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Sp!ke
Administrator
Ian at 124 works fixed it so he's most qualified to answer this one.
Ian B Walker
MB Enthusiast
- Joined
- Jun 7, 2003
- Messages
- 7,668
- Location
- Freckleton
- Car
- 300-24, saloon and coupe, E300 24 valve diesel estate
Rob has the same problem. With Sp!kes it was just a plain change of cold start valve if my memory serves me correctly. I have looked at Robs and everything seemed ok, changed the Cold start valve. However it has not cured the problem so I will need to test the input voltage. Other things I need to address are the OVP relay as the ABS light is on, Air bypass valve, again. Plug leads as they are looking a little tired and one of them is clagged up with sticky tape and fuel pressure.
kth286
MB Enthusiast
and Spike's ABS light ? Was that related ?
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Sp!ke
Administrator
Dont think so but it came back on soon after leaving Ians and its been the same since.
Its not the OVP as I've had it open and resoldered. Assuming its one of the sensors as the ABS light stays off until you reach anout 10mph.
Its not the OVP as I've had it open and resoldered. Assuming its one of the sensors as the ABS light stays off until you reach anout 10mph.
LastMinute
Active Member
I thought I'd fixed that!
We replaced the rear diff speed sensor and the ABS seemed to be fine.Like Ian, I'm pretty sure it was just the cold start valve that was causing the starting issues.
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Sp!ke
Administrator
You did... and then it broke itself again 
Over the last 4 years my ABS light has only ever not been on for the weeks surrounding MOT time. I'm so used to it that I have the image of the ABS light permanently etched into my eye.
Over the last 4 years my ABS light has only ever not been on for the weeks surrounding MOT time. I'm so used to it that I have the image of the ABS light permanently etched into my eye.
LastMinute
Active Member
It might be worth checking the wire from the speed sensor under the car... I seem to remember a slightly hurried application of self-amalgamating tape over the soldered joint as you were on your way to pick it up... maybe it's become damaged?You did... and then it broke itself again
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Sp!ke
Administrator
I will add it to my weekend to do list - thanks for the heads-up. :bannana:
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Sp!ke
Administrator
Just a follow up on this, I fixed my abs problem thanks to lastminute.
Yesterday I put the car up on stands and unplugged the rear abs sensor one end in the diff and the then at the other under the passenger seat base.
Ian replaced the sensor with a second hand unit some time ago and for some reason the sensor wire was cut and soldered to the existing feed down near the diff.
Anyway, with the sensor and cable on the bench I set out to find the cause. Sure enough, the heat shrink separating the two inner cables had worn through causing a short thus tripping the ABS light. I've now resoldered it and put copious amouts of insulation and for the first time in 6 months or so I have ABS.
Nice quiet wet road, stomp on the brakes and yep, its all working properly. Hopefully the splice will hold this time around.
I still can't quite fathom why it was fitted this way in preference to just plugging it in at both ends though.
Yesterday I put the car up on stands and unplugged the rear abs sensor one end in the diff and the then at the other under the passenger seat base.
Ian replaced the sensor with a second hand unit some time ago and for some reason the sensor wire was cut and soldered to the existing feed down near the diff.
Anyway, with the sensor and cable on the bench I set out to find the cause. Sure enough, the heat shrink separating the two inner cables had worn through causing a short thus tripping the ABS light. I've now resoldered it and put copious amouts of insulation and for the first time in 6 months or so I have ABS.
Nice quiet wet road, stomp on the brakes and yep, its all working properly. Hopefully the splice will hold this time around.
I still can't quite fathom why it was fitted this way in preference to just plugging it in at both ends though.
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Sp!ke
Administrator
Guess what....
I got my cold start problem back again.
It feels like fuel... almost like Ive ran dry and am having to pump fuel all the way up from the tank. It needs about a minute of cranking before it fires and when it does eventually fire, it doesn't fast idle.
