Can I fix harness problem?
 

Just changed the headgasket on our 95 E320. Car is running great and no leaks, but now the climate control and reverse lights don't work (16A red fuse, maybe #7, blows as soon as the key switch is turned to ignition.) The heat definitely worked the day before we started the job...so I'm thinking its a harness issue from moving it a bit. I didn't remove it from the car, just unplugged everything necessary and moved it out of the way. You can see the larger plug on top of the water pump that runs the two electric fans in the grill has cracked insulation and some has come off completely where the actual wires are exposed near the plug. However, nothing appears to actually be touching. A few inches back in the harness you can see cracking of the insulation but no insulation has come off because the black cloth wrap holds everything tightly in place. Everything else in the car works fine. Rather than shell out for a new harness, (this is our 4th car) how can I find and fix this problem? Basically, how can I find which of the circuits on fuse #7 is actually causing the short? Anybody got electrical diagrams?

Thanks

I recommend putting the crowbar to your wallet and replacing the engine wiring harness. The harness movement during the head gasket change caused the insulation to flake off and now you have naked wire against naked wire. For these models it's not a matter of repairing the wiring, but replacing it so this problem doesn't fry other delicate electronics.

I agree with Kestas on this, listen to what he says, he knows!!

When i did my head gasket, my car had the updated/ rebuilt wiring harness (thankfully) so i had no issue with that. where i had an issue was with the lower engine wiring harness (goes to oil pressure sender, oil level sender, starter ,,etc). that one had not been touched before...

I was shocked -to say the least - by the amount of degradation on these wires. this is not your simple brake-down of insulation, on the contrary, the insulation turns to powder once moved!!! literally powder!!! Once i tried to tape the broken insulation, the movement caused even more insulation to break and disappear, and the wires where touching everywhere......see my headgasket thread for pictures...

the idea here is that this is NOT doable as a patch job..either a new harness, or a harness rebuilt from scratch with new wires will solve the problems...with all the moving-around of wires during the headgasket job, you will be surprised by the amount of degradation taking place.....

good luck !!!

If you dig around on this site- several ppl have rebuilt their harnesses. The E500/420's ran hot. You can replace all the wire with some mil-spec high temp wire- teflon coated, silver or tinned copper. Pins in the plugs are available new for pennies from the dealership if you need some-same for the plastic connectors.
It's about 2 evening job- 6hrs and a few beers to rebuild a harness. It won't look like the mercedes factory one-but if you do it right, it will function perfectly and outlast the car.


Michael

Well thanks for the advice. Really trying not to pay the dealer so much for something they should have recalled. I may try rebuilding the entire harness myself. But initially, I may try to repair just what needs to be for now. Keep in mind this is the 2nd time the headgasket has been replaced...the harness survived fine 3 years ago. And even unwrapping it a bit reveals severely cracked but perfectly intact insulation being held in place by the tight black tape wrap. Nothing is turning to dust...I don't think my harness is in as critical condition as some apparently are. The only areas where the insulation has basically fallen off the harness is the few inches where the plug on top of the water pump is, maybe one other exposed spot. There is absolutely no misfiring or running concerns at this point, just the one circuit affected. I'll update on the progress...

Fixed!
 

Get this...unplugged the air pump, and the fuse doesn't pop anymore. Not even on the same circuit, weird. No MIL, so guess the pump stays unplugged in favor of heat and reversing lights. Harness survives its second headgasket change!

brittle wiring harness
 

Your solution will be temporary. I too thought a simple tape job would suffice, but not soon after, ASR lights, check engine lights, engine sputter all began with my 1995 SL500. I took a surgical scalpel and cut through that tape on the wiring harness all the way back to the firewall, the wires are all brittle and exposed along the pathway. You run the risk of frying your computer or engine fire. I just ordered a new harness and it is a do it yourself job with the advice and help from this forum. Good Luck, Steve

What's a known-good used late harness worth?

Just wondering, considering another diesel conversion (late '95).

My car doesn't have ASR / ASD, so no electronic throttle actuator. Less wires to go bad. Making some cuts further up the harness revealed cracking in the insulation but no loss of insulation. Right up near the firewall the wiring looks perfect...no cracks. I don't think mine is as bad as some. Wiring going to spark plugs / coils is fine. Kind of doubt the possibility of an engine fire even if the harness starting shorting. Fuses would pop, and for the most part the harness runs along the top of the engine, which is completely oil free and clean. Plastic and insulation might melt, but a fire I doubt. BTW, fixed air pump wiring and that is working again. Right now changing rear accumulators, what a fun messy job...

Rebuild the wiring harness with wire that is larger than stock by a gauge or two. Solder all your connections (rosin core solder only, not acid core). Wrap the new harness in fibreglass heat insulation tape (the kind that hot rodders use on their headers).

