W124 Non Cruise Control - Is there a fuse to check first - PeachParts Mercedes-Benz Forum

Warren C · #1 10-24-2007, 01:13 PM

1987 300TDT

Just starting to diagnose my non-working cruise control.

I've read the GDL site and will follow the recommendations.

But I was just wondering,...when anything electrical doesn't work..the first thing to check is the fuse. Is there a fuse to check for the CC?

Warren

rrgrassi · #2 10-24-2007, 05:58 PM

I do not have the '87, but check your fuse box lable. Usually the CC is grouped with things like the radio, etc.

gsxr · #3 10-26-2007, 06:17 PM

This is 99.44% guaranteed to be a dead cruise control amplifier. You can check fuses, but there is not a fuse for the cruise alone... you would have other things not working also. Check the card in the fuse box, I forget which fuse number feeds the cruise circuit. But plan on getting a rebuilt cruise amp... I have yet to see a W124 with the original cruise amp still functioning (V8 models excepted, they use a different cruise system.)

prakersa · #4 10-27-2007, 08:49 AM

the fuse groups with brake lamp,
and another fuse is combine with a relay, it's behind your battery

Quote:

Originally Posted by Warren C

1987 300TDT

Just starting to diagnose my non-working cruise control.

I've read the GDL site and will follow the recommendations.

But I was just wondering,...when anything electrical doesn't work..the first thing to check is the fuse. Is there a fuse to check for the CC?

Warren

soypwrd · #5 10-27-2007, 09:06 AM

Quote:

Originally Posted by gsxr

This is 99.44% guaranteed to be a dead cruise control amplifier.

Ours is intermitently working. Might the amplifier be the culprit?

gsxr · #6 10-27-2007, 11:26 AM

The fuse in the OVP relay by the battery is a very rare failure. If the OVP relay is acting up, there will usually be more symptoms, like the ABS light on, or tach dead, or something. It's worth checking but don't get your hopes up.

Soy, yes, if the cruise is intermittent that is VERY likely to be the amp failing. Especially if it's original (there is a date code painted on the outside - check it out.)

psfred · #7 10-27-2007, 03:16 PM

Replace both stop lamps with Osram bulbs -- a bad stop lamp will cause the cruise control to fail because the ground for the control circuit is the stop lamp filaments. When you step on the brake, you cause the voltage drop across the control circuit to go to 0 V and it turns off (rather cleaver, I think). A stop lamp out or with too high a resistance can cause the CC to fail to engage.

Most likely it's the amplifier though -- the solder cracks and makes intermittant contact so that the actuator fails to hold speed (and may "switch" the CC on and off for you, too). Eventually it quits altogether. However, the actuators CAN to bad, and when they draw too much current, they kill the amplifier.

Quick fix is to pull the amplifier and re-solder (this will usually ruin it for a core, though) and re-install. If it fixes the CC, then fails again, likely you need to have the actuator tested for current draw, too.

Peter

TTCummins · #8 10-27-2007, 09:08 PM

What does the OVP Relay actually run? My cruise is working intermittently and my ABS light is on, I know it is probably a wheel speed sensor, but the relay could be another piece of the puzzle. Also, what does OVP stand for?

gsxr · #9 10-27-2007, 11:00 PM

OVP = Over Voltage Protection relay. It's supposed to save the computer brain boxes, ABS controller, etc from over voltage damage. But it also makes things act up when the OVP relay fails, or intermittently drops out.

mario 3 · #10 11-01-2007, 08:32 PM

Where is the cruise control amplifer located? '87 300TDT

babymog · #11 11-01-2007, 09:05 PM

Above the knee panel, to the left of the steering column, aluminum box.

mario 3 · #12 11-02-2007, 07:15 AM

Thanks, this clears things up for me. I was confused as to shat was what and where, thanks again.

88300D · #13 11-12-2007, 05:30 AM

Hi, i very recently grabbed an 87-88 300D, the cruise never works,
I was told that a guy with a commadore fitted led stops and his cruise stoped working, this fits with what you are saying and my 300D that has led stops.
do you know the wattage of the standard globes? that way i can set some up and try my cruise.
Also I am trying to follow another thread to work out my AC / tacho problem, it works some times, it has been suggested to me that it is the OVP but i cant find any relay in the batt compartment, do i need to remove the battery to find it?
I am also trying to locate the tacho amp and is sensor, there is a device on the back of the injector pump that may be the tacho sender?
If some one could help me it would make my day, I know i am prob asking what has been posted but being a newbee i dont seem to be winning.

gsxr · #14 11-12-2007, 10:08 AM

The OVP is behind the battery, behind the plastic splash shield. It has a red top with a GM-style fuse. If it's acting up, usually the ABS light will come on.

The tach sensor is near the starter, below the oil filter, and bolts in to the side of the tranny housing. It senses the flywheel movement. It has no connection to the IP. There is no amp either, the sender goes straight to the tach, AFAIK. The sender is a semi-common failure... I replaced mine several years ago and it fixed my intermittent tach & AC problem.

Brake bulbs - use the OE/OEM bulbs.

88300D · #15 11-12-2007, 05:06 PM

dave thanks sooooooooo much you have made it much easier for me, will attack it this weekend