Looks like I'll be under the bonnet this weekend trying to work it out. Failing that, I have a Boch specialist centre up the road from work, who hopefully will be able to pinpoint this with ease.
I got my cold start problem back again.
It feels like fuel... almost like Ive ran dry and am having to pump fuel all the way up from the tank. It needs about a minute of cranking before it fires and when it does eventually fire, it doesn't fast idle. Looks like I'll be under the bonnet this weekend trying to work it out. Failing that, I have a Boch specialist centre up the road from work, who hopefully will be able to pinpoint this with ease.
television
Hardcore MB Enthusiast
What year is your car I have the service books in print form up to 1990 and I am trying to copy off
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Sp!ke
Administrator
1991
grober
MB Master
On the early cars the cold start valve and possibly the older fashioned auxiliary air device are powered via one of the connections in the the fuel pump relay so I would check this out, as you have already checked the OVP. The fuel pump relay may still power the fuel pump OK but not the cold start devices!
Spike,
I've still got that spare OVP relay sitting here if you ever wanted to try it? (I guess with the ABS working it probably wont be that) Or you could try a fuel pump relay etc from another car (Howard?)
Does sound as though it's not running 'closed loop', might be chugging away on the injection purely mechanically - ie, no electrical assistance (cold start, idle stabilisation etc)
Good luck,
Will
I've still got that spare OVP relay sitting here if you ever wanted to try it? (I guess with the ABS working it probably wont be that) Or you could try a fuel pump relay etc from another car (Howard?)
Does sound as though it's not running 'closed loop', might be chugging away on the injection purely mechanically - ie, no electrical assistance (cold start, idle stabilisation etc)
Good luck,
Will
hi this sounds like a faulty one way valve in the fuel pump , thus having to crank fuel up over a period of a few engine turn over cycles before firing and then when it does runs rough and takes a while to stabilize
the valve screws into the fuel pump and is available seperatly although it does also come with a new pump , regards kevin
the valve screws into the fuel pump and is available seperatly although it does also come with a new pump , regards kevin
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Sp!ke
Administrator
Interesting theory Kevin... could be I guess, although once it fires its fine.
LastMinute
Active Member
Sounds like I caused your problem...
Apologies for doing such a gash job.
That's interesting - and a little concerning...
It wasn't heat-shrink but chloroprene tubing that I used. I'd have expected it to give good mechanical protection, but maybe it doesn't work well in that environment?
IIRC it was because the replacement sensor we used had been cut off the donor car.I still can't quite fathom why it was fitted this way in preference to just plugging it in at both ends though.
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Sp!ke
Administrator
Today I gave up trying to start the old girl after ten minutes of trying (seems to be worse when its damp or rainy) and resorted to the trusty limo to get to work. I love that old girl 
So I've been doing some research on the web and its really surprising how many threads google throws up actually lead to MBclub. It just shows how big this forum has become. Certainly up in the top 3 Benz forums worldwide if google results are anything to go by.
Anyway I digress, having already checked for a good spark and correct timing my chief suspect is the cold start valve. As I understand it, this valve should receive a 12v feed and supply extra fuel assuming certain conditions are met. Feel free to correct me if my thoughts on the subject are incorrect in anyway.
1. Temperature sensor reports low ambient temperature
2. OVP Relay is providing voltage
3. Fuel pump relay is behaving
So the quick and dirty test would be to apply 12v to the cold start valve and see if it starts - this would at least prove I'm not barking up the wrong tree if it fires up?
If it fires, go to A) and if not go to B)
A)
Check if there is a 12v supply to the cold start valve with the starter motor turning.
If not, bypass fuel pump relay (somehow?) and check again
If relay bypassed and still no 12v suspect thermoswitch
Test thermoswitch (somehow?)
B) In the absence of anything else, suspect cold start valve itself is faulty; unscrew CSV and crank engine whilst carefully checking fuel delivery into a container (be careful of fires). No fuel, replace cold start valve
So assuming my intended trouble shooting technique is not totally flawed, I still have two somehows.
