500E Electrical gremlin and Chat GPT

[JCM1]

Hi All,
I went to start my 1992 500e this morning and was unable to start the motor. I turned the key in the ignition, the panel lights lit as usual but when I turned the key there was one click and nothing. The panel lights went out so I checked the battery which checked out as "good" according to my battery tester. It's a 2 year old AGM with a 5-year guaranteed life so I was puzzled. Not being a Luddite, I of course went to Chat GPT and presented my symptoms and got several paragraphs of suggestions. One of which was to run my jumper cable from the negative terminal to the motor lift bracket which I did. I twisted the key in the ignition switch and presto the motor turned over and started right up.

According to the ChatGPT, the problem is a bad chassis to engine ground strap which it reports is a common problem with W124 and the 500e in particular. My question for you all, the real experts, is where is the engine to chassis ground strap located and is the diagnosis correct?


Many thanks,

Joe

 

[sheward]

I've never heard it mentioned as a common W124 problem. Clean both terminals and try starting it again without the jumper cable.

The engine to chassis ground strap is located under the car, drivers' side, at the front crossmember. Close to the left exhaust crossover pipe. It's a woven strap. Fairly easy to clean if you can get the car up a bit.

 

[JCM1]

Many thanks. I'll have to wait for the rain to stop before I crawl under.

 

[gsxr]

Chat GPT is wrong that this is a common problem on the 500E, or any W124, or any Mercedes. This would be an insanely rare failure. Drew is correct about the location near the steering gearbox.

In the meantime, double check that the battery terminals are clean and tight.

 

[JCM1]

Thanks gsxr! That's why I checked in with the forum. Battery terminals are clean and tight. I do have a battery isolate switch on the ground from the battery and will remove that and connect the battery ground directly to the frame bolt in the battery box. Maybe the isolate switch is the weak link.

A wise person once said.."It's not the not knowing so much that contributes to the sum of human ignorance as it's the knowing that that's not so". Chat GPT is often wrong.

All the best,


Joe

 

[TerryA]

It’s possibly a bad starter I had to change my starter when I first purchased my 93 500E back in 2005. I drove it to the barbershop about 2 miles away from home with no indication of a problem. When ready to go back home all I got was a click from the starter. I had it trucked home on a flatbed. I bought a ReMan starter from Mercedes and installed it laying on my back in the driveway. 20 years later no problems.

BTW; It’s a pita to install you need lots of different size extensions in both 1/2” & 3/8”

 

[JCM1]

Thanks Terry,
Hope it's not the starter but your symptom is exactly what I experienced. However, when I ran the jumper cable from the battery negative to the engine hoist bracket, it started right up on the first try. Got some detective work to do.

All the best,

Joe

 

[stfallet]

Did your battery tester just test voltage or actually the health of the battery, that it actually delivered the current and holds the energy? I have had plenty of challenges with AGM batteries in the W124s and remember AGM did not exist when these cars hit the market. It might be worth just connecting a battery in parallel with jumper cables and see if it reacts differently. If the voltage drops too low when the starter turns, the electronics will reset.

 

[gerryvz]

Good that you checked the terminals, and they are visually OK -- no corrosion, and tightly connected.

For shits and giggles, loosen the terminals, remove them, and re-install them. Clean the posts and the insides of the terminals, just to be sure.

If that doesn't change anything, trying to start after removing and physically cleaning the terminals (and yes, they can have a bad connection or corrosion that isn't necessarily visible to the naked eye), the number-one candidate for the problem is the battery being weak.

Measure voltage at the terminals. If it's less than 12.5V, the battery is bad.

Put a starter battery box on it (or a charged spare battery) and see if you can get it started. That will tell you right away if it is the battery.

If you had a bad alternator, you would probably be able to get the car started, but it would run on the battery's stored power (at least for a while).

If you are able to start the car with a spare battery or battery box, check the battery terminals again while the car is running. If the measured voltage isn't at least 13.5-14V while running, then it is likely that the alternator is bad.

Check back with us once you run these checks. Again, DO NOT DISCOUNT that the terminals could be corroded. Please remove, wire-brush and then re-install them and let us know if that solves the problem. A very easy, 10-minute first step.