I prefer to use boat cable, and upsizing the wiring isn't necessary from anything I've seen, but it will make the harness huge.

If the harness acts up further I'll probably rebuild it myself. Have one fitting left on the accumulator job and then its ready for the road (of course its the hardest fitting!)

"I prefer to use boat cable, and upsizing the wiring isn't necessary from anything I've seen, but it will make the harness huge."

You don't need to use any special wire if you wrap the harness in fibreglass heat insulation tape because then heat can't get to the wire in the first place and the insulation will never break down.

Using larger-gauge wire greatly reduces the voltage drops on the harness. Voltage drops are dissipated as heat - and heat what caused your insulation to break down in the first place. The factory used the smallest gauge wire they could get away with. Also, the heat created by voltage drops overheats the copper wire, slowly converting it to copper oxide, which is not a good conductor. It's a vicious circle: the more of the wire that is converted to copper oxide, the worse the voltage drop gets and the more heat is dissipated until the wire gets really hot and either burns up and/or the insulation fails. Then usually your alternator output gets shorted to ground and your alternator burns up. Have you priced a M-B alternator recently?

So what if the rebuilt harness is bigger? After all, this isn't a beauty contest, and if you are going to wrap it in fibreglass insulation tape it isn't going to look like a stock harness anyway. And it will work much better! By using larger-gauge wire and fibreglass insulation tape, you virtually eliminate overheating of the wire.

Okay, the heat broke down the insulation, not the copper. The copper can take much more heat than the insulation, it is oxydized because it was in contact with OXYGEN, not heat, and the failure mode is shorting between the adjaecent un-insulated wire sections. Excessive heat causes copper to become brittle, usually characterized by a brown appearance and how it breaks easily without its normal flexibility. Further, the insulation was a degradable type of insulation by design, it just wasn't supposed to do that (for recyclability) while still in the car. Since the OP will be using conventional insulation wire and not degradable, it'll be fine.

Boat cable is pre-tinned, easier to work with, less prone to corrosion where exposed, and more flexible which all makes for a better harness and easier for the OP to work with. The normal household THHN etc. will not be flexible enough for automotive use, and the stuff you buy at autozone et al can be of dubious quality.

Mercedes didn't use "the smallest gauge wire they could get away with", they used the correct wire gauge for the loads and fusing, and calculated any voltage drop into the electrical systems. I'd like to see your voltage-drop calculations on the wiring to the sensors etc. in this car. Most of these wires carry a fraction of what they're capable of carrying and are sized for the fusing (catastrophic failure / short). Mercedes-Benz employed Electrical Engineers, not posters on a web forum. The same gauge wire as the original harness is all you need.

As far as I understand it, the insulation on the wiring deteriorated because of European environmental mandates in the early-mid 90s that the insulation on such wiring be of a biodegradable grade. Not because of heat exposure, though it probably exacerbates the imminent failure.
A poorly considered and engineered situation that led to premature deterioration on cars with superior longevity. Other German and European cars from the mid 90s suffer the same problems.

"The wire oxydized because it was in contact with OXYGEN, not heat"

You're setting up a straw man. Nobody claimed that oxygen wasn't involved in the transformation of copper metal to copper oxide. Any chemical reaction, such as the oxidation-reduction reaction that copper wire experiences when it becomes copper oxide and begins to conduct poorly, proceeds much more readily with increasing temperature. Indeed, in this case, the conversion of copper metal to copper oxide is caused by overheating. Anything you can do to prevent overheating will forestall the conversion of the copper metal to copper oxide

"...and the failure mode is shorting between the adjaecent [sic] un-insulated wire sections.

This statement just begs the question of what caused the insulation to break down in the first place. Heat causes it. The heat has two sources: convection from the hot engine, and voltage drop in the wire. You can prevent convection from the engine by wrapping the harness in fibreglass heat-insulating tape, and you can prevent heating due to voltage drop in the wire by using a larger-gauge wire.

"Excessive heat causes copper to become brittle, usually characterized by a brown appearance and how it breaks easily without its normal flexibility."

Another straw man. That's exactly what I said, but you're trying to make it appear that I said something different. That brittle, brown stuff is copper oxide.

"Boat cable is pre-tinned, easier to work with, less prone to corrosion where exposed, and more flexible which all makes for a better harness and easier for the OP to work with. The normal household THHN etc. will not be flexible enough for automotive use, and the stuff you buy at autozone et al can be of dubious quality."

I never said you shouldn't use good quality wire; just that if you're going to wrap it in fibreglass heat insulating tape, you don't need to use mil-spec or boat wire because it is not going to be exposed to any heat in the first place. For example, I wrapped the headers on my hot rod with fibreglass heat-insulating tape. When the engine was running full-tilt on the dyno and headers were red hot, the heat-insulating tape was only warm to the touch.

