924Board.org

[Morrie]

Hi folks,

I am tinkering with my car, trying to get cruise control to work. I note that my shift light does not work, glows very dim all the time. My Cruise will not set or do anything at all. Finally, my MPG meter does work, but tends to swing to 0 with even the slightest tap of the gas, then back to over 30 when releasing the gas. My tach and speedo work fine. I have checked and reflowed all the solder joints in the cruise computer, but it certainly could still be bad.

My question is, there is apparently a hall effect sensor in the right front wheel that delivers a signal to the cruise computer telling it you are traveling in excess of 25mph. IF that were bad, it would explain the cruise not engaging. My question is, does the speedometer use the same sensor? How about the MPG meter and shift light?

My thinking is that if the speedo uses a different source, then the MPG meter could be operating in GPH mode all the time, the shift light would likely not work properly, and the cruise would not engage.

I'd appreciate any help from those who have chased these circuits more than I.

Best Regards,

Morrie
87 924S
5 speed, Guards Red with Black interior, bone stock.

[gohim]

The speedo is driven by a cable driven by the left front wheel. Not the right front wheel, and not Hall-Effect.

Factory Cruise Control is vacuum operated, not controlled by wheel speed driven off the front wheels.

MPG meter is a worthless do nothing. It operates strictly off vacuum, on many 87s, the MPG meter is non-functional, as Porsche introduced the components of the MPG system in three stages in production. Some cars got some of the parts, some none of the parts, and some all of the parts.

[masterdave]

my shift light glows also but works, the glow is simply from a small amount of electricity getting through but when its on you will see its on. The shift light comes from its own circuit in the computer that measures acceleration. it actually has nothing to do with RPM as many people think. Chances are the circuit that controls it is bad. or the wire that drives it is bad, the other issue with the 924s which i also have, is that its 924 gauges on a 944 computer and harness so when consulting schematics they will not match, the tack is driven by a red wire with a green chaser how ever that connects to a red wire with yellow chaser out of the comp. check the connections but sounds like the computer has an issue.

[Morrie]

Hi Gohim,

Here is what I do know for sure....

Speedometer: You are spot on. I looked at my spare speedometer. It is cable driven. This answers my first question. The speedo can work without the tach signal or hall effect sensor.

Factory cruise control: Not vacuum driven. Servo under hood in front of the battery actuates a cable that drives a bell crank behind the accelerator pedal. This system is same as an 85.5 and earlier 944. It takes data from the tach signal, and the HALL EFFECT SENSOR on the back of the speedo. (mine was not hooked up)

MPG meter is more complex and uses EFI input, speed, and a couple others.

Now that the hall sensor is hooked up, everything works... I am thrilled!!

Morrie

87 924S

[mehall]

Where is the hall effect sensor? Any pics or web info avail?

I have the parts to add cruise, but haven't started with it. I do know the entire infrastructure is there for it, so I'm gonna put it in after doing some much needed electrical repairs. (old harness, loose/dirty grounds)

Thanks
_________________
Mark

'87 White 924s manual
'94 Lexus LS400

www.kcws.org

[Morrie]

The sensor is attached to the back of the speedometer with two philips head screws and is located just above the cable connection. It is a small green pc board with two wires coming off of it. These wires come across and attach to the tach connector's exposed pins not used by the main connector, but within the same housing.

I just completed the retrofit of cruise control to my car so I can answer any questions you might have. I can tell you it is pretty straightforward so long as you have a little help with how to and where to stuff.

Morrie

[mehall]

Thanks for the offer. I want to get it installed, but I have a few other projects that have higher priority. (FOeS, heat is stuck ON, etc.)

The car runs good enough for daily driving, but I wouldn't take it on the track yet.

I'll dial back in when I'm ready. I will go through the stuff I have and make sure I have everything. At least I'll have more confidence.

How many hours do you think you put into this project?

Mark
_________________
Mark

'87 White 924s manual
'94 Lexus LS400

www.kcws.org

[Morrie]

Actual installation with what I learned should be about 1 hour. Took me more than 4 hours but that was the first time through.

[mehall]

Thanks. They man who sold me the pieces/parts gave me a basic intro to it, but he's a track car builder who strips the cars clean. He doesn't normally install the parts.
_________________
Mark

'87 White 924s manual
'94 Lexus LS400

www.kcws.org

[Morrie]

I take it you got the parts from my friend Doug?

[mehall]

Actually, I got most of the parts from a local race car builder in Kansas City.

They usually don't try to save those parts unless they have a specific reason.
_________________
Mark

'87 White 924s manual
'94 Lexus LS400

www.kcws.org