Cruise control issue - intermittently stops and starts

Lumps
1

Hi guys,

My cruise control has developed an odd situation; I’ll be cruising along at 60-65 and suddenly the cruise control will drop out, then turn back on, drop out and eventually cut out completely.

The issue seems to happen when the car has to go uphill (or is going downhill), thereby modifying how the module is adjusting the throttle… but it may just be a coincidence.

I’m using a GM cruise control module; the system has been bulletproof for several years. I don’t know why this would happen. Does anyone out there have any thoughts on this?

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Tyler:

Don’t stay away so long. Kinda getting lonely around here. Good in way, the lumps are OK!!!

I suspect a loss of vacuum in some way. No vacuum, no pull on the throttle cable by the bellows.

Or a stray ground wire, Simulates brake application and opens the power circuit to the CC.

Decades ago, the after market CC I installed on my IHC Scout II “kicked out” on climbing a rather steep hill. I thought a bout it and realized, lots of throttle and slowing speed depleted vacuum. Should have dropped down a gear.

Carl,

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Ha, yeah! I’ll try not to - been focused on my TR lately and the Jaguar is stored out in the country so I don’t get to drive it as much as I’d like… so less problems to report!

The GM unit isn’t vacuum operated, instead it’s operated with a stepper motor. It was working fine and then suddenly… started giving me problems. I suspect it has something to do with the readings it’s getting for speed - perhaps a faulty reading from the VSS - so the cruise control doesn’t know what to do and then just cuts out. But I have no idea…

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@Ray_Livingston… any hints?

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Another idea away from the complexities of
pulse matching. although so long as the pulses are steady, the CC should react. A clue might be in the speedo. Steady needle or erratic. Each relies on VSS.

But, I sense an issue at the brake light. errant closing of the circuit, kicks out CC!!

Carl

6

The thread is a year old, but I finally figured it out, along with a couple other problems I kept having.

I logged my data and it would always come back with strange LT/ST fuel trims. The car ran OK, so I never really thought much of it, until the cruise control started giving me trouble.

Then I noticed my fuel pump was whining. I replaced it. Since I don’t get a chance to drive the car that often, I don’t put too many miles on it and it was weird that I already burned through a pump. Then that second pump went bad after about 20 miles.

These were cheap pumps mind you, so I figured OK, I’ll try one more time… but I’m going to replace all the fuel filters first. The filters were all fine, the lines were all fine, clean gas.

But I did get to thinking about my data; the LT/ST fuel trims only were problematic when the car was accelerating and under load.

I purchased a fuel pressure gauge from Harbor Freight; sure enough, at idle, the pressure was fine… but I decided to tape the gauge to the window and took the car out for a ride.

Ah Ha! I was getting pressure drops under load, which coincided with the cruise control dropping out. I assume the LT/STs went haywire due to the pressure drop. Because the OBD1 system does not have a trip code for low fuel pressure, I never threw a code.

Now to figure out why I was getting low fuel pressure… and this was 1 of 2 things, or so I thought. Fuel pressure regulator or bad pump.

I had a brand new pump on there, so it couldn’t be that. I had a spare fuel pressure regulator and swapped it on to see if it made a difference, and STILL kept getting the same problems.

Right when I was about to give up, I remembered something - the fuel tank.

About 10 years ago, I had sealed the drivers-side fuel tank with a Por-15 gas tank sealant. I thought to myself, hmm… I wonder if I clogged something with the sealant. So I did a little poking around. And found my problem.

Drumroll…

There is a small piece of rubber fuel line which connects to a metal line, which then connects to another rubber line in the trunk which then goes to the tank switch module, then fuel pump, then lines, etc. The FIRST piece of rubber line, going from the nipple on the tank to the metal line which goes into the trunk had been KINKED.

the tank either shifted slightly (the wedge shim must have moved) or the hose had softened over the years and kinked. This caused a fuel starvation issue with the pump, which is why the pumps kept going bad.

Anyways, long post… but just in case this happens to someone else (weird LT/ST readings coupled with fuel pump failures), check this area to make sure the rubber hose from the tank to the metal line is free from kinks. You can’t see it unless you go under the car - it’s hidden in the gas-tank well.

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