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Old 11-12-2000, 02:09 PM
240Joe's Avatar
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Join Date: Nov 2000
Posts: 525
I'm trying to figure out why my 1980 240D won't always downshift from 4 to 3 when you put it to the floor running at about 30 to 40 mph.

I put my vacuum guage on the line that runs to the transmission and this is what I saw.

At idle, I get 20 inches of vacuum. Just a little off idle, it immediately switches to 9 inches. From just off idle to full throttle, it goes from 9 inches all the way to 0.

It seems to vary nicely with throttle position, but what I'm not sure about is the immediate jump from 20 to 9 just a little off idle. I know that is when the vacuum switch on the valve cover switches to the vacuum control value on the injection pump.

So three questions...
1. Does 20 inches seem ok for my vacuum pump, and

2. Is the immediate switch from 20 to 9 inches when the switch kicks in the vacuum control valve on the injection pump normal?

3. and if this is all normal, what is next, change the modulator in the transmission, or adjust somehow??


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Old 11-12-2000, 04:43 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Your vacuum seems about right. If anything you might have too large a drop when the proportioning valve cuts in (the 20 to 9 in drop). This would cause the opposite shifting problem though so I would say your problem is in the electric kickdown.

This is done by the small switch under the gas pedal and I don't remember where it goes from there on a 240D. On gas cars it is handled by the fuel pump relay, but I forget on yours. It will be apparant on the wiring diagram.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 11-12-2000, 06:11 PM
240Joe's Avatar
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Join Date: Nov 2000
Posts: 525
Steve...thanks for the comeback. Are you sure there is a electric switch on an 1980 240D? I was unaware that there is an electric switch, but I'll look.

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Old 11-12-2000, 06:19 PM
240Joe's Avatar
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Join Date: Nov 2000
Posts: 525
Steve...never mind the previous message...there is a switch there. I just checked. The problem is that the accelerator pedeal never hits the switch. It looks like the pedal runs out of travel before the switch is actuated.

I'll have to look it some more.

Thanks again.

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Old 04-10-2001, 10:13 AM
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Join Date: Apr 2001
Location: Houston, TX
Posts: 27
I'm the same way!

I have the same year car, and have the same problem. Don't know if you figured out a solution since the 12th of november; I'm new to the site. I looked at the linkage, and it just seems impossible to adjust so the pedal can hit the switch. I'm not sure what to do. There's a similar post, and one of the senior guys said to just take it to a MB mechanic to adjust. I feel it's too simple to take it to a mechanic, but i still haven't figured out a solution. But as soon as I do, I'll be posting my findings.
'80 240D (sold) wish I didn't.
'85 300D (240K miles)
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