• The Forums are now open to new registrations, adverts are also being de-tuned.

Correct resistance for ht leads?

Stringer

Active Member
Joined
Sep 16, 2012
Messages
144
Location
Lebanon
Car
300E 1987
Would i be correct in guessing that this is a disputed issue? I have read on other sites that a 1987 300e should have leads which have a 5k ohm resistance - but then others have said 1k.
I would be very grateful if someone could put my mind straight as im about to replace mine and they are all 1k ohm and very, very old. I still haven't solved my misfiring issue btw although i dont think it is either the coil (checked with the meter when hot) nor the ignition modul, which i cooled down with some freeze spray but with no change.
thanks indeed for your advice about the leads though.
M in Beirut
 
Not sure on the value your looking for but some Times if you pull off the ht lead from plug put it near the plug but not touching a few mm gap it will increase the resistance in the circuit and you will get rid of miss this will tell you if it's the lead of coil of that cylinder
 
thanks for that. I was actually talking about resistance of the ht leads themselves, i.e the measurement which uses ohms as a value
 
on my later 124 (1995) the wires themselves are straight through with NIL
resistance.

The end plugs which the leads screw into (and then the plugs fit onto the spark plugs) contain 2k ohmn resistors.

The Merc dealer parts dept make up the new leads (to appropriate length of each one) using the various components such as screw-in ferruls and end caps etc etc.

The actual leads themselves hardly cause a problem.

It is the end caps or resistor plugs that cause the problem, and you can buy all the seperate parts from the dealer to refurbish your own leads.

Give full VIN to dealer.
 
Hi KTH
that was enormously helpful. Tell me though, if i tested the leads on the ohmeter from one end to the other (including the plug), and it only reads about 1k, then would you say that was normal? If you say the plugs themselves should be 2k, then have mine lost their resistance?
 
I have genuine M-B leads on my W201 2.6 of similar vintage. I checked the resistance on them all and they were within a midges of 1k - handy that as that's the resistance stamped into the metal plug cap.
 
Would i be correct in guessing that this is a disputed issue? I have read on other sites that a 1987 300e should have leads which have a 5k ohm resistance - but then others have said 1k.
I would be very grateful if someone could put my mind straight as im about to replace mine and they are all 1k ohm and very, very old. I still haven't solved my misfiring issue btw although i dont think it is either the coil (checked with the meter when hot) nor the ignition modul, which i cooled down with some freeze spray but with no change.
thanks indeed for your advice about the leads though.
M in Beirut

Hi Stringer,

There are two types of HT leads that may have been used or fitted on your vehicle, depending upon whether or not they are in fact the originals as fitted from day one. The first one being the original 7mm silicone sleeve with the conductor wire constructed of only “tinned copper”, which has a negligible or zero resistive conductive core per metre in length and solely relies on the resistive plug cap as mentioned earlier by kth286 and ss201 for suppression only. This is the correct one for your vehicle by the way. The resistive value of the plug cap itself is actually1k ohm +/- 10%, or between 0.9k ohms - 1.1k ohms. If yours are within this range, then they are considered as serviceable, provided, the insulation of the lead itself is not “breaking down” resulting in “tracking” or loss of HT voltage.

The second type of lead is a 9mm silicone sleeve with Rayon strands - “impregnated with carbon as the conductor”; the value of this type of lead is considerably higher in comparison and is approx 25k ohms per metre length. (40 inches) and is not for your system at all.

The ignition coil, if you wish to test this to determine if you have the right one fitted anyway and its serviceable, should measure between 0.8 ohms to 1.2 ohms across the coils low tension (primary winding) terminals, pin numbers 15 and 1, if this value is OK and within the spec I’ve mentioned, then move any one of the test leads, in turn, from the multimeter to the centre coil tower to measure the (secondary windings). Pin number 15 and coil tower and then pin number 1 and coil tower. You may need to use a screwdriver or suitable object to insert down into the coil tower to make contact with the brass terminal at the bottom. The secondary winding values should be between 5k up to 9k ohms.

Note, the TCI coil should be cold during these initial tests, you can re-check these figures when the coil has warmed up after the vehicle has been run for approx 15 minutes, as often they do fail during warm-up periods. Alternatively, if you wish to speed up this process whilst the engine is running, then you can use a hair dryer to simulate the conditions under “bench test” processes by gently warming the coils casing. I said a hair dryer and not a hot air gun for paint stripping; you don’t want to melt the coil.

On a more serious note, do not “open circuit” a spark plug lead on an electronic ignition system to test if it is actually producing a spark, unless, you place an old serviceable spark plug in the end of the lead first, and put it to ground, if you do “open circuit” the system, then you will certainly increase the chances of inducing the HT voltage into the 12 volt primary circuit as it will track down the coil tower to any one of the low tension terminals on the coil and back to the ignition module, resulting in damage to the switching transistor and circuitry.

