87 300d turbo - cruise question for someone technical
 

ive been thru like 5 cruise amps, resoldered them an all. no luck getting my cruise to work.

i tested actuator and it works great. im wondering if my speed sensor is screwed. i replaced the one mounted to the speedo, no luck. but i notice the wires go from that speed sensor to a hall sendor mounted behind the cluster. starting to think that is screwed up.

i hooked up a multimeter to green wire with yellow stripe - the speed wire. tried to read frequency, but it just bounces all over the place. ive heard it should read in hz roughly same in kph. so 100 kph sould read 100 hz. im definitley not getting anything readable coming off the multimeter.

i also notice there is like 3 wires coming out of hall sender, all green with yellow stripe. i think 1 goes to ACC, 1 goes to cruise, 3rd goes i dont know where, maybe radio or something? the 3rd mystery wire is disconnected since PO installed a aftermarket radio.

Questions -

1. where do the 3 green/yellow wires go off the hall sender? does it matter that 1 is not connected?

2. should all 3 of those wires coming off hall sender be reading same frequency?

3. is there some trick to hooking up a multimeter to read frequency? am i doing something wrong here? I probe the positvie lead to the green/yellow wire and negative to a ground.

4. can someone verify what frequency readings should be coming off that speed wire?

5.given fact ive spent countless hours on cruise and not gotten it working, is there an aftermarket cruise system i can hook up and just use the MB actuator? I know audiovox makes a universal thats always for sale on ebay for like $100.

if the dam gas pedal didnt weigh 50 lbs i wouldnt care so much, but its down right tiring to drive this thing for long periods of time!

thanks!!

Paul
pi2060@yahoo.com

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I'm not familiar with your particular car, but Hall effect sensors themselves don't put out much of a usable signal on a multimeter - they are cleaned up and amplified before distribution. The amplified pulse on my car(the same era) is about 10-12V, a fairly clean pulse. I THINK, but don't know for certain, that 1 rev of the driveshaft would equal 1 pulse - a little math involving the rear gear and the circumference of the tires would turn that into MPH.

Does your speedometer work OK? Is it mechanical or electrical?

much thanks for reply!

The speedo in my 87 300d turbo is mechanical. There is sensor mounted in the cluster that reads a little metal wheel geared to the speedo cable. From there it sends signal to hall sender, from hall sender it goes out to cruise amp, several other places. Im trying to read freq coming off hall sender with no luck.

How would I verify speed senser and hall sender are both working as they should?

The speedo bounces a little up to about 40 mph but is stable after that. Odo not working though.

You're not going to like the answer to your question. :rolleyes: The best way to test the speed sensor is to have a good independant mercedes repair shop connect the cruse control tester and verify all of the circuits. The tester plugs into the harness in place of the amplifier. It will verify the power and ground, all of the switch positions, the speed sensor signal as well as the actuator. A cruise diagnosis should cost around an hr at the shop. Compare that to the price of replacing 5 cruise amps and you may realize that DIY can cost more sometimes, especially when trying to repair complex sub-systems. :greedy:

yeah you are right i dont like that answer :)

I already tested the combi switch - it is working, because i get proper voltage at the amp plug when going thru set, accel, decel, coast. I also already tested the actuator by jumping it in per the GDL website instructions. The actuator moves throttle cable just as it should.

Im just confused as to if the speed sensor and hall sender are sending correct signal to the amp, I know the rest is correct.

Ive got an arsenal of cruise amps now from various old benz at the junkyard (hell there $5 ea so I bought 5 of em). Nothing even remotely works, no signs of life.

What I really need to understand is :

what signal should be coming off the speed sensor ?

what signal should be coming off the hall sender going to amp?

and, how do you read that with a Fluke?

Im really considering the audiovox aftermarket cruise at this point, and just hacking the amp into the benz actuator system. I just really dont want to go taping magnets to drive axle if speed sensor is OK.

I really think its either the speed sensor, hall sender, or amp. If I can KNOW for a fact that speed sensor and hall sender are good, I will send cruise amp off to GDL for the $150 rebuild.

thanks for all help!!