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Fuel Pump Relay 1988 300E

9K views 6 replies 4 participants last post by  benzborg 
#1 ·
1988 300E Mercedes 6 cyl gas 220K.
Trying to track down a low idle (to stall) problem.

Original breakdown due to cold soldered pin on the board of the fuel pump relay which we resoldered. Found this "fix" in another thread in this forum, seems to be a common soldering problem for this control. Works great in every way but I now have a low idle problem.

Replaced OVR and fuse. Replaced fuel filter and checked fuel pump thoroughly.

Prior to breakdown I never had an idle problem. Why after the "fix" would I have this problem with idling now? Alternator fairly new and good battery, but with lots of load on electrical the low RPMs get worse. Putting the car in neutral allows the RPMs to increase enough to avoid a stall. (what a pain)

I'd replace the unit but my FPR numbers are different than any I've seen on other 88s. What is the story with these little headaches? I've tried swapping a working FPR from another 88 just to see if the idle increases again, but pins and numbers are different than mine. The numbers on mine are 003 545 00 05. Is this the original unit or an aftermarket? Not marked "Klima".
(I bought the car used) Other 88/87/89 300E MBZs seem to have various FPRs - if I plug one in from another 88, can it cause any damage? Why are pin layouts so different?

A new unit costs $150 plus tax here in Canada, and I'd really like to avoid having to fork that over if it might not even be the problem.

Thanks in advance - any replies I've asked to be fwd'd to my email so I won't miss any ideas clues or input.
 
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#2 ·
I would suggest that you put a multi-meter on your battery (with the motor running) and see what voltage reading you get. If the voltage regulator is up to par, you should get a reading of at least 13.4 + volts. If not, regardless of the alternator being somewhat new, you need to replace the regulator. It is mounted on the back of the alternator and held in place with two phillips screws. Takes about 10-15 min. to change out. Not an expensive part either. About $40 or less US depending on where you purchase it. Correct voltage is very important. If you would, be kind to take a few minutes and complete your profile. It's very helpful.

Good Luck,

Jayare
 
#3 ·
I would suggest that you put a multi-meter on your battery (with the motor running) and see what voltage reading you get. If the voltage regulator is up to par, you should get a reading of at least 13.4 + volts. If not, regardless of the alternator being somewhat new, you need to replace the regulator.

Thanks for the reply and I'll have a look sometime today for sure. Makes sense that load like headlights etc making it more prone to stall indicates a voltage drop somewhere.

Can't figure out how that would all of a sudden start to act up over an unrelated matter like the FPR cold solder getting repaired.
 
#4 ·
There is a thread somewhere that mentions troubleshooting fuel pump problems by installing a jumper across a couple of pins in the FPR socket. This might also be a good way to eliminate/confirm your relay as the source of the current problem. I know this isn't a scientific approach but it might be worth a try.
 
#5 ·
Yes I saw that get-around too but ended up trying another FPR today which made no improvement on the low idle. The voltage reading is 13.6 across the battery so it's not the regulator or alternator. According to some reading up on the OVP and input from a mechanic this is likely the cause.

From
Mercedes-Benz Area: Mercedes-Benz Driveability Evaluating Electronic Engine Controls

The number one intermittent problem we see is caused by the over-voltage protection (OVP) relay. Watching the idle valve current allowed me to understand how the OVP can cause stalling.Since a disconnected idle valve limps home at a higher than normal rpm, I always found it hard to understand how removing the idle valve?s power could cause a stall. I finally saw it, though, while driving and monitoring the idle valve current. The normal current of 600-700ma dropped to 350-450ma and the idle dropped to about 450 rpm. A thump on the OVP relay caused the current, as well as the idle, to instantly jump to proper value. The current was being controlled not by the KE controller, but rather by the bad internal power feed connection of the OVP.

I'm swapping mine for another tommorrow - wish me luck. :thumbsup:
 
#7 ·
Mmmm, what a headache. I swapped a working OVP (from another 88 300E that runs well), as indicated in my article yesterday. No improvement, still low idle rpm. 500 max, wants to go lower with load like headlights.

If you happened to be following this so far,

Car breakdown leads to re and re of fuel pump, no gas being pumped through line to engine, we assumed FP was to blame from square one. Cleaned contacts and test. Still no fuel, can't hear pump cycling. Check FPR and find bad cold solder connection on board. Repair as per this being a common problem for these FPRs. Fire it up - car starts, good pressure. Shut it off. Change filter. Putting it all back together, I noticed and replace burnt fuse in OVP. Fired up fine again, go for test drive - notice low idle and car will stall at a stop w/o a little foot on the pedal or putting car in neutral. Worse with load like headlights and heater. Checked alternator and VR - 13.6 volts.

Replaced FPR,(yesterday) no improvement.
Replaced OVP (today), still no improvement.

I'm almost thinking that removing the fuse from the OVP might bring my idle back up to 700 like before. There can not be *anything* else that caused that RPM to drop from 7/800 to 4/500 other than that fuse being replaced.
Removing the fuse defeats the OVP however, and still no idea why that fuse was burnt out from the onset of all this. The car was likely driven with that burned fuse over the last two years since there was never a reason to check it. Fuel pump was always fine. New day tomorrow. I'll pull the replaced fuse in the OVP to get a final ruling. If this turns out to be the case, this car runs better w/o a fuse in the OVP which would tell someone who really knows their Benz(s), what is really going on behind this low RPM deal.

Believe I've used up all my "get out the dealership" free cards and may have to hand it over for the high dollar diagnosis.

:(
 
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