Who needs power steering or headlights?

tleed · May 2, 2004

Last weekend I towed a 31' RV trailer from GA to VA with my '96 Suburban 6.5 liter diesel. Every time I got off the interstate, the power steering was gone. Driving around the exits it usually returned shortly. Once I felt a mysterious thunk in the brake pedal as I came down the exit ramp, and the power steering suddenly returned.

I was having a bit of trouble with transient power steering before I left on this trip, so I siphoned out a pint of fluid and re-filled it with Lucas Power Steering fluid, but apparently that didn't do the trick. Suggestions?

Oh, couple of other questions:

1) Should the engine drop 500-800 rpm when I let off the gas on the highway? Then it runs back up to around 2100 RPM at 65-75 mph (approximately). The motor pulls fine, I got 17.8 mpg on the highway unloaded, and the previous owner told me the transmission was replaced about 25k miles ago (vehicle shows 224k miles), but that seems like a pretty big drop in rpm to me. Is that normal?

2) Sometimes the headlights are reluctant to come on. As in, I flick the switch to "on" and the parking lights come on, but the headlights don't, until they decide they want to. What gives there?

By the way, this is my maiden post. I've recently retired my '83 Grand Wagoneer (203,000 miles) as my primary tow vehicle and just acquired this Suburban to replace it. Sniff! If this bulletin board is half as good as the IFSJA bulletin board I used to hang out on, it will be outstanding.

Thomas

blue2kzr2 · May 3, 2004

Welcome to the board. I visit all the forums and try to help where I can, but I have an S-10 so I have VERY little knowledge of trucks like yours. Having said that, I bet you have a system called "Hydroboost" that uses the power steering system to provide brake assist, as a replacement for vacuum boost. That being the case, sounds like your brake clunk and power steering problems are connected in some way, but I'll let those that know the system better handle the specifics...

tleed · May 3, 2004

Yeah, thanks for the response.

I had an experienced Ford mechanic say just about the same thing. He stared off into the distance and said, "I think the power steering and brake systems on that truck are linked together somehow, but I'm not exactly sure how."

Surely there's got to be a good Chevy man out there who knows how?

Thomas

2001 HD · May 3, 2004

NBS 2500 use "Hydro-Boost" brake sysyems. The power steering pump provides boost instead of a vaccum brake booster. Not sure about OBS. If you see lines running from th PS pump to the master cylinder and there is no brake booster behind the master cylinder, then you have hydro-boost. There have been a few reports of power steering shafts shearing off inside the pump. It's hard to tell since the shaft is turning, but it is broken behind the front seal. Most people have had to actually remove the pump to tell for sure. You may also have speed sesitive steering problem or debris in the line. If I'm not mistaken, there is an orifice in the line as well. If you do have vaccum brake booster and the diaphram is ruptured, this may be related to the brakes and engine speed dropping due to a loss of vaccum.

The lights are strange. A slow relay perhaps. Try swapping it out with another relay that is the same part number. On the NBS, the PCM controls the headlights since it uses a light sensor. And there is always the "Loose connection" theory.

What transmission do you have?

Good luck.

robsawatsky · May 3, 2004

Oh, couple of other questions:

1) Should the engine drop 500-800 rpm when I let off the gas on the highway? Then it runs back up to around 2100 RPM at 65-75 mph (approximately). The motor pulls fine, I got 17.8 mpg on the highway unloaded, and the previous owner told me the transmission was replaced about 25k miles ago (vehicle shows 224k miles), but that seems like a pretty big drop in rpm to me. Is that normal?

If you are cruising at highway speed, I thought the torque convertor should stay locked-up (unless you touch the brakes), therefore RPMs shouldn't drop much at all.

Can you tell if your TC is locking-up? If you accelerate hard to get it to downshift all the way to 1, you should feel the shifts up to 2, 3, OD and then feel the TC lock-up when you let up on the gas a bit or feel another shift in there if still accelerating.

tleed · May 3, 2004

Didn't all the 6.5 liter diesels have the Allison transmission? How do I tell which transmission I have?

Thomas

tleed · May 3, 2004

And I'm not sure the TC is locking up. Maybe that's really what I'm asking: what should it feel/act like? Seems like maybe it's not behaving right.

Thomas

blue2kzr2 · May 4, 2004

Didn't all the 6.5 liter diesels have the Allison transmission? How do I tell which transmission I have?

Thomas

Allison didn't come out till the NBS, so you probably have the 4L80E which is a decent tranny.

Yes, the OBS had Hydroboost as well, it's not something that started in the NBS.

TC locking/unlocking should feel like a VERY smooth shift, sometimes even undetectable. Best way to tell if it's locking is to get on the highway, hold a steady speed, keep your foot on the gas to hold the speed and tap the brake with your other foot. You should be able to feel it un-lock, brake application is one variable to unlock it. You can also watch the tach and see the RPMs go up some when it unlocks.

tleed · May 5, 2004

NBS? OBS? That's new to me. What is that?

On the side of the transmission is "Hydramatic 242033-70-5".

So I suppose it's a Turbo Hydramatic, duh?

But does that number tell me whether it's the 4L80E or the 4L60E? And what's the difference between the two?

Thomas

blue2kzr2 · May 5, 2004

NBS = New Body Style, OBS is Old Body Style. You have a GM transmission, it's a 4L80E, I can gaurantee that. The 80 is the "high-torque" transmission, the 60 is the "low torque" transmission to put it simply. www.gmpowertrain.com has info on the different units if you want to know more, including gear ratios.

tleed · May 10, 2004

OK, just yesterday while driving this happened, and the system wasn't even all that hot:

I hit the brake, and the power steering cut out. As soon as I released the brake, the power steering returned.

What gives? This is really starting to concern me.

Thomas

blue2kzr2 · May 10, 2004

Sounds like the Hydroboost for the brakes is pulling more PS pressure than it's supposed to. Maybe someone with more knowledge of the system can chime in and say eaxctly where the problem lies. This is just a wild guess, but considering the 2 systems are linked it seems like a logical one...