[iluvmyoldmerc]

Hi All, I have a 1990 300TE with an intermittent starting problem. New battery, cranks fine but sometimes refuses to start. Leave it for an hour try again and usually starts.
I have also had an intermittent problem with ABS warning light coming on.
The ABS warning light problem has been happening for over a year with no impact on starting or performance.
The intermittent starting problem has only been occurring for the past few weeks.
I don't know if this is relevant but it is summer in Australia, so it is not an extremely cold start, plus this may be coincidence but it seems worse in wet weather.
I searched the forums and common theme with starting and ABS problems was the OVP.
I checked the fuse in the OVP and it was OK.
I followed Jimarilo's link to OVP diagnosis
Mercedes Overload or Over Voltage Protection Relay - Hard Start
With the ignition on pin 30 12V, pins 87 0v.
Before I fork out my hard earnt cash on an OVP does this confirm the OVP is blown?
Would it be safe to short pins 30 & 87 to bypass the OVP just while starting the car? Doesn't sound like a good idea so I won't attempt it unless someone out has successfully tried this.
Is there anything else I should check?
Thanks
Stranded

 

[QuangsBenz]

Hi All, I have a 1990 300TE with an intermittent starting problem. New battery, cranks fine but sometimes refuses to start. Leave it for an hour try again and usually starts.
I have also had an intermittent problem with ABS warning light coming on.
The ABS warning light problem has been happening for over a year with no impact on starting or performance.
The intermittent starting problem has only been occurring for the past few weeks.
I don't know if this is relevant but it is summer in Australia, so it is not an extremely cold start, plus this may be coincidence but it seems worse in wet weather.
I searched the forums and common theme with starting and ABS problems was the OVP.
I checked the fuse in the OVP and it was OK.
I followed Jimarilo's link to OVP diagnosis
Mercedes Overload or Over Voltage Protection Relay - Hard Start
With the ignition on pin 30 12V, pins 87 0v.
Before I fork out my hard earnt cash on an OVP does this confirm the OVP is blown?
Would it be safe to short pins 30 & 87 to bypass the OVP just while starting the car? Doesn't sound like a good idea so I won't attempt it unless someone out has successfully tried this.
Is there anything else I should check?
Thanks
Stranded

I got an 89 300E with similar issue. While waiting for my engine temp sensor and fuel pump relay to arrive I decided to open up the OVP relay and see what's up. I used the solder iron and reheated all the solder joints. We'll find out what happens soon.

You might want to do the same if you want.

Here's the image:

 

[iluvmyoldmerc]

The accidental mechanic to the rescue.
My usual trick of waiting an hour and trying again didn't work and I flattened the battery.
I recharged the battery and went to check the voltage on the OVP.
I accidentally shorted two of the terminals for a split second but I'm not sure which ones.
It didn't blow the OVP fuse so, fearing the worst, I tried starting the car and it started first time.
And, guess what . . . The ABS warning light went out!!
I drove it round the block and it seemed fine.
I'll wait and see how it is behaving tomorrow before I attack the OVP with a soldering iron, but thanks for the suggestion.

 

[alexnatt]

could also be the crank positioning sensor.

 

[QuangsBenz]

could also be the crank positioning sensor.

Apparently, there 2 of them on my car. I'm not sure how many there are on the 300TE model. The front one looks easy to remove. Does anyone know if the rear one is also easy to remove?

 

[Clarkz71]

A 300E will start even with the OVP relay disconnected.

 

[QuangsBenz]

A 300E will start even with the OVP relay disconnected.

I need a second opinion on this. Will it start when the engine is cold, left over night, with the OVP relay removed?

 

[Clarkz71]

Yes. Get a second opinion if you wish however I am a
certified MB tech and have tried this.No start is either no spark or no fuel pressure.

Possible crank sensor.

 

[Guest]

:thumbsup:Listen to this man!!

 

[QuangsBenz]

I got an 89 300E with similar issue. While waiting for my engine temp sensor and fuel pump relay to arrive I decided to open up the OVP relay and see what's up. I used the solder iron and reheated all the solder joints. We'll find out what happens soon.

I put the OVP in this morning after resolding all the joints last night. The stupid car still doesn't want to start. I wired up 2 wires, one to pin 30 and the other to 87L, and carefully put the relay back with the wires hanging outside. With the voltage meter I checked pin 30 and it read 12 volts. With the ignition key off, pin 87L read 0 volts, and read 12 volts with ignition on position II. That looked like my OVP is functioning, ain't it?

Then with the ignition key off, I connected the 2 wires together I could hear some clicking noise from the ABS and ICV.

I just called a local autoparts place and they have one in stock for about $60. I'm going to pick it up and see if it's going to help.

They're telling me that the one they have has 2 fuses with 9 pins. Mine has 1 fuse and only 7 pins. They verified that it's for my car, 89 300E. Could this be an updated one?

 

[QuangsBenz]

I just looked it up on Autohaus Arizona and it looks like the updated OVP relay has 2 fuses and 9 pins.

