I have just come into an 87 300SDL and it is of course, not without issues. Most of the issues are electrical, and there are a few I am not sure where to start at. First, and foremost the tachometer is not working (as well as the idle control circuit). I found a post from a while ago that mentioned cleaning the contacts on the RPM sensor and checking the OVP relay. I have not gotten my shop tech manual yet and am not sure where to start looking for these two items.
Next the ABS light is on. I have seen that come on in my 124 when my alternator puked 2 years ago, but I checked the charging system and all seems to be up to par.
I also noticed that I wasn’t getting any boost even at higher RPM, so I believe that the relay on the boost pressure line to the ALDA from the intake is also being affected, because I did a test run with the relay bypassed and the engine had plenty of giddy-up. The only other bothersome electrical issue is the oil pressure gauge pegs high even before the engine is started. I am guessing that it is either the sensor or wiring. I also read about a bad ground connection causing similar issues, any thoughts?

Thanks for your time

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Dean

Get a 10A fuse and go to the left of the brake booster, the red deal is the OVP relay. Dollar to a donut the fuse is blown, that messes with your idle, ABS, etc.

The connection to the oil pressure sender could be loose or soiled.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

I'll check that right away,

<<<

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Dean

Well when I unplug my OVP my idle drops to nothing and my chums 300E had the ABS light on when the OVP fuse was blown, we replaced the fuse... light was off.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Dean,

Pull and clean all the fuses and relays. Checking the fuse on the OVP is a great idea too.

Sometimes the spring in the oil sender can break, so no tension. Might cause it to spike immediately. They are fairly cheap.

ABS can be power (voltage), one of the relays, or I've seen the wiring from the sensor go bad from wear. Trace them from the front wheels back to the connectors. I think one is under the radiator overflow or washer container. The other is near the fuse box on the drivers side. I think they were round and looked like a coax connector.

The lack of boost could be an oil plugged line not an electrical issue with the overboost. It sounds like all is fine when you skip it. You ought to have 0-60 in the 11-13 second range. My went to 20 seconds when plugged. Follow the boost line from the intake to the relay to the ALDA. The relay also plugs up.

Good luck!
Chuck

Chuck, I had cleaned out the lines prior to performing testing, connected them back up and tested, then thinking my issue might be electrical, I decided to bypass the "boost" relay. and that test result was positive, I got the famous 603 performance I have come to know and love.

Hitman, the fuse was blown. I hunted and found a replacement, the tach came back, the antilock light went out and the idle straightened out.

that was the needle in the present haystack.

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Dean

hitman is smarter then i look

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commercial fisherman diesel in my blood in my boats trucks and Dear old Dad has had me drivin them since i got my first license in 1968
1986 300 SDL 427654
1999 Chevy Crew Cab Dually 225423
1986 300 SDL 287000 Dad's
1987 190 Turbo 158000 Mom's (my inheritance)

Dean, that's the "overboost protection relay". It can get clogged with soot over time and block the pressure from the manifold. It's usually not an electrical problem.

You could consider removing the ALDA completely from the engine. Eliminates all that problem. I've done it on both of these and I love the driveability of them. You do lose the overboost protection, however.

Just clean the vacuum line its probably clogged with EGR tar. Once it can get the proper vacuum the relay will probably work.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Brian, did you have to plug the hole that the alda seated into? do you have any photos of what you did?

I will be working on it some more tomorrow and will try to "unclog" the relay, or i may just swap the one from my dead 300d.

The interior of this car needs some real help, it is burgundy. what years and models are compatible to the 87 300 SDL, and does anyone know of any possible donors?

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Dean

I hadn't planned to bother with a cap to replace the ALDA. It's basically a plunger that sits beneath it. However, Martin made a couple of threaded caps and I'm going to install those just to keep the dust out of the IP.

There's not much to it. Use two wrenches.......or one pair of vice grips and one wrench and loosen the nut under the ALDA. Unthread it and the ALDA will be removed. You'll need to plug the boost pressure line at the ALDA. The line is in a Y and continues onto the vacuum amplifer for the transmission. Or you can just provide boost pressure directly to the vacuum amplifer and eliminate all the pressure lines to and from the overboost solenoid.

Any W126 made from 1986-91 should work. The 89 and newer ones had the updated interior which was really nice. Start looking for a rusted out or shot 420/560 that has a good interior.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Good deal!

BTW - maroon interior parts are VERY difficult to locate!

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]