Generally speaking, this TCI ignition circuit does actually suffer, like many other sophisticated ignition circuits, from crankshaft position sensor failure, the output voltage produced from this component over time does in fact diminish resulting in non-start, bad starting or misfires. If you wish to test the sensor, then the value of that component measures 770 ohms to 950 ohms, although the resistance check in itself being within spec would not constitute this as being a good sensor, the output voltage alone would determine and be more accurate to evaluate this, once again, this would vary in comparison to “cranking” speed as opposed to actual “running” speed. You may wish to substitute it with a known working one.

I have also just read another thread of yours regarding the oxygen sensor issue, I can talk you through the testing process on this if you could tell me exactly how many wires are coming out of the rear of the sensor, although I’m certain it will be just the one and is therefore classified as a standard EGO (Exhaust Gas and Oxygen) - Zirconia Dioxide. ZrO2 unit. Have you now resolved this particular issue?

Regards,

Dash1
 
The only reason HT leads/caps/plugs are resistive is for interference suppression.
You should be able to use cable and caps that are nominally zero resistance to help diagnose any faults on the car.
 
hey Dash, that's really great and thanks for the time in explaining it all. But i'm baffled by a couple of things.
1. What the dickens is terminal 15 when refering to the coil's primary connectors? Are you locating that particular terminal inside the ignition module (one of the piins)?
2. Looking at my spark plug caps (at the spark plug end), it's difficult to tell which type i have as they are made of what looks like a hard, whiite asbestos type material, rather than silicon.

3. Where EXACTLY is the CPS as i have looked for it closely but can't find it for the life of me

I did manage to test the secondary coil circuit and got a reading of about 10k ohms

M in Beirut
 
hi again Dash
Well, to answer your question about the o2 sensor, there were two wires visibly going into it, although a third one attached (but i could not see whether it actually reached the plug itself).
Here's what happened. I got a mechanic to gas weld it in, but i admit i didn't pay too much attention. A day later i tested the ohms coming out of the fine, third lead (which connects to the green one which actually takes the reading and processes it) and found there was nothing, so iu gently tugged the lead and found it was not conncted as i pulled it out of its sleave (it was about 30cm long). If you could let me know how i reconnect this, i would be grateful.
thanks
m in beirut
 
hey Dash, that's really great and thanks for the time in explaining it all. But i'm baffled by a couple of things.
1. What the dickens is terminal 15 when refering to the coil's primary connectors? Are you locating that particular terminal inside the ignition module (one of the piins)?
2. Looking at my spark plug caps (at the spark plug end), it's difficult to tell which type i have as they are made of what looks like a hard, whiite asbestos type material, rather than silicon.

3. Where EXACTLY is the CPS as i have looked for it closely but can't find it for the life of me

I did manage to test the secondary coil circuit and got a reading of about 10k ohms


Hi Stringer


When I referred to Pin No’s 15 and 1, these are actually the two connectors on the coil itself, the two upright posts next to the coil tower. If you clean the area around the pins, you will see the numbers actually stamped into the casing at the side of each terminal post.

When I said that your plug leads were constructed of silicone, I meant the actual leads themselves and not the plug caps. The plug caps were generally made of a pressed steel construction as these are subject to excess heat being close to the exhaust manifold.

The crankshaft position sensor is actually located at the rear of the engine on the bellhousing on the right hand side behind the oil filter. Some people actually remove the oil filter to gain access to it much easier, If you shine a torch down the back of the engine, you will see it at the top of the bellhousing on the right hand side. It is actually secured in place with a 5mm allen set screw. You could use an allen key socket and a universal joint on the end of a long extension bar and remove the screw that way. Tighten it back up to 10Nm.

Are you actually saying that the O2 sensor has a wire broken off at the sensor end where it fits into the exhaust, if so, then I’m afraid that cannot be repaired and you would require a new sensor. Come back and tell me exactly what colours the wires are on the sensor. It sounds as though it is actually a HEGO sensor (Heated Exhaust Gas Oxygen. Two of those wires (should be white) are for the heating element and have a resistance of approx 2.4 to 2.9 ohms.

As a matter of interest, where did you get this sensor from, and was it the correct one for the same model vehicle. Weld it in place, why? Was it the wrong thread size, 12mm or 18mm. 12 mm are generally Japanese sensors and 18mm are generally European sensors.

Regards,

Dash1
 
hi mate
thanks for all that. Very useful. I picked up the sensor in a junk yard. The reason why i had to get it welded on was that the original hole (with thread, i guess) was not to be found on what is probably an after-market exhaust system which is on the car now. I can't remember seeing a third terminal but i do remember the two white cables, as you mentioned.
best
m in beirut

ha! i've just seen you're in merseyside. Was in LVP 2 months back and had a great time. I'm having withdrawal symptoms of industrial quantities of guiness though
 
Last edited:

Users who are viewing this thread

Back
Top Bottom