"Mercedes didn't use "the smallest gauge wire they could get away with", they used the correct wire gauge"

You're distorting what I said. Yes, they used the correct wire gauge if everything works as it did when it left the factory. But as the factory-stock wire degrades, and when the connectors get dirty, there's no conductivity headroom. That's what I meant by "the smallest gauge wire they could get away with". By using larger gauge wire, you have much more conductivity headroom to prevent excessive voltage drops.

"I'd like to see your voltage-drop calculations on the wiring"

Sure, no problem. The current-carrying capacity of a wire is proportional to its cross-sectional area, which is in turn proportional to its diameter squared. Each AWG gauge has a diameter approximately 1.12 times that of the next smaller gauge. That means that, for example, AWG 16 gauge wire has a diameter 1.25 times as large as AWG 18 gauge wire (because it's two gauges larger). Therefore, AWG 16 gauge wire has a current-carrying capacity of almost 1.6 times (1.25 squared) that of AWG 18 gauge wire before excessive voltage drop is encountered. That's a lot more headroom!

Methinks thou dost protest too much.

So, you're more interested in winning an arguement than helping the OP, I get it. You've really stated nothing new nor addressed the inaccuracies in your former posts.

To the OP: Make the harness out of the OE gauge wire, use good quality automotive or better wire, I suggest boat cable, wrap and run it as original, and you'll be fine just like the millions of Mercedes-Benz harnesses that preceeded and succeeded the bad-insulation years. Fiberglass wraps et al are not necessary nor are over-sizing the wires. Good luck with the new harness, should take a little time and patience.

I'll be a little diplomat here, both points of view add value to the thread as well as address the OP's concerns.
The point that using good quality original spec wiring would be more than sufficient to last the remaining life of the car is correct and well taken. Though, suggesting the use of materials that exceed spec supplemented with techniques and protection that address conditions that had some affect on the failure are also quite valid, not necessarily incorrect and are certainly worthwhile to point out.
While I would probably opt for the factory spec system (in this case) I'm glad to have learned about the protective quality of fiberglas tape and the explanation of wire gauges will be useful to many readers.

I'm curious though, as I'm not so electrically knowledgeable. Can't over-gauging wire pose a potential threat to some more delicate instruments and sensors? That is, some wire gauges are actually chosen to cap or restrict the flow of juice (provide resistance) and using a larger gauge wire might allow too much amperage to flow though components (particularly computers and control units) that aren't rated for such flow. Maybe I'm wrong about this or maybe it doesn't really apply here, just curious.

Although the wire does technically provide resistance, and reduce voltage, the voltage drop over a few feet of 16AWG wire carrying perhaps 20ma-100ma is negligable. The gauges are calibrated / designed for that voltage drop and could read a very small amount differently with oversized wiring, but for the most part no change, same with the computers.

As far as protection goes, fuses not wires provide that protection.

I'm a bit late to this party, but bear with me. Rebuilding, the harness may be a weekend event, but what would be the harm in fiberglass wrapping the rebuilt harness? The way I understand it, heat creates resistance ad resistance creates heat...would it hurt to minimize the affects of ambient heat at the get go?

Suit yourself on rewiring/rebuilding your engine wiring harness. The risk of engine fire alone would be worth the ~$800 to me, not to mention the aggravation of the car potentially leaving you stranded...

-GH

Thanks babymog. Hmmmm, yep, there are those pesky fuses keeping the juice pretty well under control, aren't there. :rolleyes:
Okay , I'm an idiot, but I did admit that i weren't none two 'lectrificly lerned!

Thanks for all the enlightening. In giving this a little more thought, what we do with the harness depends on how long we continue to use this car and how well it performs and how many additional repairs it needs. If we drive this car for, say two more years and all it requires is oil changes and fuel, we might be inclined to opt for a new factory harness just for convenience. If this car continues to rack up repairs or becomes unreliable, it may well just get sold. In reality, the car isn't worth much...a 15 year old mercedes with 115k miles is worth maybe 2500? So spending more than a 3rd of the value of the car on a harness doesn't make sense considering it is a 3rd vehicle anyway. So we will see. I wouldn't mind rebuilding the harness myself but maybe trading for a newer 3rd car (210 benz) is a better bet? I do like having a 'classic' mercedes though.

I just did a quick web check. The boat cable I found is only rated to 105 deg C. http://bestboatwire.com/catalog/default.php/cPath/23 Is that high enough? I would guess not for all locations in the engine, especially in the wiring located right on the engine. Is there a higher temperature spec for boat wire than this?

One of these days I am going to rig my thermocouple reader to monitor engine compartment temperatures at various locations so I can see just what is going on in there. I am just enough of a nerd to do this, too :-P I live in a hot climate so this is a more challenging application than, say, for someone living in the northeast.

That wiring is considered heat resistant, but there are other grades rated higher.
AVX: 100°C
AEX: 120°C
GXL, JXL: 125°C