Hi all, I'm trying to troubleshoot a vacuum problem in my '83 300SD that's causing hard downshifts. Measuring at the tee in the main line, before the white metering valve, how much vac should I be seeing at idle?

Right now it's got 11" at idle, rises to 15" at 2000rpm. I've seen posts mentioning anywhere from 15" to 20", is this correct?

__________________
1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)

Vacuum level is fine.

Connect a Mityvac directly to the transmission modulator and pump it up to 17" and see if it holds. If it holds.........take the vehicle for a drive at a full 17" vacuum and report back with results.

Brian,
Thanks for the quick response!

I disconnected the black line to the transmission from the Y connector in the engine compartment, sucked down to 17", it held for a ~2 mile loop around the neighborhood (lost maybe 1/16"). Downshifting was greatly improved, no clunking from the rear like I've had, going up from 1-2 and 2-3 was maybe a bit earlier than I'm used to.

__________________
1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)

With full vacuum to the transmission, quick shifts are expected. Under normal operating conditions, your vacuum control valve will vary the vacuum depending on how much you're on the go pedal. At full depression, the transmission will get no vacuum and therefore hold gears longer.
When you give it full vacuum, it thinks you're never getting on the pedal so it just shifts as soon as it can not utilizing power but maximizing economy

__________________
-Dan
1983 240D AT, 163K running on VO, "The Patience Tester"

I think vacuum only controls the firmness on the SD... frequency is controlled by the bowden... listen to Brian on this one. there is supposed to be a restrictor orifice in the modulator vacuum line. if it is gone you will get firm shifts.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

We've established that the modulator is functioning properly and would be fine with a bit more vacuum.

The best thing to do is to adjust the VCV to provide 15" of vacuum at idle and see if the problem is eliminated.

The adjustment screw is directly beneath the small domed cover on the side of the VCV. The screw needs to move very slightly (maybe five degrees) for the adjustment. Pump up the VCV with the Mityvac and turn the screw slightly. If the Mityvac drops.........you went the wrong way. If nothing happens.........you went the proper way and you must pump up the Mityvac to a higher level. Repeat until you get 15"..........then take it for a test drive with all the connections proper..........and without the Mityvac.

Pulled vacuum on the VCV where the green damper normally is, and it just won't hold. Got it to 15", then watched it slowly bleed out over about 90 seconds. I'm guessing that's not normal? Am I going to need a new VCV?

Is there any terrible harm that would come from just plumbing the transmission modulator directly to the supply tee, bypassing the VCV and 3/2 valves? Or, at least until I can get a new VCV, leaving the MityVac hooked up to the transmission line and pulling down to 15-17"?

__________________
1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)

Well, the answer is, putting the transmission vacuum modulator line directly into the supply tee at the brake booster leads to some weird things happening - shifts just fine, except I can't start the engine on the first try. It catches the second time though.

For now I'm running with the MityVac hooked up, pulling 15" of vacuum. If it's not the VCV, next best guess would be 3/2 valves? Might it be time to replace those, along with maybe the two Ys and the lines?

__________________
1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)

Follow my advice in post #6.

Brain,
Thanks again for the response. I think I've got it nailed down to a bad 3/2 switchover valve.

The VCV wasn't 'holding' vacuum by my definition - it would bleed slightly. I checked against the VCV on my '84, did the same thing. Either that's normal, or they're both failing.

This morning I got a tee so I could take readings in parallel while the engine was idling, and the VCV was getting no vacuum whatsoever. The supply line to the switchover valves doesn't hold vac when I apply it, which it does do on the '84.

__________________
1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)

Get rid of the switchover valves. Both lines to the black box can be eliminated. The T's that they go to can be eliminated. No sense in spending any money on the switchover valves when they only control the EGR on that vehicle.

Brian;

I had already ordered the new control arm levers for the switches. Should I still eliminate them? Could give me the link to your posts? I'll have vacuum readings by noon. Thanks.

John

__________________
Certified Registered Nurse Anesthetist (CRNA)
2002 F250 powerstroke with Plantdrive WVO conversion
1983 300SD 190K miles ,sold
2006 E320 CDI

Are you going to keep the EGR.........or not? If not, you don't need the switches and I'll tell you which lines to remove. If so, then you will need the switches for proper EGR function.