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What could cause this?

8

89Blazin

1/2 ton status
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Mar 20, 2001
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Location
midwest KC area
Me and Bondo have the same problem. I've got an 89, he's got a 90.... 350TBI/700R. With the cruise control on, doing 70 down the road, I come to an incline and loose 15 MPH before she drops down into drive. The cruise is pullin my acc. pedal in, but seems way too slow of a response. Bondo has been through all his cruise components and still has the problem. I have a new cruise lever/swithch in mine.

Could it be something like the throttle position sensor? Or is the cruise a closed loop w/o the sensor? Sensor only feeds the ECM?

Ideas?

85 Burb and an 89 Blazer
One just for fun, one simply for pleazer
 
Probably just a quirk of the FI Blazer. Blazer's do tons of wierd stuff when its in good working order. In the morning the shoot out water and soot. The speedometer needle likes to shake. The wipers go on like they were possosed. The slip yoke on all the np-208/241s leak. I wouldn't be worried about it because you'll give yourself gray hairs from worry.

febbb8ba.jpg
 
Thanks Blazer Boy....it's probably the windshield wipers or speedometer causin it? Wow...no wonder under the dash looks like a birds nest of wiring....lol

See you're in SUX....go there for busineess on occasion. Love the smell of IBP...makes me hungry for hamburgers....

85 Burb and an 89 Blazer
One just for fun, one simply for pleazer
 
I have an '89 (Stock 350, 700R4, 3:73's, 33" tires) , and when I have cruise control on at that speed, it only drops 1-3 mph on a low to moderate incline. I don't know what's wrong with your cruise control, but it isn't functioning normally. Hope this gives you an idea of how it should perform. Good Luck.

Did a test for you guys today. I crossed the San Mateo Bridge (in the SF Bay Area), which is level from the Hayward side for the first 2/3rds, and then has a sizeable rise or hump in the last 1/3rd (The bridge is about 7 miles long). While on level road, in OD, I set the cruise at exactly 65 mph indicated (69 mph corrected for tire size error). On the rise I lost 2 mph (the torque converter unlocked), and on the way down the other side I gained 1 mph (the torque converter locked). When I reached the level stretch on the San Mateo side, it returned to 65 on the speedometer.

'89 K5 Silverado..."You can’t have a real adventure, unless the outcome is truly uncertain".<P ID="edit"><FONT SIZE=-1>Edited by Gandolf on 04/06/01 02:37 AM (server time).</FONT></P>
 
I would check all your vacume lines. You will be amazed at the difference if you go through and take the time to just go and get some new lines and replace the old ones. The cruise runs off some vacume lines... Later LittleJimmy....
 
Hey 89Blazin, check you other post on this subject, I just wrote a mini-novel there...

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Semper Maintenance!
 
I checked into this and they blame it on the tire size to gear ratio tweaking the speed readings for the cruise control to fix it they said to regear to the same rpm as stock and get a speedo gear.

88k5 silverado w4"lift soon to have <font color=orange>d44 and corp14ff</font color=orange> with 4.56s
 
I replaced all of the vacuum lines, the servo, and the turn signal lever/cruise switch in mine and none of it worked. The gearing thing won't work for me as I still have stock tires and gears. It has to be something else........
Any other ideas?

BONDO WILL FIX ANYTHING!!!
 
I would do a vacuum test on the servo... new or not... also would check the the entire vac system... does it hold when the symptoms hit? Hook a gauge up and go driving... Another thought is proper kickdown and throttle cables adjustments. Last of all I'd drive it while a metter is hooked up measureing the speed sensor if nothing else worked, but I would lean towards the other items.

ND55856.jpg

1987 Chevy K5 Blazer- 350 TBI
 
My new servo has been tested and is fine. The vac system tests fine as well. How do you check the kickdown and throttle cable adjustments?

BONDO WILL FIX ANYTHING!!!
 
electrically tested? mechanically tested? or vacuum tested... I suggest the latter. it may be leaking a slight bit. You can check the linkage by having someone nail the accelerator to the floor. The kick down is a TV and has a tab/braket you adjust al;l the way back.

ND55856.jpg

1987 Chevy K5 Blazer- 350 TBI
 
So you say when you get to an incline, it won't downshift? Perhaps it isn't your cruise causing the problem.....
Yes, cruise is pulling the acc pedal down to give the engine more revs to pull up the incline. But, you say it drops almost 15 mph before it downshifts? I'm afraid that sounds like a tranny problem. Cruise only holds the throttle plate at a preset position to maintain the associated engine rpm. This in turn, holds your speed. Cruise is nothing more than an automatic acc pedal. Try this:
On that same stretch of hwy, drive along as steady as you can without cruise. Just keep your foot on the gas. When you hit the incline, slowly apply the acc pedal until the tranny downshifts. I bet you'll lose about 15mph before it does.
If I'm anywhere near right(and I'm usually not), it sounds like the throttle valve is out of adjustment(I'm pretty sure the 700r4's had them) or the converter won't come out of lockup. My 84 700r4 is doing that now. No matter I have a diesel. When the mph drops, my tranny begins to shudder because it won't back out of lockup. Instead, I have to press the pedal until it downshifts to 3rd. If you can feel cruise pulling your pedal down, it's working fine. It only has one connection and that's to the throttle plate. My Ford AOD had a similar problem being reluctant to downshift.
 
No, my tranny works fine. It'll downshift geat when holding the pedal at the same spot with your foot when coming to a hill, just not with the cruise. Also, theres no vibration, shifts are smooth, no noises, etc. It has done it since I bought the truck two years and 50,000 miles ago. Seems if there was a tranny problem I would've known it by now. As for the servo being tested, it was both vacuum and electrically tested and checks out fine.

BONDO WILL FIX ANYTHING!!!
 
Ditto Bondo....great shifts, downshifts fine with cruise if the hill is steep (Bondo...u 2 ?).....a gradual hill with cruise seems my only nusance... I think my TC is staying locked.

85 Burb and an 89 Blazer
One just for fun, one simply for pleazer
 
Yeah, same thing with mine. Just seems to be some little quirk.

BONDO WILL FIX ANYTHING!!!
 
The TV cable need adjustment? Locked in OD maybe the engine just lugs until it finally downshifts .. it's a simple adjustment if its out and easy to check.
 
Does your tranny shift out of lock up, when accelerating or going up a hill? That is when out of cruise?
 
Has anyone solved the problem yet?

'89 K5 Silverado..."You can’t have a real adventure, unless the outcome is truly uncertain".
 
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