 

[JCM1]

View attachment 233572

Did your battery tester just test voltage or actually the health of the battery, that it actually delivered the current and holds the energy? I have had plenty of challenges with AGM batteries in the W124s and remember AGM did not exist when these cars hit the market. It might be worth just connecting a battery in parallel with jumper cables and see if it reacts differently. If the voltage drops too low when the starter turns, the electronics will reset.

I am familiar with the difficulties involved with AGM batteries. I have a pretty good battery tester - I've had issues with AGMs in the past and found that a good tester and battery charge is critical. Here is a screen shot of the last reading. Unfortunately, I don't have a spare battery to hook up in parallel.

 

[JCM1]

Good that you checked the terminals, and they are visually OK -- no corrosion, and tightly connected.

For shits and giggles, loosen the terminals, remove them, and re-install them. Clean the posts and the insides of the terminals, just to be sure.

If that doesn't change anything, trying to start after removing and physically cleaning the terminals (and yes, they can have a bad connection or corrosion that isn't necessarily visible to the naked eye), the number-one candidate for the problem is the battery being weak.

Measure voltage at the terminals. If it's less than 12.5V, the battery is bad.

Put a starter battery box on it (or a charged spare battery) and see if you can get it started. That will tell you right away if it is the battery.

If you had a bad alternator, you would probably be able to get the car started, but it would run on the battery's stored power (at least for a while).

If you are able to start the car with a spare battery or battery box, check the battery terminals again while the car is running. If the measured voltage isn't at least 13.5-14V while running, then it is likely that the alternator is bad.

Check back with us once you run these checks. Again, DO NOT DISCOUNT that the terminals could be corroded. Please remove, wire-brush and then re-install them and let us know if that solves the problem. A very easy, 10-minute first step.

Thanks Gerry! I've done all of the above except installing a spare battery in parallel. I also used Deoxit 100 on the terminals and ground connection. I've got to wait for the rain here to stop so I can inspect/clean the chassis to motor ground connection which unfortunately looks like it will be a week or so before we get a sunny break. I posted a shot of the battery tester above and I think it's in pretty good shape. Will revert once I've had a chance to check the ground strap. I'm also going to bypass the isolate switch on the negative battery cable.

All the best,


Joe

 

[stfallet]

You always have to be careful with Voltage readings from batteries as they can be deceiving when there is no load. Having said that, taking measurements at different points, both in the off position and while trying to start may help you narrow down things. A starting point could be to take the plus from the fusebox and measure the minus from the chais and as a second measurement, the point you connected the jumper cable, etc. and compare this with the results you get when directly reading from the batterie with the jumper cable. Still that you all of a sudden have a ground problem surprises me

 

[JCM1]

You always have to be careful with Voltage readings from batteries as they can be deceiving when there is no load. Having said that, taking measurements at different points, both in the off position and while trying to start may help you narrow down things. A starting point could be to take the plus from the fusebox and measure the minus from the chais and as a second measurement, the point you connected the jumper cable, etc. and compare this with the results you get when directly reading from the batterie with the jumper cable. Still that you all of a sudden have a ground problem surprises me

Thanks for the response and good points all. I too am surprised with this sudden issue - potentially chassis-ground issue or perhaps something else (starter/battery/ignition switch) I'll post results once all items have been investigated.

All the best,

Joe

 

[JCM1]

Well it looks like the culprit is not the chassis - ground. I checked the battery this morning at the terminals and got a "Good Battery" reading as in the pictured in my previous post. I then connected the tester to the battery's "+" terminal, closed the battery isolate switch and checked again the chassis ground in the battery box. This time I got a "Replace Battery" reading.

I removed the battery isolate switch and using the battery tester, check again at the chassis ground post in the battery box and got a "Good Battery" reading!! I tried the ignition and the car stared instantly!!

I have attached a picture of the isolate switch and it is immediately evident that the corrosion at the connect point (the annular metal ring in the green screw down) was not letting current flow. I have removed the isolate switch from the ground line and will look for an alternate replacement.

Thanks to all for the suggestions.

All the best,


Joe