OVP Relay

[The Big Man]

Hi Guys
Can anyone tell me where the tacho signal is taken from on a 300D W124 606 engine or have a wiring layout for OVP connections. The engine idles as if the ovp relay was removed but ABS light only comes on if I remove the OVP indicating it is switching.
What is controlled from the OVP that affects idle.

Thanks

Big Man

 

[grober]

The OVP feeds 1. the ISC module =idle speed control module and 2 the EGR valve/resonance intake manifold control module. (Think in the later models this is one combined unit) Either of which might effect your idle. In the petrol engined cars the OVP merely supplies 12V to the idle control unit and the engine ECU controls its action by a controlled path to earth thus regulating/switching the current passing thro it. I don't know for certain but I imagine the diesel ISC does something similar. Usually its the idle device (a variable electromechanical throttle plate bypass) goes faulty or gets gummed up ( it controls the idle by bypassing the closed throttle plate) but sometimes the control unit itself goes faulty.
I think the engine speed is derived from the crank position sensor triggered by the starter ring gear on the flywheel. This has a feed to the isc/egr/vrm control unit.

 

[The Big Man]

Thanks Graeme

The idle seems better once engine is warm although its hard to tell as the rev counter is now only working above 4k and getting worse. Is rev counter fed from the alternator or is this a myth?

 

[grober]

Thanks Graeme

The idle seems better once engine is warm although its hard to tell as the rev counter is now only working above 4k and getting worse. Is rev counter fed from the alternator or is this a myth?

It comes from the crank position sensor via the EGR part of the ISP/EGR/RIM control module. The feed to the instrument panel is designated as a GREEN insulated wire with a YELLOW Tracer in the circuit diagram . The closest I have come across an RPM feed from an alternator was a cable drive connection from the back of a dynamo on a Mark 1 Austin Healey Sprite! so probably doesn't count.
ADDED
It does sound as if you have a problem with circuitry down stream of the OVP. Check the supply voltage to the OVP and instrument panel are OK FUSES 5,6,7,9 in the main fuse box--- check for corrosion on the contacts. Remove the instrument cluster http://www.mercedesshop.com/Wikka/W124InstCluster and check the earthing point on the inner bulkhead is free from corrosion also. CHECK any earthing point for the control unit also---- Mercedes earth wires are usually brown.

 

[The Big Man]

Ok, thanks

Whats an Austin? LOL

Before my time

 

[grober]

I added a bit to the previous post of things to try.

 

[Number_Cruncher]

This could be a wiring harness problem.

Although the wiring from the crank sensor to the idle/EGR controller does not form part of the main engine wiring harness, the wiring between the idle speed controller and the actuator which is on the injector pump does form part of the main engine harness.

This harness can degrade, and the wires can short. On my wiring harness, this area under the inlet manifold, towards the rear of the engine was the worst affected area.

 

[grober]

This could be a wiring harness problem.

Although the wiring from the crank sensor to the idle/EGR controller does not form part of the main engine wiring harness, the wiring between the idle speed controller and the actuator which is on the injector pump does form part of the main engine harness.

This harness can degrade, and the wires can short. On my wiring harness, this area under the inlet manifold, towards the rear of the engine was the worst affected area.

Good point the car is probably the right age too.

 

[The Big Man]

Thanks guys

I shall check the wiring on the weekend. I have had the inlet manifold off recently to renew the glow plugs so maybe something has been disturbed

 

[Smiley]

Thanks guys

I shall check the wiring on the weekend. I have had the inlet manifold off recently to renew the glow plugs so maybe something has been disturbed

Let me know how you get on please.
I have had exactly the same experience with a 95 e300 diesel. Removed the im to replace glow plugs and ever since then a rough idle on cold. The starter motor also turns over more slowly when cold - never fails to start though and the battery is perfect. The idle is perfect within a short period of running. I have replaced all the fuel lines and connections but no joy. It has been like this for the last 50k miles (now on 200k) so finding a cure would be great - the good news is that it has not got any worse over that time.

 

[The Big Man]

Just an update

I disconnected the RPM sensor from the flywheel housing to see if I could get a reading across the terminals but it is a sealed unit and then found that the cable connection is up beside the fusebox. I was getting some variations in readings with the meter on AC volts then DC volts but nothing on Frequency as I would have expected. I traced the same cable on a friends 300D 603 engine a nd the terminals are beside the battery. With the engine running i disconnected his cable and straight away the car idle dropped like mine. I measured the frquency on his unit and the readings were way higher. I reconnected and idle improved

I have ordered the sender and shall post my results as soon as.

 

[Smiley]

Just an update

I disconnected the RPM sensor from the flywheel housing to see if I could get a reading across the terminals but it is a sealed unit and then found that the cable connection is up beside the fusebox. I was getting some variations in readings with the meter on AC volts then DC volts but nothing on Frequency as I would have expected. I traced the same cable on a friends 300D 603 engine a nd the terminals are beside the battery. With the engine running i disconnected his cable and straight away the car idle dropped like mine. I measured the frquency on his unit and the readings were way higher. I reconnected and idle improved

I have ordered the sender and shall post my results as soon as.

sounds interesting - where is this sender located exactly? And how much is a new one?
Just out of interest did your car turn over slowly when starting from cold?

 

[The Big Man]

The sender is located under the starter motor just before the bottom and a new one is £37+vat. Its easier to get at from the bottom to save removing the IM and 1 allen bolt holds it in place. Mine currently runs like a pig in the mornings with smoke everywhere untill warm, but this onlystarted around the time of the tacho and idling fault. I`m hoping the`re connected

 

[Smiley]

The sender is located under the starter motor just before the bottom and a new one is £37+vat. Its easier to get at from the bottom to save removing the IM and 1 allen bolt holds it in place. Mine currently runs like a pig in the mornings with smoke everywhere untill warm, but this onlystarted around the time of the tacho and idling fault. I`m hoping the`re connected

OK mine is not that bad - just a bit of a lumpy idle for 2 or 3 mins. More annoying than a big problem - would be nice to find the fault though.
Like you mine started after the IM came off and new glow plugs so I am convinced it is something that got disturbed in the process - it is too much of a coincidence.

 

[230K]

Hi

The Big Man is a neighbour of mine and he used my ramp to fit the sensor, i took a few pics, here goes.

Location just below starter

What the sensor looks like

New cable passing along bulkhead

Where it connects beside the brake master cylinder

Sensor fitted and rev counter working again and odle correct and less smoke at start up
Hope this helps someone in the future



230K