engine sound at idle as well. Pump also whines a good bit when reaching steering limits as well.

After replacing the pump, hydroboost, the low pressure lines, and adding the cooler and the filter, I added power steering fluid to the reservoir, left the engine off, and turned the steering wheel back and forth a bunch. That definitely moved fluid through the lines, as it did take topping off the reservoir a few times. After doing that until the fluid level no longer changed, I started up the engine, moved steering wheel many more times, applied the brakes many times as well. This made only one small change in fluid level, and it has remained constant ever since then. I had the engine running long enough that I could feel both the top loop of the new power steering fluid cooler, and the hydroboost return lines getting warm, so I figured that was a sign that fluid is circulating as it should and no more air trapped in the lines. Is there any other signs I should look for to confirm that the power steering system is fully bled? I ask that, as it doesn't seem like the pump should be whining like it is. I've thought about swapping it back to the previous one that was on it, I hardly ever heard that one whine, and now I'm curious how that would affect the brake pedal feel.
 

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Discussion Starter · #2 ·
I went back to the pump that was on it before my swap, as I found out it was already changed not long before I got the truck from my dad last year. Power steering feels better now and significantly less noise from the “old” pump compared to the new one.
I wore out my son’s leg while I bled the heck out of the brakes. I don’t have much experience with that, it surprised me with how small the air bubbles were that I got out of the lines, but now I have a good firm pedal feel. So I got that portion of my problems resolved.

For not being able to shift out of park, I figured out it is between the brake light switch and the hydro boost pushrod. I took the switch off the pedal, pushed the switch by hand and had the shifter was allowed to move out of park every time. I took dremel and small grinding stone to the hole in the pushrod to make that slightly larger. It seemed todo the trick, shifted out of park better than 20 times in a row. Over the next few hours,I started it up and moved it around in my yard several times, all without a problem. But a few hours later, I started up to take it out for a test drive, couldn’t get it out of park.
Is the relationship between the brake light switch and the hydro boost pushrod really that finicky? I don’t get how I’m having so many problems with this. The pushrod on the old hydro boost and the new look identical. I’ve seen references to going to a newer version of the hydro boost and having to drill out the pushrod hole to 43/64” or 17mm. But that would make the hole a good bit bigger than it was, so it made me think the version I have is different than what others are referring to. Am I wrong on that?
I guess I’ll do some more grinding on the hole. Need to get this sorted out ASAP, have another trip planned for this coming weekend.
 

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See attached description for a 2009 model. You may need to look for the ELECTRICAL MANUAL for your 2007 if you need more accurate information. mI didn't think that our older trucks had such a shift lock mechanism going on.
 

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Discussion Starter · #4 ·
I don’t think I’ve been in any vehicle with an automatic transmission that didn’t require applying the brake to shift out of park. Definitely have it on my ‘07. When it does work right, I can hear a soft “click” when it activates to release the shifter from park.
The file you posted shows what I’m concerned about- there are extra components between the switch and the solenoid. Makes the troubleshooting a bit more complicated if it is something else besides the fit between the pushrod end and the switch contact.
 

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I guess you just haven't been in any old vehicles. I've owned several cars where the foot brake did not need to be applied to shift out of park, but of course in park or neutral to start.
 

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I'm having the interlock issue after a new hydroboost (2010 version on 2006). I did not enlarge the hole as it fit over the pin just fine. Does it need to have some play in it? I have a new brake light switch coming tomorrow. Any tips? Anyone know what size drill to punch it out if required?
 

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Search for hydroboost and there are quite a few posts about this. Someone else recently (in the last week) posted that his problem was the connector to the brake switch. Connector pigtail replaced and the sun shines in the east now.
 

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Discussion Starter · #8 ·
Hmm, well, I've tried finding the post you've mentioned, Ron, but I can't seem to find it. Just a general search comes up with 434 results for hydroboost. I played around with the advanced search function to sort by recency, posts 9 & 10 mention this wire harness a bit. Cruise control not working

Looking a pictures to purchase a new pigtail from the brake pedal switch, they only show the end that plugs into the switch. What is the other end? Is it a plug, or just have to splice wires together? Guess I'll figure that out either way in a few days when a new one arrives. A new brake switch, connector, and shift interlock solenoid are only about $75 combined on amazon, so we'll see which gets me going for good here soon.
 
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