Cruise Control problems
Thread Starter
Registered User
Joined: Apr 2023
Posts: 17
From: PA
Cruise Control problems
My 1979 Ninety Eight with the standard 350, 4 bbl, has a working cruise control system that was optional on the car. The problem is the lunging and surging and trail-off of acceleration followed by more surges and luges, et.
Any insights into diagnosing and repairing the system would be appreciated.
Thanks in advance!
Any insights into diagnosing and repairing the system would be appreciated.
Thanks in advance!
Registered User
Joined: Jun 2009
Posts: 15,116
From: southeastern Michigan
Just off the top of my head, I would start by checking all vacuum connections to make sure there are no leaks. The servo unit is controlled by vacuum, and the transducer provides this vacuum, the amount of vacuum being determined by whether or not the throttle needs to be increased or decreased (going up or down a hill, say, or "resuming" the set speed from a slower speed). The amount of vacuum is adjusted by the transducer metering a small mount of air into the vacuum line, as needed, to increase or decrease the vacuum felt by the servo. If there are any leaks in the vacuum lines, this could hinder the transducer's ability to control the vacuum, and it may keep trying to compensate by bleeding in more or less air as it fights the leak.
If there are no vacuum leaks, I might check the servo unit itself, which is the vacuum diaphragm assembly that is actually connected to the throttle linkage. If there is a leak through the diaphragm, that would have the same effect as a vacuum leak anywhere else, I would think.
If there are no vacuum leaks, I might check the servo unit itself, which is the vacuum diaphragm assembly that is actually connected to the throttle linkage. If there is a leak through the diaphragm, that would have the same effect as a vacuum leak anywhere else, I would think.
Registered User
Joined: May 2009
Posts: 2,179
From: Northern California
I don't know the late 70's system, but the mid-60's system was fed its speed signal from the front left wheel via a mechanical spiral cable. At least in those years, if the cable didn't spin freely, it would cause an inconsistent speed signal to the unit.
Your car must have some kind of wheel speed cable - either from the transmission or maybe even in that late date from the wheel. Probably the trans.
If so, disconnect the cable at the transmission and the cruise head and see if it spins freely. If it doesn't that may be part of your problem. I use "Powerlube" spray oil down the cable housing on my car to lubricate the spinning cable.
Where ever your cable goes, sometimes they come unwound and bind or you get a loose strand and don't spin freely. The best fix in that case is to replace the cable.
Hope that helps
Chris
Your car must have some kind of wheel speed cable - either from the transmission or maybe even in that late date from the wheel. Probably the trans.
If so, disconnect the cable at the transmission and the cruise head and see if it spins freely. If it doesn't that may be part of your problem. I use "Powerlube" spray oil down the cable housing on my car to lubricate the spinning cable.
Where ever your cable goes, sometimes they come unwound and bind or you get a loose strand and don't spin freely. The best fix in that case is to replace the cable.
Hope that helps
Chris
Old(s) Fart
Joined: Mar 2007
Posts: 50,539
From: Northern VA
I don't know the late 70's system, but the mid-60's system was fed its speed signal from the front left wheel via a mechanical spiral cable. At least in those years, if the cable didn't spin freely, it would cause an inconsistent speed signal to the unit.
Your car must have some kind of wheel speed cable - either from the transmission or maybe even in that late date from the wheel. Probably the trans.
If so, disconnect the cable at the transmission and the cruise head and see if it spins freely. If it doesn't that may be part of your problem. I use "Powerlube" spray oil down the cable housing on my car to lubricate the spinning cable.
Where ever your cable goes, sometimes they come unwound and bind or you get a loose strand and don't spin freely. The best fix in that case is to replace the cable.
Hope that helps
Chris
Your car must have some kind of wheel speed cable - either from the transmission or maybe even in that late date from the wheel. Probably the trans.
If so, disconnect the cable at the transmission and the cruise head and see if it spins freely. If it doesn't that may be part of your problem. I use "Powerlube" spray oil down the cable housing on my car to lubricate the spinning cable.
Where ever your cable goes, sometimes they come unwound and bind or you get a loose strand and don't spin freely. The best fix in that case is to replace the cable.
Hope that helps
Chris
Oldsdruid
Joined: Dec 2007
Posts: 10,600
From: Southside Vajenya
Check all the vacuum hoses first. Then check transmission to transducer speedometer cable for correct routing with no kinks or burns.
Actually your unit has all the symptoms of a bad transducer and, if it's a resume system, a wonky resume solenoid. Rebuilt units used to be available; DK about now as the last one I replaced was at least 12 years ago, for the same symptoms.
Actually your unit has all the symptoms of a bad transducer and, if it's a resume system, a wonky resume solenoid. Rebuilt units used to be available; DK about now as the last one I replaced was at least 12 years ago, for the same symptoms.
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