1. How to bypass the fuel pump relay (which pins to short)
2. How to test the thermo switch (resistance reading at different temperatures perhaps?)
Anyone care to share further thoughts on the matter?
So I've been doing some research on the web and its really surprising how many threads google throws up actually lead to MBclub. It just shows how big this forum has become. Certainly up in the top 3 Benz forums worldwide if google results are anything to go by.
Anyway I digress, having already checked for a good spark and correct timing my chief suspect is the cold start valve. As I understand it, this valve should receive a 12v feed and supply extra fuel assuming certain conditions are met. Feel free to correct me if my thoughts on the subject are incorrect in anyway.
1. Temperature sensor reports low ambient temperature
2. OVP Relay is providing voltage
3. Fuel pump relay is behaving
So the quick and dirty test would be to apply 12v to the cold start valve and see if it starts - this would at least prove I'm not barking up the wrong tree if it fires up?
If it fires, go to A) and if not go to B)
A)
Check if there is a 12v supply to the cold start valve with the starter motor turning.
If not, bypass fuel pump relay (somehow?) and check again
If relay bypassed and still no 12v suspect thermoswitch
Test thermoswitch (somehow?)
B) In the absence of anything else, suspect cold start valve itself is faulty; unscrew CSV and crank engine whilst carefully checking fuel delivery into a container (be careful of fires). No fuel, replace cold start valve
So assuming my intended trouble shooting technique is not totally flawed, I still have two somehows.
1. How to bypass the fuel pump relay (which pins to short)
2. How to test the thermo switch (resistance reading at different temperatures perhaps?)
Anyone care to share further thoughts on the matter?
Last edited:
Number_Cruncher
Active Member
The thermo switch should be just that, i.e., a switch.
When cold, the switch is closed, and current can flow.
However, the combination of the heating effect of the current flowing through a winding in the switch, and the coolant temperature makes the bi-metal leaf in the switch move to open the contacts. Hence, you get a longer duration of cold start injection when the temperature is really low.
However, I would be really surprised if the cold start / thermo time switch is the sole cause of your poor starting problem. Extra cold start fuel is usually only needed in *really* cold conditions, and most of the time in the UK, you should start just fine without it.
Remember that most of the reason that old cars needed choke was fuel condensing in the manifold - with injectors spraying onto the back of the inlet valves, the need for cold start enrichment is vastly reduced, and in most UK cases, the enrichment required is handled very well by the warm up regulator.
What I would do instead, is pull out an injector, and plug the hole in the manifold with something solid and secure, to make a seal. Point the injector into a bottle, and try to start from cold. Is fuel being sprayed or not?
In a similar vein, pull off a plug lead, and rig it so you can see if there's a spark while cranking from cold - is there a spark or not?
Answering these extremely basic questions will get you a long way towards finding the fault. IMO, the cold start injector is a red herring.
When cold, the switch is closed, and current can flow.
However, the combination of the heating effect of the current flowing through a winding in the switch, and the coolant temperature makes the bi-metal leaf in the switch move to open the contacts. Hence, you get a longer duration of cold start injection when the temperature is really low.
However, I would be really surprised if the cold start / thermo time switch is the sole cause of your poor starting problem. Extra cold start fuel is usually only needed in *really* cold conditions, and most of the time in the UK, you should start just fine without it.
Remember that most of the reason that old cars needed choke was fuel condensing in the manifold - with injectors spraying onto the back of the inlet valves, the need for cold start enrichment is vastly reduced, and in most UK cases, the enrichment required is handled very well by the warm up regulator.
What I would do instead, is pull out an injector, and plug the hole in the manifold with something solid and secure, to make a seal. Point the injector into a bottle, and try to start from cold. Is fuel being sprayed or not?
In a similar vein, pull off a plug lead, and rig it so you can see if there's a spark while cranking from cold - is there a spark or not?
Answering these extremely basic questions will get you a long way towards finding the fault. IMO, the cold start injector is a red herring.