W124 Ammeter DIY - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 06-01-2010, 12:41 AM

I thought it might be nice to have an ammeter in my '87 300D Turbo (W124, OM603) for diagnostic purposes (I have a voltmeter in my dash panel for daily monitoring). Since I had an inexpensive ammeter that had been in my junkbox for years, I decided to experiment. (Obviously this mod isn't unique to the W124 model, it will work on any car.) You can do the same job with the inductive ammeters that some forum members have but I don't have one of those and did have this one.

Rather than run heavy-gauge wires into the passenger cabin or buy an expensive ammeter that uses an external shunt, I mounted the ammeter next to the battery and spliced into the main cable from the battery to the junction (X35) on the rear of the right shock tower (not the starter cable).

I made a bracket for the ammeter from a piece of heavy aluminum scrap and mounted it to the factory bracket that holds the EDS test connector (X29/4). The main cable then runs from the battery to the negative terminal of the ammeter and a second heavy cable (from the local Pick and Pull) goes from the positive ammeter terminal to X35. The increase in wire length is only about a foot so the voltage drop is negligible.

So what good is a gauge that you can't see while driving? Well, this gauge is for monitoring the charging/discharge rate of the battery while you work on the car. Are you trying to diagnose an electrical problem by disconnecting things or pulling fuses? For large loads that your DVM can't handle, the ammeter will show the increase or decrease of the load on the battery. Is your OM617's alternator belt slipping? The ammeter will let you watch the rate of charge increase (or not) as you tighten the belt. I'm sure you can think of many other uses.

Like all of my modifications, this one can be reversed by simply removing the added components. Moving the ammeter out of the way to work on the monovalve requires removing only two screws. The hardest part of the whole job was removing the battery so I could get at junction X35 just in front of it.

I'm still searching for rubber boots to cover the terminals on the back of the ammeter -- a bad place to have a short circuit!

Jeremy

Sev · #2 06-01-2010, 01:04 AM

dude you're missing the plastic cover that goes over the diagnostic port. otherwise not a bad idea. does it work on 500e500?

Jeremy5848 · #3 06-01-2010, 01:19 AM

Quote:

Originally Posted by Sev

dude you're missing the plastic cover that goes over the diagnostic port. otherwise not a bad idea. does it work on 500e500?

Sev, you're right, but I haven't been able to find a cover for my year-model. The later years have a different shape piece of plastic for the port and the covers won't fit my port.

My ammeter mod should work on any car so long as you can get at the positive cable. You can't put the ammeter in the starter cable and some cars use the big terminal on the starter as a junction. That makes it a lot more work because you have to run wires from the starter up to the ammeter. I'm not familiar with how Mercedes built the 500E -- aren't there models with the battery in the trunk? My wife's E300D (1996) has the battery under the rear seat.

DieselPaul · #4 06-01-2010, 01:27 AM

I happen to be parting out a 1987 300D... want one?

Chas H · #5 06-01-2010, 01:50 AM

I have an inductive ammeter, but I keep it in my tool box so I can use it on other cars.
Normally cars don't have a charging issue.

Jeremy5848 · #6 06-01-2010, 01:52 AM

Quote:

Originally Posted by DieselPaul

I happen to be parting out a 1987 300D... want one?

Please! How much and all that?

Jeremy

Jeremy5848 · #7 06-01-2010, 01:55 AM

Quote:

Originally Posted by Chas H

I have an inductive ammeter, but I keep it in my tool box so I can use it on other cars.
Normally cars don't have a charging issue.

Absolutely right! Hopefully I'll never need to use mine. Then again, I've been surprised at how many battery-charging, alternator-failing threads show up on this forum.

What is the range of your inductive ammeter? I know that several ranges are available.

Chas H · #8 06-01-2010, 02:22 AM

Quote:

Originally Posted by Jeremy5848

Absolutely right! Hopefully I'll never need to use mine. Then again, I've been surprised at how many battery-charging, alternator-failing threads show up on this forum.

What is the range of your inductive ammeter? I know that several ranges are available.

It's been so long that I've used it I don't remember, but 30 amps comes to mind. It's stored away with a Mac "full fielder" that allows the tech to easily apply full voltage to the field circuit. It's a great diagnostic tool for older cars with a discrete voltage regulator.