 

[slk230red]

I need a second opinion on this. Will it start when the engine is cold, left over night, with the OVP relay removed?

No OVP relay? Yes, it will start.

Defective or unplugged CPS? NO, it won't start.

Did you read post #4 & 7? You proved your OVP was putting battery voltage on pin 87 when the ignition is on. Next, you should have checked the resistance on the Crank Shaft Position Sensor. Then, check the output AC voltage of the CPS while someone is cranking the engine.
If your Ignition Control Module is not receiving correct output from the CPS, you will have weak or no spark.

My CPS test:

Cold: 814 ohms, .842v.
Hot: 900 ohms, .986v.

 

[QuangsBenz]

No OVP relay? Yes, it will start.

Defective or unplugged CPS? NO, it won't start.

Did you read post #4 & 7? You proved your OVP was putting battery voltage on pin 87 when the ignition is on. Next, you should have checked the resistance on the Crank Shaft Position Sensor. Then, check the output AC voltage of the CPS while someone is cranking the engine.
If your Ignition Control Module is not receiving correct output from the CPS, you will have weak or no spark.

My CPS test:

Cold: 814 ohms, .842v.
Hot: 900 ohms, .986v.

I just put in a new OVP relay and it still doesn't want to start. It feels as if it wants to start but it never completes start. Aren't there two CPSes? According to online catalogs there are two of them, but I only see the front one on my car. Does your CPS test apply to the front or back CPS?

 

[QuangsBenz]

Yes. Get a second opinion if you wish however I am a
certified MB tech and have tried this.No start is either no spark or no fuel pressure.

Possible crank sensor.

Online catalog Your Parts Search Returned 2 Part(s) shows that my car has 2 of them. Which one are you referring to? I hope it's the front one since it's easier to access and cheaper. I couldn't find the other one on my car.

 

[slk230red]

Online catalog Your Parts Search Returned 2 Part(s) shows that my car has 2 of them. Which one are you referring to? I hope it's the front one since it's easier to access and cheaper. I couldn't find the other one on my car.

No, it's the rear one. Look at your Ignition Control Module and you'll see a coax cable that goes to the rear of the engine. It connects to the upper left hand corner of the ICM in this photo:

 

[slk230red]

I just put in a new OVP relay and it still doesn't want to start. It feels as if it wants to start but it never completes start. Aren't there two CPSes? According to online catalogs there are two of them, but I only see the front one on my car. Does your CPS test apply to the front or back CPS?

At least test the CPS....the rear one. We all hate throwing parts at a problem. Also, did you go over all of your vacuum hoses, especially the ones connected on both sides/ends of your Idle Control Valve?

 

[QuangsBenz]

At least test the CPS....the rear one. We all hate throwing parts at a problem. Also, did you go over all of your vacuum hoses, especially the ones connected on both sides/ends of your Idle Control Valve?

I double-checked all the vacuum lines I could find. They are all okay. I'm going to test the CPS and see what happens.

Thanks,
Quang

 

[slk230red]

I double-checked all the vacuum lines I could find. They are all okay. I'm going to test the CPS and see what happens.

Thanks,
Quang

Especially check the vacuum hose that goes under the air intake.

 

[QuangsBenz]

No OVP relay? Yes, it will start.

Defective or unplugged CPS? NO, it won't start.

Did you read post #4 & 7? You proved your OVP was putting battery voltage on pin 87 when the ignition is on. Next, you should have checked the resistance on the Crank Shaft Position Sensor. Then, check the output AC voltage of the CPS while someone is cranking the engine.
If your Ignition Control Module is not receiving correct output from the CPS, you will have weak or no spark.

My CPS test:

Cold: 814 ohms, .842v.
Hot: 900 ohms, .986v.

Okay, I tested the CPS and here's what got. The resistance is 824 ohms and the voltage is about 0.4 AC volts. My crappy meter only shows tenths for AC voltage. For DC it can display thousandths, but I had to use AC for this test. So my CPS gets right about half of what it's supposed to. Does it mean it's hosed?

I pulled a plug, connected it to the wire, and held it to the metal while had an asistant crank the car I noticed sparks but I thought the sparks were weaker than I saw on a different car. Maybe this is a sign of a bad CPS?

 

[slk230red]

Okay, I tested the CPS and here's what got. The resistance is 824 ohms and the voltage is about 0.4 AC volts. My crappy meter only shows tenths for AC voltage. For DC it can display thousandths, but I had to use AC for this test. So my CPS gets right about half of what it's supposed to. Does it mean it's hosed?

I pulled a plug, connected it to the wire, and held it to the metal while had an asistant crank the car I noticed sparks but I thought the sparks were weaker than I saw on a different car. Maybe this is a sign of a bad CPS?

I got an 89 300E with similar issue. While waiting for my engine temp sensor and fuel pump relay to arrive I decided to open up the OVP relay and see what's up. I used the solder iron and reheated all the solder joints. We'll find out what happens soon.

I see you replaced your fuel pump relay, are the fuel pumps operating?
When you pulled the spark plug to check for spark, was the plug wet with fuel?